Zahia Bidai
email: bidai.zahia@univ-oran.dz

Moufida Maimour
email: moufida.maimour@univ-lorraine.fr

Multipath Routing for High-Data Rate Applications in ZigBee Wireless Sensor Networks

Keywords: IEEE 802.15.4/ZigBee, Cluster-tree Topology, ZigBee Multipath Routing, Tree Routing, Wireless Sensor Networks

In this paper, we propose ZM R (ZigBee Multipath Routing), a novel hybrid multipath routing protocol for Zig-Bee/IEEE 802.15.4 networks. The main goal is to improve the global throughput which is basically insufficient in ZigBee-based Wireless Sensor Networks to satisfy high bandwidth requirements for high traffic applications like structure health monitoring systems, human health care systems and multimedia surveillance systems. The solution is based on the parent-child links and the availability of hierarchical address allocation process of the cluster-tree topology and the neighboring links. We compare our proposed multipath routing to the ZigBee one-path hierarchical tree routing T R through detailed simulations which showed that the proposed multipath routing enhances application performance in terms of effective bandwidth needed for high data rate applications, packet delivery ratio, incurred delay and network lifetime.

I. INTRODUCTION

Wireless sensor networks (WSNs) have appeared as one of the emerging technologies that combine automated sensing, embedded computing and wireless networking into tiny embedded devices. While the early research on WSNs has mainly focused on monitoring applications [START_REF] Wark | Transforming agriculture through pervasive wireless sensor networks[END_REF] [START_REF] Mainwaring | Wireless sensor networks for habitat monitoring[END_REF] based on low-rate data collection, current WSN applications can support more complex operations like building automation, industrial automation [START_REF] Milenkovic | Wireless sensor networks for personal health monitoring: issues and an implementation[END_REF] and personal health care [START_REF] Gungor | Industrial wireless sensor networks: challenges, design principles, and technical approaches[END_REF]. Besides these, the availability of low-cost CMOS cameras and microphones where a tiny sensor can be equipped with modules for collecting visual and audio informations [START_REF]Waspmote video camera, technical guide[END_REF] [START_REF]CSIRO multimedia node[END_REF], have enabled a new class of WSNs: multimedia or visual wireless sensor networks (WMSN/WVSN) [START_REF] Akyildiz | A survey on wireless multimedia sensor networks[END_REF] which has significantly enhance a wide range of applications like object detection, recognition, tracking and surveillance, etc. What is common in these emerging application domains is that performance and quality of service (QoS) assurance are becoming crucial as opposed to the besteffort performance in traditional monitoring applications. The importance of WSN has been enforced by the delivery of the most popular IEEE 802.15.4/ZigBee technology. ZigBee aims at handling low data rate, low cost devices and long-life batteries making it very suitable for wireless sensor networks [START_REF] Akylidiz | Wireless sensor networks: A survey[END_REF].

However, ZigBee technology suffers from its limited bandwidth (250 kbps at 2.4 GHz) and extending it to meet requirements of high data rate applications such as multimedia ones is a real challenge. Few works [START_REF] Garcia-Sanchez | Feasibility study of mpeg-4 transmission on ieee 802.15.4 network[END_REF][10] [START_REF] Garcia-Sanchez | A cross-layer solution for enabling real-time video transmission over ieee 802.15.4 networks[END_REF] [START_REF] Qin | Technologies to improve the performance of wireless sensor networks in high-traffic applications[END_REF] already considered multimedia and high data applications over IEEE 802.15.4 at PHY and MAC layers.

Routing is a key process to consider in WSNs when dealing with QoS requirements as routing decisions can impact the network lifetime, packet delivery rates and end-to-end packet delays. Zigbee specification [START_REF]Zigbee alliance[END_REF] provides basic recommendations on how to perform routing tasks. However, these are based on a simplified version of AODV and Tree Routing [START_REF] Kim | Shortcut tree routing in zigbee networks[END_REF][15] [START_REF] Ha | EHRP: Enhanced hierarchical routing protocol for zigbee mesh networks[END_REF] [START_REF] Tsai | Self-learning routing for zigbee wireless mesh networks[END_REF] that do not comply with the high data and multimedia application requirements. Having only one available route means that, in high data rate application scenarios with varying radio channel characteristics, connectivity is likely to be lost. Indeed, node/link failure causes packets to be dropped and may cause a delay in delivering the data to the destination in time; thus the requirements of these applications are not met which often making this approach to routing unsuitable for such applications. It is desirable to find alternative methods of reducing the effect of node/link failure on performance. Motivated by this, the aim of the work presented here is to promote the use of WSN ZigBee in high data rate applications by designing an efficient multipath algorithm. It is based on the parent-child relationships of the ZigBee cluster-tree topology and the neighboring links of the ZigBee mesh topology where direct transmissions between non parent-child devices is allowed. Multipath routing is desirable because an increase in the number of possible routes provides load balancing and increases the robustness and the bandwidth requirement of the application [START_REF] Ganesan | Highly-resilient, energyefficient multipath routing in wireless sensor networks[END_REF] [START_REF] Wan | K-multipath routing mechanism with load balancing in wireless sensor networks[END_REF]. Also, since there is more bandwidth available, a smaller end-to-end delay can be achieved [START_REF] Huang | End-to-end delay differentiation by prioritized multipath routing in wireless sensor networks[END_REF]. Given the considerable advantages of the multi-path routing technique, we focus on guaranteeing the required soft quality of service through multipath routing.

To the best of our knowledge, there has been no earlier work that deals with handling multipath routing in ZigBee WSN on top of the tree-based topology.

This paper is the extension of the authors previous publications [START_REF] Bidai | Node disjoint multi-path routing for zigbee cluster tree wireless sensor networks[END_REF] [START_REF] Bidai | Multipath extension of the zigbee tree routing in cluster-tree wireless sensor networks[END_REF]. The new contributions of this paper consist of presenting an enhanced ZigBee multipath routing to improve the QoS metrics required by the high data rate applications such as multimedia applications. The hybrid nature of the multipath routing proposed allows to achieve this purpose. Firstly, The proactive part of the routing protocol is activated in order to reduce the end-to-end latency and to increase the network bandwidth, the reactive part of the protocol is triggered to discover other node disjoint paths. Extensive simulations have been carried out to evaluate the improved multipath routing from different aspects include the offered multipath effective bandwidth. More details will be given later. This paper is structured as follows. Section II briefly introduces the IEEE 802.15.4 /ZigBee standard and mainly the cluster-tree topology and Tree Routing protocol. Our proposed ZigBee multipath routing protocol is described in Section III and Section IV provides our simulation results. Finally, Section V concludes the paper.

II. IEEE 802.15.4 AND ZIGBEE

We consider a WSN using the ZigBee technology, composed of static sensor nodes deployed in a field of interest. ZigBee is a robust wireless communication standard managed by the ZigBee Alliance [START_REF]Zigbee alliance[END_REF] and based on the IEEE 802.15.4 physical and MAC layer standard [START_REF].4: Wireless medium access control (MAC) and physical layer (PHY) specifications for low-rate wireless personal area networks (LR-WPANs)[END_REF]. It defines a network layer, application framework as well as security services. The MAC layer defines two types of nodes: Reduced Function Devices (RFDs) and Fully Functional Devices (FFDs).

The ZigBee network layer defines how a network address is assigned to each participant ZigBee device and how a data packet is routed. ZigBee specification extends the basic star topology of an IEEE 802.15.4 [START_REF].4: Wireless medium access control (MAC) and physical layer (PHY) specifications for low-rate wireless personal area networks (LR-WPANs)[END_REF] to a mesh or a cluster-tree topology. In a cluster-tree topology, the root, called the ZigBee coordinator (ZC), and all internal nodes refered to as ZigBee Routers (ZRs) are FFDs. RFDs can only be leaf nodes and are designated by ZigBee End Devices (ZEDs). A cluster-tree ZigBee network is characterized by three topological parameters (L m , C m , R m ) fixed by the ZigBee coordinator where L m is the maximum depth of the tree, C m and R m are respectively the maximum number of joining children (ZEDs or ZRs) and ZRs a parent can have. According to ZigBee specification, the ZC is at depth 0 and the end devices are at depth L m and can only be ZEDs. Based on its depth d (d ∈ [0, L m -1]) in the tree, a (ZC or ZR) assigns for every joining ZR a range of consecutive addresses with size Cskip(d). Assume that for a given parent node at depth d with network address A p , the addresses in [1, R m ] and [R m +1, C m ] are assigned to its children ZRs and ZEDs respectively, then the k th ZR and the n th ZED will be assigned the network addresses :

A k = A p + Cskip(d).(k -1) + 1, k ∈ [1, R m ] (1) A n = A p + Cskip(d).R m + n, n ∈ [1, C m -R m ] (2)
Cskip(d) is given by :

Cskip(d) = 1 + C m .(L m -d -1) if R m = 1 Cm.R (Lm-d-1) m +Rm-Cm-1 Rm-1 otherwise (3)
Two routing protocols are considered in ZigBee depending on the adopted topology. The table-driven routing (similar to AODV : Ad hoc On demand Distance Vector) [START_REF] Baronti | Wireless sensor networks: A survey on the state of the art and the 802.15.4 and zigbee standards[END_REF][25][26] [START_REF] Ran | Zigbee routing selection strategy based on data services and energy balanced zigbee routing[END_REF] is used in the mesh topology while tree routing (T R) [START_REF] Nefzi | performance analysis and improvement of zigbee routing protocol[END_REF][14] is adopted for a tree topology. T R can directly infer the routing paths from network addresses based on the parentchild relationships. No extra memory and broadcast overhead are required. However, routing paths could be longer because the data packets follow the hierarchical tree topology to the destination even if the destination is close to the source node. When a packet is received by a ZigBee router (ZR), it decides on whether the next hop node for the destination address is up or down the tree. Since T R is simple to implement and is lighter in terms of memory and processing requirements than the AODV routing, it is more suitable for the ZigBee limited resources devices.

III. ZIGBEE MULTIPATH ROUTING

Our proposal consists in ZM R (ZigBee Multipath Routing), a multipath routing protocol built on top of the ZigBee T R protocol. On one hand, we take advantage of T R's simplicity and limited required resources. On the other hand, the single path T R's failure drawback can be avoided since other alternative paths can be used. Indeed, in ZM R, more than one path (if available) are used simultaneously, so the aggregate bandwidth may satisfy the bandwidth requirement of high data rate applications. Moreover, these paths are node disjoint which allows additional resources and higher fault-tolerance. In what follows, we begin by giving some preliminaries before presenting how ZM R builds its node disjoint paths.

A. Preliminaries

In this work, we only consider the case of routing toward the Sink (a central controller or a gateway) of the WSN. The cluster-tree topology is assumed to be rooted at the Sink that plays the role of the coordinator. The ZM R is applied on a steady state network where all the devices are well associated to their parent devices. To make its forwarding decisions, ZM R relies on this tree structure mainly on the already parent-child established relationships in addition to neighborhood relationships. In ZM R, each node is assumed to maintain an up-to-date neighbors table that contains all nodes that are physically in its radio range (parent, child or adjacent nodes). This neighbors table can be built simply during joining process when a node scans its neighborhood to find potential parent. This information is used during the routing task. We further assume that neighboring links are symmetric. That is, if node N a is a neighbor of node N b then node N b is a neighbor of node N a .

In order to be able to check, based only on parent-child relationships, if two paths are node disjoint or not, we introduce the ZigBee Tree Path information. For a given node C at depth d c ∈ [1, L m ], the ZigBee Tree Path information of node C noted ZT P c is an integer sequence (Z c 1 , Z c 2 , ..., Z c dc ) that defines the parent-child path in the tree from the Sink to

C. Each element Z c k (1 ≤ k ≤ d c
) is simply the rank of the child node located at depth k in the path from the Sink to C.

The value of Z c k components of the ZigBee tree path information can be computed using :

Z c k = ⌊ A c -k -k-1 i=1 Cskip(i -1).(Z c i -1) Cskip(k -1) ⌋ + 1 (4)
where A c is the network address of C and can be deduced using the recursive form of (1) as follows :

A c = Ap of C (d c -1) + dc-1 i=1 Cskip(i -1).(Z c i -1) +Cskip(d c -1).(Z c dc -1) + 1 (5)
Note that the first value Z c 1 of the ZigBee tree path information of node C represents the tree branch number to which belongs the node C, where a branch is a subtree rooted at the child number Z c 1 of the PAN coordinator (Sink). Therefore, two different nodes N a and N b belong to two different subtrees if and only if their first values Z a 1 and Z b 1 , respectively, are different. In this case, we say that the two parent-child paths of N a and N b , respectively, are two node disjoint paths.

B. Multipath Routing Algorithm

ZM R is a hybrid node disjoint multipath routing. When the source detects an event and in order to reduce the end-to-end latency, it starts immediately transmitting data packets to the Sink using the proactive hierarchical routing T R where the parent-child path is already established during the association phase at the MAC layer. Also, it starts the route discovery phase which represents the reactive part of ZM R. Unlike the traditional discovery phase with its heavy traffic of control messages (for instance RREQs), in our case, a light discovery phase is used in order to estabilish additional paths based on the parent-child and neighboring links and the ZigBee Tree Path information.

Given a source node S, the basic idea to build subsequent paths is as follow: Initialy, the source chooses as the nexthop node an adjacent neighbor from its neighbors table. If the neighbor is the Sink, noted by N S, or a node which belongs to a branch (subtree) not already used by T R, noted by N T R, then a new path of one link (in the case of N S neighbor) or composed of one neighbor link and the parent-child links from N T R to the Sink, can be directly used without triggering the discovery process. Otherwise, the source sends an Explore message ExploreM sg which is forwarded (unicast and never broadcast) from node to node using forwarding decisions. Whenever a node forwards the ExploreM sg, it records the next node and the node from which it has receveid the ExploreM sg in its routing table. When an intermediate node C at depth d c receives ExploreM sg, it starts searching for the next-hop node among its neighbors. If more than one candidate neighbor exist then the one with the smallest depth is chosen. Three cases are considered :

The intermediate node C has a N S or a N T R neighbor node, in this case, the ExploreM sg is not fowarded and the new disjoint path established is the one followed by the ExploreM sg concatenated to either (i) the last link between C and the Sink, or (ii) the tree path from the N T R node to the Sink, respectively. In this case, C node sends a ResponseM sg message towards the source using the reverse path already established during the discovery phase.

Based on the depth of the first common node between the ZT P c of node C and the ZT P Z c 1 of the node which has firstly used T R routing in C'branch, node C chooses the next-hop node, two cases are possible :

-The first common node is at depth (d c -1). This common ancestor is not considered as the next-hop node and an adjacent node N is chosen from the neighbors table of node C.

-The first common node is at a depth less than (d c -1).

In this case, T R is applied by the intermediate node C. The intermediate node C has no candidate node as its nexthop node. It sends a ErrorM sg message to its predecessor node. This latter, tries to forward ExploreM sg to a qualified node among its neighbors; otherwise, in its turn, it sends an ErrorM sg to its predecessor. This feedback mechanism continues until an intermediate node finds a good candidate node to forward the ExploreM sg or the ErrorM sg arrives at the source. In this case, if another candidate node is available and a new disjoint path is required, then the source can initiate a novel discovery process.

When the source node receives the ResponseM sg from its neighbor on which the discovery path was initiated, it means that another valid disjoint path is established and the source can use it for transmitting its data packets.

We assume that the source has adjacent nodes (are not parent, child, Sink and N T R neighbors) and sends ExploreM sg in order to discover subsequent disjoint paths. Algorithm 1 describes the discovery process at the intermediate node C .

Illustrative Example

Figure 1 shows a network topology with parameters (L m , C m , R m ) = [START_REF] Milenkovic | Wireless sensor networks for personal health monitoring: issues and an implementation[END_REF][START_REF] Gungor | Industrial wireless sensor networks: challenges, design principles, and technical approaches[END_REF][START_REF] Gungor | Industrial wireless sensor networks: challenges, design principles, and technical approaches[END_REF]. Where all the sensor nodes are evenly distributed on a square-shaped area. The Sink is the ZC located at the center. The transmission range (T r) chosen in this topology allows to cover at least three neighbors and at most eight neighbors. The Cskip(d) values are respectively 21, 5 and 1 at depth d where d = 0, 1, 2 (Equation 3). For a given node, the two numbers between brackets are respectively the address (that results from equation 1) and the Cskip(d) value. The other numbers denote only addresses of nodes (equation 1). In this example, the source node s is at depth 2 and has address 17 and nodes a, c, b, f and k as neighbors (one parent neighbor and four adjacent neighbors). By applying equation 4, Its ZT P s = (1, 4) (node s is the fourth child of the node a, It finds that the first commun node is at depth 1 (it is the node e) and can't be taken as the next-hop node (it already belongs to the path P 2). The node k searches another candidate from its neighbors table. Since it can't choose the node c because it belongs to the path P 2 (thanks to the inclusion operation) so it takes y as its next-hop node. The node y chooses the node z, which is its N T R neighbor, as the next-hop node. The node z applied T R until the Sink. The resulting node disjoint path is

P 3 = s → k → y → z → t → Sink.

IV. SIMULATION RESULTS

Different simulations have been carried out to evaluate the performances of the proposed ZigBee multipath routing protocol ZM R with respect to the classical single ZigBee tree routing (T R). Simulations are conducted using NS2 simulator with the IEEE 802.15.4 implementation provided by Zheng [START_REF] Zheng | NS2 simulator for IEEE 802[END_REF]. Even if ZM R is able to build more than two paths, we limited our simulations to using only two paths simultaneously. It has been shown that using more than two paths does not achieve further throughput gain. The number of nodes in the network topology is fixed to 101 stationary nodes with one sink (the PAN coordinator) at the center of the area and 100 FFD nodes. The communication range is of 11m. The ZigBee Parameters L m , C m and R m are 7, 4 and 4 respectively. The non-beacon mode with unslotted CSMA/CA mechanism of the IEEE 802.15.4 standard is used at the MAC level. Propagation model used was two-ray ground model. The size of the packets payload is 80 bytes. Poisson traffic model is considered in our simulation and the traffic load is varied from 1pps (0.625 kbps) to 100pps (62.5 kbps) to evaluate different metrics for different routing strategies.

Each scenario is simulated 12 times and the total number of simulation runs is 96 (12 × 8). Results over all the network scenarios are averaged to produce the packet delivery fraction, throughput, average end-to-end delay and network lifetime as a function of the traffic load.

Packet Delivery Fraction: Figure 2 shows that when the traffic load is in the range of 1 to 20pps all protocols have approximately the same behavior, they are able to handle the traffic quite well, delivering more than 99.5% of the packets to the destination. However, when the traffic load exceeds 20pps, the multipath routing ZM R outperforms the single routing T R due to the insufficient bandwidth capacity of T R to support such traffic load over one path and to the fact that the packet loss due to the intra-path collisions and interferences becomes higher for a single path. Average end to end delay: The average end to end packet delay, depicted by Figure 3 increases with the data rate since packets incur more queuing delay. Moreover, due to MAC collisions and interference, packets need to be retransmitted and so their delivery time increases. However, in the multipath routing, where the data packets are spread onto two node disjoint paths, the number of intra-path collisions and intrapath interference is reduced leading to less retransmissions and hence improve the end to end delay.

Network lifetime: The network is considered as not operational when the Sink stops receiving packets. Figure 4 shows that the use of more than one path prolongs the network lifetime and hence gives better throughput performance. Beyond the saturation value 20pps, T R suffers from excessive retransmissions (the entire traffic load would route along only one path), greatly degrading its performance, while the multipath routing (total network load is distributed evenly into two paths), can still enjoy the integrated capacity of multiple paths and achieves longer network lifetime. Effective Bandwidth: Figure 5 shows that the multipath routing ZM R outperforms the single path T R. To calculate the effective bandwidth of the ZM R strategie, we need to consider the maximum throughput achievable by this technology (theoretically support 250 kbps at 2.4 GHz). The simulation results of figure 5 indicate a maximum of throughput of 40756bps ≃ 41kbps is achievable when operating at a rate of 67pps ≃ 42kbps under this technology (This is equivalent to 1/6 of the total bandwidth). However, utilizing this bandwidth is possible if we expect a QoS of 96% delivery of data. Therefore, if we expect a QoS of atleast 99% delivery of data, the throughput at lower datarates need to be considered. Thus referring to graphs 5 and 2, to achieve a delivery ratio of 99%, the traffic rate should be = 50pps(≃ 31kbps) and the achievable throughput at this rate of operation is ≃ 31kbps. However, in the case of the single path routing T R, the maximum throughput achievable is about 12.5kbps (equivalent to 1/18 of the total bandwidth) for a QoS of 99% delivery of data expected, which is very insufficient for handling high data rate applications. In this paper, we proposed ZM R, a hybrid node disjoint multipath routing for ZigBee WSNs to handle high data rate applications. The proposed ZM R protocol combines the proactive and the reactive approaches in order to construct multiple node disjoint paths. For this aim, it exploits judiciously the address assignment scheme of ZigBee and the parent-child and non parent-child links. Results showed that the multipath routing ZM R provides the best performances in terms of packet delivery fraction, end to end delay, network lifetime, throughput and bandwidth with regard to the single path routing T R and satisfies soft QoS provision requirements of the high traffic applications.

Algorithm 1 7 : 17 :

 1717 Discovery process at an intermediate node C Inputs :P : this node parent, C : the current node, N : the adjacent neighbor node of C. N S : the adjacent neighbor of C is the Sink. N T R : the adjacent neighbor of C is the node which belongs to a subtree (branch) not already used by T R. F a 1 : the node which has firstly used the T R routing in branch number Z a 1 of node A . ZT P F a 1 : the ZigBee tree path information of node F a 1 . Intersection(ZT P a ,ZT P b ) returns the rank of the first different element between ZT P a and ZT P b . Inclusion(ZT P a ,ZT P b ) test if the node B belongs to the parent-child path of A.Output :ExploreM sg.X : a field in the ExploreM sg packet that contains the next-hop node.1: search in the neighbors table of C for a candidate neighborN 2: if (N is a N S or a N T R) then 3:update the routing table of C; send ResponseM sg packet to the source over the reverse path; 4: else 5: t = Intersection (ZT P F c 1 ,ZT P c ) 6: if (t = d c ) then {the first common node is at depth d c -1} 8: search in the neighbors table of C for an adjacent node N such that Inclusion (ZT P F n 1 , ZT P n ) = f alse 9: if (N exist) then 10: ExploreM sg.X = N ; update the routing table of C; forward ExploreM sg packet to the next-hop node N else if (t < d c ) then 16: {the first common node is at depth < d c -1} ExploreM sg.X = P ; update the routing table of C; forward ExploreM sg packet to the next-hop node P 18: end if 19: end if which is the first child of the Sink node (PAN)). As shown in the figure, three node disjoint paths are established from the source node s to the Sink node. The first one is the classical path based on parent-child links (no discovery phase) and is the shortest one P 1 = s → a → Sink. To construct the second path, node s takes node c as its next-hop node. Since the intermediate node c with ZT P c = (3, 4) is a N T R neighbor of the node s (3 = 1 and the T R routing is not yet applied on the branch of c), consequently, T R is applied from node c until the destination node Sink and gives the second path P 2 = s → c → e → Sink which is established automatically without triggering the discovery process. The node s takes the node k at depth d k = 2 as the next-hop node to build the third node disjoint path. Since k is not a N T R neighbor of node s (since c and k belong to the same branch, so T R is already applied in this branch by node c), this case requires then a path discovery. The intermediate node k compares its ZT P k = (3, 2) with the node c one ZT P c = (3, 4) (the node which has firstly used the branch of the node k).

Figure 1 .

 1 Figure 1. Example of node disjoint paths discovery from the source node s to the Sink

Figure 2 .

 2 Figure 2. Packet delivery fraction in the different routing strategies

Figure 3 .

 3 Figure 3. Average end to end delay in the different routing strategies

Figure 4 .

 4 Figure 4. Network lifetime in the different routing strategies

Figure 5 .

 5 Figure 5. Throughput in the different routing strategies


Médard Thiry
email: medard.thiry@mines-paristech.fr

Paul Fernandes
email: paul.fernandes@paleotime.fr

Tony Milnes

Jean-Paul Raynal
email: jpraynal@wanadoo.fr

Driving forces for the weathering and alteration of silica in the regolith: implications for studies of prehistoric flint tools

Keywords: silica, weathering, solubility, recrystallization, flint, prehistoric artifacts

A range of geochemical and mineralogical factors trigger the weathering and influence the stability of various silica phases under near-surface temperatures and pressures. Based on current knowledge of the crystallochemical basis of silica behaviour in near-surface environments, the mineralogical and petrographical transformations of silica under weathering conditions in the regolith are reviewed using data from wide-ranging and detailed investigations of siliceous materials. Some of the investigations are of siliceous materials that evolved in nearsurface environments over geological time and thus the weathering and alteration features are relatively clear and well expressed. Examples include weathering of tightly cemented sandstones, the formation of silica duricrusts (pedogenetic silcretes), and the weathering of flintstones. The various micromorphological and mineralogical forms of silica as determined by high resolution petrographic and electron-optical techniques can be related to geochemical and hydrological conditions in particular microenvironments. However, new studies have demonstrated that silica alteration and transformation mechanisms can also occur within prehistoric and historic time intervals. These are also described but are more difficult to observe, principally because of the microcrystalline character of the alteration phases and the inframillimetric scale on which they develop.

Of many potential applications of knowledge about the near-surface alteration of silica in regolith materials and the environmental conditions under which it occurs, our studies can be used as a basis determining the provenance and post-disposal history of prehistoric flint tools and artefacts, in part to determine the source of the primary raw material, and secondly to test the integrity of archaeological excavations. We suggest that the mineralogical and petrographical characteristics of the cortex of the flint (specifically the recrystallization, dissolution and deposition of the silica phases, and the spatial arrangement of surfaces that have undergone successive transformations) are related to specific and identifiable weathering environments.

Introduction

Rates of dissolution and precipitation of silica in weathering environments are relatively slow and few detailed investigations have been undertaken to document the outcomes of these alteration processes. Although the effective solubility of silica at room temperature was measured during the 1950's [START_REF] Krauskopf | Dissolution and precipitation of silica at low temperature[END_REF][START_REF] Siffert | Some reactions of silica in solution: formation of clay[END_REF] it was twenty or more years before complementary studies were published [START_REF] Marshall | Amorphous silica solubilities. II Effect of aqueous salt solutions at 25°C[END_REF] to assist research on weathering, and there have been no data since! Our studies undertaken over many years have examined the weathering of siliceous materials on many continents in order to illustrate the behaviour of forms of silica under different environmental conditions and go some way to explain the responsible mechanisms and the time frames in which they occurred. The alteration processes and outcomes are complex. The influence of variations that occur in microenvironments, for example the porosity of the siliceous material, the percolation rate of water through the material, and repeated mechanical shocks, are some key controls.

In this paper we review the mineralogical and micromorphological transformations of silica in regolith materials. Although these transformations have occurred in a geological time frame, new data has demonstrated that similar transformations of silica can occur much more quickly within prehistoric and historic time frames. Consequently, this information and the concepts derived may be applied in archaeological investigations where the provenance and post-disposal weathering and alteration history of prehistoric flintstone artefacts is a recurrent question. Some initial studies have been undertaken in this context.

Crystallo-chemical basis of silica behaviour

Mineralogical forms of silica

Under near-surface conditions silica forms several minerals that differ in the spatial ordering of silica tetrahedrons. Quartz is the most common silica mineral and forms at high as well as low temperatures. Chalcedony is a microcrystalline and fibrous variety of quartz [START_REF] Drees | Silica in soils: quartz and disordered silica polymorphs[END_REF]. Moganite [START_REF] Flörke | Moganite. A new microcrystalline Silica-Mineral[END_REF][START_REF] Rodgers | The occurrence, detection and significance of moganite (SiO2) among some silica sinters[END_REF]) is a widespread microcrystalline silica-mineral [START_REF] Heaney | The widespread distribution of a novel silica polymorph in microcrystalline quartz varieties[END_REF] but is not commonly identified in the descriptions of siliceous materials. Cristobalite and tridymite have a sheet-like crystalline structure [START_REF] Ix | Uber Faserbau und opstische Eigenschaften von Chalzedon[END_REF], similar to that of clay minerals, and in supergene environments they are often bound to chemical (evaporitic) deposits. Opal CT has also a sheetlike crystalline structure, with alternating cristobalite-and tridymite-like sheets [START_REF] Flörke | Opal CT crystals[END_REF]. It is relatively frequent in buried marine deposits and in confined continental environments. Opal is a petrographic variety of silica defined at the optical microscope level as amorphous because the crystallites are too small to be distinguished [START_REF] Arbey | Les formes de la silice et l'identification des évaporites dans les formations silicifiées[END_REF]Folk and Weaver, 1952). It may correspond to true amorphous silica, but is most often composed of quartz, moganite, cristobalite, opal-CT, or a mix of these minerals. Amorphous silica has no crystalline organisation: silica tetrahedrons pile up in a disordered fashion and form chains limited to a few tetrahedrons.

Solubility of the various forms of silica

Silica in solution forms mono-molecular silicic acid [Si(OH)4]. The solubility of amorphous silica in water is about 120 mg/L SiO2 and remains independent of the pH for values below 9 but increases strongly above 9 [START_REF] Krauskopf | Dissolution and precipitation of silica at low temperature[END_REF]. The solubility of all other silica varieties is lower than that of amorphous silica (Fig. 1). A fundamental law that rules geochemistry is that the most soluble phases sustain the precipitation of the less soluble ones. In this way, amorphous silica in equilibrium with a solution is able to sustain the crystallisation of every other crystalline silica variety, but these phases do not necessarily precipitate. Furthermore cristobalite, tridymite, chalcedony and all opal varieties are able to sustain the crystallisation of quartz [START_REF] Garcia-Hernandez | Interprétation cinétique de la géochimie d'altération de la silice à basse température[END_REF][START_REF] Gislason | The difference between the solubility of quartz and chalcedony: The cause?[END_REF][START_REF] Iler | The chemistry of silica[END_REF][START_REF] Siffert | Some reactions of silica in solution: formation of clay[END_REF]. Moreover, thermodynamic laws predict that as long as a more soluble phase is present, the less soluble phases do not dissolve, but kinetic dynamic equilibrium may partly disturb this rule. At room temperature the equilibrium between silica varieties and water is reached very slowly, sometimes only after 1 or 2 years [START_REF] Dove | The influence of the alkaline earth cations, magnesium, calcium, and barium on the dissolution kinetics of quartz[END_REF][START_REF] House | Dissolution kinetics of silica between 5 and 35°C[END_REF][START_REF] Siffert | Some reactions of silica in solution: formation of clay[END_REF]. It should be noted that most ground waters around the world have silica content between 12 and 18 ppm SiO2 [START_REF] Garrels | Solutions, Minerals and Equilibria[END_REF][START_REF] White | Chemical composition of subsurface waters[END_REF] that is roughly in equilibrium with clay minerals. This means that most groundwaters are oversaturated with respect to quartz (solubility 4-7 mg/L) and are thus potentially able to precipitate quartz. This oversaturation in silica with respect to quartz is thought to result from a kinetic dynamic equilibrium since the dissolution kinetics of clay minerals are much higher than those for the precipitation of quartz [START_REF] Palandri | A compilation of rate parameters of water-mineral interaction kinetics for application to geochemical modelling[END_REF].

It is possible to demonstrate a specific sequence of crystallization and particular relationships between the silica phases that form in surficial regolith environments. For example, siliceous duricrusts that have formed in soils (pedogenic silcretes) and occur over wide regions of the world display several distinct horizons with systematic micromorphological and mineralogical structures [START_REF] Thiry | Diversity of continental silicification features: Examples from the Cenozoic deposits in the Paris Basin and neighbouring basement[END_REF]. Detailed mineralogical and petrographic studies show that, consistently from the base to the top of the profile (Fig. 2): This distribution of silica phases reflects the chemistry of the solutions from which they precipitated (Williams and Crerar, 1985). Microcrystalline, amoebic and euhedral quartz in the upper horizons developed from dilute infiltrating soil water solutions that dissolved pre-existing more soluble silica phases. The leached silica migrated down towards the base of the profile and precipitated microcrystalline quartz as a result of the progressive concentration of silica in solution by evaporation. In this sequence, each mineralogical and petrographical silica species is derived from the previous generation by in-situ dissolution and recrystallization, indicating the re-equilibration of the mineral with its environment. The recrystallizations are irreversible and ultimately favour development of quartz [START_REF] Thiry | Mineralogical forms of silica and their sequence of formation in silcretes[END_REF].

The influence of organic compounds (i.e. humic acids, oxalic acid, alcanes) on silica solubility in the regolith is poorly known and has been generally neglected in the past. Recent studies have shown that silico-organic complexes raise silica solubility and influence dissolution kinetics [START_REF] Bennett | Quartz dissolution in organic-rich aqueous systems[END_REF].

Additionally, the influence of temperature on the solubility of quartz is important. Quartz solubility decreases exponentially with temperature [START_REF] Rimstidt | Quartz solubility at low temperatures[END_REF], Wiliams et al., 1985). Between 12.5 and 0°C quartz solubility decreases to half its initial value (Fig. 3).

Figure 3 -Quartz and chalcedony solubility versus temperature [START_REF] Bethke | The geochemist's workbench release 4.0: a user's guide to Rxn, Act2, Tact, React, and Gtplot[END_REF]. Quartz solubility decreases to more than half of its initial value between 12.5 and 0°C.

Stability of forms of silica vs crystallinity

The crystallinity of the various forms of silica relates to the geochemistry of the solution from which they precipitated. Solutions with high silica content favour the blooming of crystal nuclei and crystal defects that restrict the growth of crystals and consequently amorphous or low crystallinity silica varieties will form (Lee, 2007a). On the other hand, in dilute solutions, the number of nuclei, as well as impurities, remains limited. As a consequence, elementary crystallites form regularly and large quartz crystals develop or, under less favourable conditions, microcrystalline or fibrous silica varieties are formed.

An empirical relation exists between the stability (the reverse of solubility) of silica varieties and their degree of structural order and crystallinity. Large silica crystals, which have few structural defects, are the most stable; for example euhedral quartz and quartz overgrowths on matrix grains. Silica varieties such as opal (formed of nanoquartz), microcrystalline quartz, moganite and fibrous silica varieties with numerous structural defects form small elementary crystallites (coherent crystal domains). They are relatively more soluble than quartz, in terms of thermodynamic solubility and kinetics [START_REF] Gislason | The difference between the solubility of quartz and chalcedony: The cause?[END_REF]Williams et al., 1985), and so the microcrystalline silica varieties, especially moganite, are preferentially dissolved in the first stages of weathering [START_REF] Gislason | Kinetic and thermodynamic properties of moganite, a novel silica polymorph[END_REF] (Fig. 4). Silica solubility may even vary within a single crystal due to domains of various degrees of crystallinity: increased crystallinity correlates with a larger size of the coherent (without defect) domains of its atomic arrangement, or is inversely proportional to the frequency of crystal defects. The variation in crystallinity of quartz, for example, can be evaluated by several methods [START_REF] Graetsch | Structural defects in microcrystalline silica[END_REF]Herdianita et al., 2000b;Lee, 2007a;[START_REF] Moxon | Agate recrystallization: evidence from samples found in Archaean and Proterozoic host rocks, Western Australia[END_REF][START_REF] Murata | An index of crystallinity for quartz[END_REF][START_REF] Smykatz-Kloss | Das Hoch-Tief-Umwandlungsverhalten mikrokristalliner Quartze[END_REF] including the full width at half-maximum intensity of an X-ray diffraction band, the intensity of peaks in X-ray diffraction patterns (quantitative analyses), and the sharpness/intensity of the thermal transformation of quartz-α into quartz-β as determined by Differential Thermal Analysis.

Detailed studies of the quartzite pans of the Fontainebleau Sandstones show the relationship between solubility and crystallinity. These pans are tightly cemented and have a friable rim that results from partial dissolution of the quartzite [START_REF] Thiry | Diversity of continental silicification features: Examples from the Cenozoic deposits in the Paris Basin and neighbouring basement[END_REF]. As clearly indicated in thin sections (Fig. 5), the dissolution preferentially affects overgrowth quartz aureoles: primary quartz grains are hardly ever corroded [START_REF] Thiry | Silicification et désilicification des grès et des sables de Fontainebleau. Evolutions morphologiques des grès dans les sables et à l'affleurement[END_REF]. This contrasted dissolution behaviour highlights the occurrence of two different kinds of quartz crystals in the quartzite, although there appears to be no distinction between primary quartz grains and secondary quartz overgrowths in the optical microscope. The overgrowth quartz, which precipitated at low temperature within a near-surface water table, has a higher solubility than the inherited magmatic quartz grains, presumably because the former has a lower crystallinity and more structural defects than the primary quartz. It is widely observed in diagenetic and weathering environments that successive transformations of silica phases generally results in increasing crystallinity and decreasing solubility [START_REF] Goldstein | Recrystallization in quartz overgrowths[END_REF][START_REF] Graetsch | Microstruture of flint and other cherts raw material[END_REF][START_REF] Knauth | A model for the origin of chert in limestone[END_REF][START_REF] Lee | Characterisation of silica minerals in a banded agate: implications for agate genesis and growth mechanisms[END_REF][START_REF] Willey | Effects of ageing on silica solubility: a laboratory study[END_REF]Williams et al., 1985). Once precipitated the silica crystal begins to increase in size, resulting in a decrease in surface defects and a more compact structure [START_REF] Moxon | Agate recrystallization: evidence from samples found in Archaean and Proterozoic host rocks, Western Australia[END_REF].

Spatial and temporal aspects of weathering and alteration of silica in near-surface environments

In natural systems silica generally behaves in a much more complex way than in laboratory experiments. This is mostly due to the specific spatial distribution of the various forms of silica in relation to water flows and because weathering and alteration reactions occur according to changes in ground water (and soil water) chemistry and conditions. Thiry and his colleagues have undertaken detailed investigations of several examples of the development, weathering and alteration of siliceous materials in the regolith in Europe and Australia, focussing particularly on mineralogical and micromorphological features [START_REF] Thiry | Diversity of continental silicification features: Examples from the Cenozoic deposits in the Paris Basin and neighbouring basement[END_REF][START_REF] Thiry | Development of tightly cemented sandstone lenses within uncemented sand: Example of the Fontainebleau Sand (Oligocene) in the Paris Basin[END_REF][START_REF] Thiry | Pedogenic and groundwater silcretes at Stuart Creek Opal Fields, South Australia[END_REF][START_REF] Thiry | Ground-water silicification and leaching in sands : Example of the Fontainebleau Sand (Oligocene) in the Paris Basin[END_REF][START_REF] Thiry | Interpretation of palaeoweathering features and successive silicifications in theTertiary regolith of Inland Australia[END_REF]. Here we review two examples of weathering and alteration in France that illustrate the complex of spatial and temporal processes that seem to act together but in fact occur alternately.

Weathering and alteration of silica in outcropping sandstones

Numerous outcropping sandstones around the world show specific weathering features at their surface and provide invaluable information about weathering of siliceous materials [START_REF] Willems | Existence de karsts en roches cristallines silicatées non carbonatées en Afrique sahélienne et équatoriale, implications hydrologiques[END_REF][START_REF] Wray | A global review of solutional weathering forms on quartz sandstones[END_REF][START_REF] Young | Sandstone landforms[END_REF].

For example, in outcrop, quartzite pans in the Fontainebleau Sandstone break down and yield quartzite blocks that progressively acquire rounded dome-like morphologies [START_REF] Thiry | Weathering morphologies of the Fontainebleau Sandstone and related silica mobility[END_REF][START_REF] Thiry | Silicification et désilicification des grès et des sables de Fontainebleau. Evolutions morphologiques des grès dans les sables et à l'affleurement[END_REF]. The domal structures are often coated with a 0.5 to 2 cm-thick hardened silica crust (Fig. 6a). The crust is restricted to the top of the domes, and does not occur on lower parts or on overhanging surfaces. The crust appears in relief as a result of differential alteration at the periphery of the domes. Thin sections across this hardened silica crust show that quartz overgrowths on original grains are split off by tiny cracks filled by brown opal (Fig. 6b).

Polygonal groove patterns about 5 to 10 cm in diameter develop on the flanks of the domes and on overhanging surfaces (Fig. 6c). Such grooves are never observed in the silica crusts on the top of the blocks. The layout of the overhanging structures clearly indicates that they are related to a former soil that has been stripped off by erosion. Thin sections across the grooves show the development of large intergranular voids that are related to preferential dissolution of the quartz overgrowths (Fig. 6d &6e) that occurred within the former soil. These typical quartzite weathering morphologies result mainly from concomitant and contrasted weathering processes, in particular silica dissolution and, to a lesser extent, silica deposition. It is important to recognise that even on the relatively small scale represented by individual quartzite domes, silica dissolution and precipitation do not occur in the same place or at the same time in weathering environments. Silica dissolution is enhanced on the flanks of the dome where there is contact with humid and organic-rich soils, and is probably due to complexation with organic compounds. Silica deposition occurs on the top of the domes and originates from pore waters from the quartzite blocks that are in equilibrium with quartz and which concentrate by evaporation during warm sunny days (Fig. 7). All quartzite blocks display these features, even those on very steep and unstable sandy slopes. Quartzite weathering is thus a current and rapid phenomenon and a millennium time scale (10 to 100 ka) is envisaged for its development as sandy slopes date back to the last glacial period [START_REF] Thiry | Periglacial geomorphological evolution of the Fontainebleau Massif (France)[END_REF].

Weathering and alteration of silica in siliceous duricrusts

Siliceous duricrust profiles in France, Australia and elsewhere [START_REF] Dubroeucq | Indurations siliceuses dans des sols volcaniques. Comparaison avec des silcrètes anciens[END_REF][START_REF] Thiry | Pedogenic and groundwater silcretes at Stuart Creek Opal Fields, South Australia[END_REF][START_REF] Thiry | Tertiary paleoweatherings and silcretes in the southern Paris Basin[END_REF] display vertical and horizontal joints that determine their unique columnar structure (Fig. 8a). The voids linked to horizontal joints show specific features of successive reorganisation (Fig. 8b) [START_REF] Thiry | Sédimentation continentale et altérations associées : calcitisations, ferruginisations et silicifications[END_REF][START_REF] Thiry | Mineralogical forms of silica and their sequence of formation in silcretes[END_REF]. At the base of the joints silica accumulates in fine illuviation laminae. The youngest laminae, at the top, always consist of opal. The oldest laminae, at the base, commonly have a nodular structure and are formed of microcrystalline quartz. This sequence indicates a progressive recrystallization of the opal. The recrystallization is accompanied by a loss of silica which induces the destruction of the primary structures. The ceilings of the joints are lined with mammillary deposits of titania resembling stalactites. Preferential dissolution of microcrystalline quartz takes place in the ceilings of voids but coarse quartz grains are preserved and the resistant titania and impurity traces of the former microcrystalline quartz matrix have remained and accumulated in the mammillary deposits. The bases and ceilings of voids behave in different fashions according to the hydraulic regime. Opal deposits form in the base of the voids where water flows slowly or is stagnant and is concentrated by evaporation, while the microcrystalline matrix dissolves in the ceilings through which water seeps as a consequence of the percolation of soil water through the profile.

However, it is important to recognise that this close link between leached and confined environments does not imply a strict synchronism of degradation and construction. In fact, the two systems work alternately between humid (wet) periods during which silica dissolves and is leached downwards and dry periods during which the solutions that have accumulated at the base of the profile are concentrated by evaporation and opal crystallises. In between these fluctuations of the soil water regime, the secondary silica deposits may undergo partial dissolution, and successive dissolution-precipitation may trigger recrystallization into various microcrystalline petrofacies. Again, as in the studies of sandstone weathering and alteration, a complex of spatial and temporal conditions are implicated.

Influence of mechanical stress on weathering and alteration of silica

Prehistoric polishing features are known on numerous quartzite blocks in the Fontainebleau area [START_REF] Hofstetter | Les polissoirs de Faÿ-les-Nemours[END_REF][START_REF] Loiseau | Le massif de Fontainebleau. Géographie, histoire, généralités : légendes, préhistoire, géologie, faune, flore. Vigot Frères Ed[END_REF]. They are polished abraded surfaces that are strikingly hardened and in relief, raised above the non-polished sandstone by weathering (Fig. 9a). This arrangement clearly points out that the "worked" surface is indurated and probably re-silicified. Such features are known world-wide on a variety of siliceous rocks used by prehistoric peoples. It is well known that rubbing surfaces together with a fine powder between them develops a "vitreous" film of amorphous character that coats the irregularities of the materials and gives the shiny aspect that is called the "Beilby layer" [START_REF] Bowden | Physical properties of surfaces. IV. Polishing, surface flow and the formation of the Beilby layer[END_REF][START_REF] Jones | Amorphous coatings on mineral surfaces[END_REF]. This amorphous film probably provides the silica for later recrystallization leading to hardening of the surface.

One of us (Thiry, unpublished work) has found that similar surface hardening is also developed on paving stones made of Fontainebleau quartzite. The paving stones can be found at several historical sites and show a convex surface due to wear (Fig. 9b). This surface is indurated with a millimetric crust that flakes off. Thin section studies show that quartz grains within the crust are intensely crushed and display microfractures spaced at about 10 µm and to a depth of 2 to 4 mm beneath the surface (Fig. 10a). This is the effect of repeated mechanical shocks. With such fractured grains one would expect that the exposed surface may be friable, but it is not. This is due to concomitant cementation and hardening as the microfractures have refilled with brown opal (Fig 10b &10c). The opal has formed from silica dissolved from amorphous fracture edges in the crushed quartz and cements the quartz splinters together to form the hardened crust. occur within a few decades, as testified by the history of the paving stones. The development of an indurated crust on prehistoric polishing stones may have occurred in a similar time scale: although the polishing stones may have been used by several generations of prehistoric peoples, they were only used intermittently.

Weathering and alteration of silica in flintstones

The microcrystalline texture of flint makes it very difficult to observe and analyse its weathering and alteration in near-surface environments. Nevertheless, there are specific case studies of flint deposits that provide evidence of the geochemical and mineralogical mechanisms at play [START_REF] Burroni | The surface alteration features of flint artefacts as a record of environmental processes[END_REF][START_REF] Bustillo | Is the Macroscopic Classification of Flint useful? A Petroarchaeological Analysis and Characterization of Flint Raw Materials from the Iberian Neolithic Mine of Casa Montero[END_REF][START_REF] Catt | The nature, origin and geomorphological significance of clay-with-flints[END_REF][START_REF] Glauberman | Flint patina as an aspect of "Flaked stone taphonomy": A case study from the loess terrain of the Netherlands and Belgium[END_REF][START_REF] Hurst | Patination of cultural flints: Flint artifacts can be dated by cortical changes in mineralogy and texture[END_REF]Kelly, 1961, Fayolle, 1979;[START_REF] Laignel | La désilicification des silex des formations résiduelles à silex de l'ouest du bassin de Paris[END_REF][START_REF] Rodgers | Silica phases in sinters and residues from geothermal fields of New Zealand[END_REF][START_REF] Rottlander | The formation of patina on flint[END_REF].

Alteration after reworking of flint from chalk deposits

A comparison of more than a hundred samples of flints in-situ within chalk deposits, and a similar number of samples of flints inherited in clay-with-flints and reworked into alluvium and sedimentary deposits, in France, was undertaken by [START_REF] Trauth | Silex et chailles du bassin de Paris : modifications minéralogiques lors de leurs altérations[END_REF]. From a strict mineralogical point of view, flints embedded in chalk contain quartz as well as tridymite, cristobalite and opal-CT. These minerals are easily identified by X-ray diffraction and differential thermal analysis (Fig. 11). In contrast, flints reworked from chalk deposits and weathered are exclusively composed of quartz. In addition, all relevant indexes show that quartz in reworked flints is of higher crystallinity than quartz in flints embedded in chalk (Fig. 12). Figure 12 -The crystallinity of quartz (proportional to the size of the coherent crystal domains) is lower in the quartzose flints than in magmatic quartz. Moreover the reworked and weathered flints show quartz of a higher crystallinity than that in flints in-situ in chalk (after [START_REF] Trauth | Silex et chailles du bassin de Paris : modifications minéralogiques lors de leurs altérations[END_REF].

The mineralogical composition of flints starts to change after they are removed from the host chalk, even if the changes are not visible by means of petrographic techniques. The changes always favour a more stable secondary mineralogical structure, less soluble and of better crystallinity than the primary one. This process involves recrystallization triggered by the external environment of the flint and physico-chemical mechanisms like Ostwald ripening through which the smallest crystallites dissolve and contribute silica for the growth of larger crystals [START_REF] Crerar | Growth and ripening of silica polymers in aqueous solutions[END_REF]. The mass balance exchange is minimal and there are no dissolution features. Recrystallization probably occurs through water films between the crystals and exchange by diffusion rather than by convection [START_REF] Cady | HRTEM of microcrystalline opal in chert and porcelanite from the Monterey Formation, California[END_REF]. Such recrystallization of silica phases may occur in a relatively short time as shown by primary opal deposits that have transformed to microquartz over periods of 20 to 50 000 years (Herdianita et al., 2000a).

Dissolution of flints

Often flint cobbles have a friable white cortex. Flints altered in this way are common at the top of clay-with-flints sections [START_REF] Laignel | La désilicification des silex des formations résiduelles à silex de l'ouest du bassin de Paris[END_REF][START_REF] Quesnel | The Neogene and Quaternary Clay-with-flints north and south of the English Channel: Comparison of distribution, age, genetic processes and geodynamics[END_REF] and in fluvial terrace gravels [START_REF] Coutard | Nouvelle approche de l'altération des galets de silex dans les dépôts marins pléistocènes : contribution à l'étude des plates-formes marines du Val de Saire (Normandie, France)[END_REF]. Similar alteration affects some flint gravels interlayered in sand formations [START_REF] Thiry | Ground-water silicification and leaching in sands : Example of the Fontainebleau Sand (Oligocene) in the Paris Basin[END_REF]. The friable white cortex is generally about 1 to 10 mm thick and the shape of the flint cobble is generally preserved. However, in some regolith environments the flints are more extensively altered and have contorted shapes (Fig. 13), with obvious solution features (cavities and hollows), and can be easily broken by hand. The friable white cortex is associated with progressive dissolution and development of porosity which may reach 35%, and a concomitant increase of the pore size and specific surface area, which may reach 15 m2/g in the most altered, powdery samples [START_REF] Coutard | Nouvelle approche de l'altération des galets de silex dans les dépôts marins pléistocènes : contribution à l'étude des plates-formes marines du Val de Saire (Normandie, France)[END_REF].

These weathering features illustrate the sensitivity of the flints to silica leaching, with the progressive development of a friable white cortex. As reworked flints are primarily exclusively composed of microcrystalline quartz (in contrast with primary flints in chalk that contain some tridymite and cristobalite), the development of the friable texture reveals the preferential or relatively rapid dissolution of some of the microquartz varieties. These are most probably quartz microcrystals that have the lowest crystallinity and the smallest grain size (and thus the highest specific surface area). Once the dissolution has started, porosity develops, promoting water penetration into the flint and enhancing dissolution. The less soluble quartz microcrystals remain and are preserved, or at least partly preserved. These have the greater grain size with less structural defects and thus better crystallinity. A few occurrences of flint cobbles embedded in geological formations allow us to estimate the rate of development of the friable white cortex. In the Paris Basin, leached and bleached sandstone profiles that contain weathered flints are related to valley incision, and thus the duration of the alteration producing the cortex is probably several 10 ka [START_REF] Thiry | Ground-water silicification and leaching in sands : Example of the Fontainebleau Sand (Oligocene) in the Paris Basin[END_REF]. Elsewhere, along the Channel shore in Normandy, flints with a thick, friable cortex are found in association with wave-cut platforms of Saalian and Eemian ages, and so the cortex is considered to have developed in less than 100 ka [START_REF] Coutard | Nouvelle approche de l'altération des galets de silex dans les dépôts marins pléistocènes : contribution à l'étude des plates-formes marines du Val de Saire (Normandie, France)[END_REF].

Recrystallization of flints

Studies of geological formations that contain flints show that silica alteration and recrystallization phenomena occurred over long time periods. Weathering and alteration features no doubt occur in shorter time frames but are incipient and difficult to recognise.

Flint cobbles embedded in silcrete profiles of Lower Eocene age in the Paris Basin have been weathered and altered in-situ [START_REF] Thiry | Sédimentation continentale et altérations associées : calcitisations, ferruginisations et silicifications[END_REF][START_REF] Thiry | Tertiary paleoweatherings and silcretes in the southern Paris Basin[END_REF]. In the upper parts of these silicified formations the flints show peripheral dissolution with a white and porous centimetric cortex and a dull core: only small primary translucent zones of the original flint remain. In thin sections the altered flints show microcrystalline quartz forming a microspherulitic texture with spherules about 10µm in diameter (Fig. 14). The microspherulites become more pronounced progressively from the core of the flint towards the cortex. Voids occur within the porous cortex and even larger voids can be observed at the outer rim. The microspherulites are not visible in thin section in unweathered, translucent flints formed of very fine microcrystalline quartz. Nevertheless, it is obvious that these microspherulites are primary constituents of the flint. They have been described in fresh flints within chalk deposits [START_REF] Cayeux | Contribution à l'étude micrographique des terrains sédimentaires. Deuxième partie : craie du Bassin de Paris[END_REF][START_REF] Knauth | Silica, physical behaviour, geochemistry and materials applications[END_REF]. The microspherulites become progressively more distinct because they have a lower solubility than the matrix between the spherulites that is composed of a different form of microcrystalline quartz, possibly mixed with opal, and that is more soluble and dissolves preferentially. These features reveal two successive transformations of silica in flints: (1) the presence of microspherulites show that recrystallization of the flint to microcrystalline quartz after it has been reworked from the chalk nevertheless preserved at least some of the primary petrographic features; and (2) weathering of flints results not only from simple dissolution processes but progresses as a result of differential dissolution and reorganisation of silica phases progressing from the surface inwards towards the core of the flints.

Application to investigations of flint artefacts

Determining the origins of lithic raw materials has long been considered essential to understand the collection behaviours of prehistoric societies. Petro-archeological studies of flints have difficulty in establishing a correlation between the artifact material and the potential sources of that material [START_REF] Bressy | Caractérisation et gestion du silex des sites mésolithiques et néolithiques du Nord-Ouest de l'arc alpin. Une approche pétrographique et géochimique[END_REF][START_REF] Briggs | The Past and Future of Irish Implement Petrography[END_REF][START_REF] Deflandre | Techniques micropaléontologique appliquée à l'étude des silex[END_REF][START_REF] Kars | Petrography and geochemistry of flint from the Lanaye chalk (Rijckolt-ST. Geertuid) and some other Neolithic sources[END_REF][START_REF] Masson | Pétroarchéologie des roches siliceuses[END_REF][START_REF] Mauger | Les matériaux siliceux utilisés au paléolithique supérieur en Ile-de-France[END_REF][START_REF] Sieveking | Prehistoric flint mines and their identification as source of raw material[END_REF][START_REF] Valensi | Micropaléontologie des silex du Grand-Pressigny[END_REF]. Studies have been mostly on primary (in situ) and/or sub-primary sources, whereas gatherings of flints by prehistoric peoples were often probably made from secondary, redeposited materials such as glacial till and waterlaid gravels [START_REF] Fernandes | Pétroarchéologie du silex: un retour aux sources[END_REF][START_REF] Hughes | Sourcing flint from Sweden and Denmark A pilot study employing non-destructive energy dispersive X-ray fluorescence spectrometry[END_REF]Lavin andProthero, 1987, 1992;[START_REF] Verri | Flint mining in prehistory recorded by in situ-produced cosmogenic 10 Be[END_REF][START_REF] Williams-Thorpe | Geochemical provenancing of igneous glacial erratics from southern Britain, and implications for prehistoric stone implement distributions[END_REF]. Transformations of the flint cortex that can be identified from detailed petrographic studies are specific to some sedimentary environments [START_REF] Burroni | The surface alteration features of flint artefacts as a record of environmental processes[END_REF][START_REF] Fernandes | Itinéraires et transformations du silex : une pétroarchéologie refondée, application au Paléolithique moyen[END_REF][START_REF] Fernandes | Origins of prehistoric flints: The neocortex memory revealed by scanning electron microscopy[END_REF][START_REF] Hurst | Patination of cultural flints: Flint artifacts can be dated by cortical changes in mineralogy and texture[END_REF]. Recognition of these petrographic characteristics may be the key to characterizing secondary flint deposits and could help to locate such deposits worked by prehistoric peoples and permit the drawing of maps of subsistence areas and potential itineraries [START_REF] Fernandes | Middle Palaeolithic raw material gathering territories and human mobility in the southern Massif Central, France: first results from a petro-archaeological study on flint[END_REF].

As in the case of flintstones reworked from their sedimentary host, the silica minerals in flint tools made from flintstones and later buried when discarded by their users are affected by dissolution and precipitation (Fig. 15) [START_REF] Arrighi | Surface modifications of flint tools and their functional meaning[END_REF][START_REF] Burroni | The surface alteration features of flint artefacts as a record of environmental processes[END_REF][START_REF] Glauberman | Flint patina as an aspect of "Flaked stone taphonomy": A case study from the loess terrain of the Netherlands and Belgium[END_REF][START_REF] Hurst | Patination of cultural flints: Flint artifacts can be dated by cortical changes in mineralogy and texture[END_REF][START_REF] Masson | Pétroarchéologie des roches siliceuses[END_REF][START_REF] Vannest | Patination of Knife River flint artefacts[END_REF][START_REF] Vignard | Altérations des silex préhistoriques de Nemours sous l'influence des climats qui se sont succédés du Périgordien Gravettien au tardenoisien locaux[END_REF]. Silica dissolution in flints buried within soils or sediments commonly occurs at the periphery due to water seeping through the surrounding material. On the other hand, silica precipitation may occur either by concentration of pore waters during dry periods (seasons), or by cooling of warmer groundwater rising to near-surface, for example at times of periglacial environments in Europe during the cold periods of the Quaternary [START_REF] Woo | Progress in permafrost hydrology in the new millennium[END_REF]. When transported flint clasts or worked tools are buried, a gradient of percolation is established between the essentially impermeable core of the flint and the host sediment or soil. Reactions within the margins of the flint occur via diffusion processes through water films between component silica crystals. Any primary interstitial water bound within the flint core will have had a long contact time with the silica minerals: in the outer rim, which is in contact with percolating water in the surrounding soil or sediment, the contact time will be shorter. This equates to a gradient of silica concentration in the solutions. Whereas the interstitial solutions and the water films between crystals within the core of the flint are in equilibrium with the silica minerals and no mineralogical change occurs, solutions at the surface of the flint are renewed by percolation and are thus undersaturated in silica so that the silica phases in the flint are dissolved. Beneath the surface layers of the flint, in the zone termed the cortex, solutions have intermediate silica concentrations that may trigger recrystallization (Fig. 16). Flint weathering always leads to a loss of silica. However, in turn, the crystallinity and therefore stability of the silica minerals increases, leading to more stable phases.

Si dissolution

Si deposits Si deposits

Figure 16 -Relationship between solution concentration and rate of percolation through the flints.

Interplay between the flow rate of the solutions and their silica concentration steers the mineralogical and petrographical transformations in the flint cortex.

Of particular importance in unraveling the petrographic evolution of flint tools in nearsurface environments in relation to the geochemical and kinetic behaviour of silica minerals is recognition of the remnant natural surfaces that are inherited from the primary chalk deposits. The detailed characteristics of these natural surfaces will reflect the environments in which the flints were deposited and contained before they were collected for tool-making. The first step in determining the provenance of flints is to decipher the spatial arrangement of the successive natural surfaces (flat surfaces, edges, depressions, cupules, etc) and their relative chronology (Fig. 17). Secondly, the wear marks and the neo-cortex formed when the sample was exposed, removed and transported have to be characterized for each surface. This requires a multi-scale approach by macroscopic and binocular observations, then examination of the microfacies, textures and porosity by optical microscopy, and finally a clarification of various phases of the silica dissolution and precipitation by scanning electron microscopy (SEM) [START_REF] Fernandes | Itinéraires et transformations du silex : une pétroarchéologie refondée, application au Paléolithique moyen[END_REF].

Pre-gathering weathering and alteration

Successive cortex transformations occur during the geological reworking process, for example in flints from colluvial deposits downslope from flint-bearing limestone outcrops and, further downstream, flints bound within a mature alluvial conglomerate. Studies so far have been undertaken on about 3500 flint cobbles and 8000 flint artefacts, and included 200 thin sections, and Scanning Electron Microscope analysis of 80 specific samples originating mainly from Massif Central, Rhône Valley and Aquitaine Basin in France and from several sites of Morocco [START_REF] Fernandes | Itinéraires et transformations du silex : une pétroarchéologie refondée, application au Paléolithique moyen[END_REF].

The first stage of alteration after release of the flint from the source limestone consists of the development of three differentiated zones within the cortex (Fig. 18a). An inner microporous zone develops as a result of dissolution of the most soluble and less crystalline phases (diverse opal). An outer friable cortex with large pores develops with further dissolution. In between, a tight zone cemented by secondary crystalline silica (mainly microquartz) has formed.

In colluvium, most of the surface of the flint is remoulded by a thin layer of silica underneath which quartz grains remain visible (Fig. 18b &18c). The silica layer is thicker in surface depressions and frequently shows shrinkage cracks. The most soluble mineral phases like calcite and opal CT have disappeared from the matrix of the flint. The cortex of the flint contains secondary silica deposits, abrasion on bulges and split features (Fig. 18d).

This complex weathering ultimately ends in the development of a thick silica layer that completely changes the surface of the flint. The cortex becomes smooth and the grains that form the flint underneath the silica layer are no longer visible. Sometimes, the silica coats biogenic elements, like diatom frustules, that record deposition in a quiet environment.

The development of silica coatings appears to be characteristic of flint weathering. It is a consequence of differential alteration: the most soluble silica phases are dissolved whereas more stable phases and of better crystallinity than in the primary one are preserved and/or precipitated [START_REF] Graetsch | Microstruture of flint and other cherts raw material[END_REF]. The silica coating remains partial and discontinuous on flints contained within colluvium. It thickens up and the flints become somewhat rounded in downgradient fluvial networks, until they become cemented entirely as well rounded pebbles in alluvial conglomerates. There appears to be a relationship between the thickness of the silica coating and the intensity and duration of the weathering and reworking processes. It is possible that mechanical stresses due to repeated friction, abrasion and impact of the flints during fluvial reworking gave rise to amorphous silica that has reorganised into silica coatings that harden the surface of the pebbles, in a similar way to that demonstrated above in paving stones and grinding stones. The neo-cortex formed after knapping is the most demonstrative example of the alteration of flints. The alteration is obvious and can be dated, which allows an assessment of its rate of development. The alteration may also indicate the sedimentary and climatic processes that acted on the archaeological unit. Most commonly there are three zones superposed from the surface to the interior (Fig. 19): (1) an outer tightly cemented rim of a few tens of microns thickness, without porosity, which consists of a film of secondary silica that coats the microquartz and blurs entirely the granularity of the matrix; (2) a more or less regular underlying zone formed of microquartz without fibrous silica phases (chalcedony) that has relatively highly interconnected porosity and; (3) an inner zone in which the primary fibrous texture of chalcedony is preserved. These neo-cortex alterations are frequent on flint flakes and are always characterised by a very fine silica phase covering the microquartz of the matrix. This is separated from the primary flint matrix by a network of open pores that indicates dissolution and recrystallization of the matrix as testified by the disappearance of the fibrous silica phases in the primary flint facies. This zonal arrangement may not have developed simultaneously but possibly successively or even in alternative fashion. The outer silica casing has most probably developed from amorphous silica compounds resulting from the knapping of the flints, or later mechanical shocks.

Implications

Weathering and alteration of silica in regolith environments has specific relevance to studies of the alteration of flint tools in near-surface environments. In particular, interpretations of the geochemical and kinetic behaviour of the constituent silica phases permits the forecasting of changes that may occur at the surface of a flint during its history (Fig. 20). For example:

(1) dissolution always occurs at its surface; (2) percolation rates of soil water or groundwater through the core are very slow, and changes probably occur over geological time-scales (Ma); Flint alteration and cortex development are mainly triggered by the nature of the component silica phases that have differing solubilities. The most soluble phases dissolve first and at the highest rate resulting in relative accumulation of the most stable phases. This is the first process of alteration. The crystallinity of the silica phases also plays a main role: it controls solubility which decreases with increasing crystallinity. Significant parts of the alteration process are the recrystallizations that occur in the solid state via diffusion in water films between crystal grains. Silica solubility is also temperature dependent: if groundwaters rise to the surface during cold periods they may precipitate silica and re-silicify cortex textures as they cool down.

The chemistry of interstitial water depends on the host material and on the water flow regime. In soils and in the zone of aeration, percolating water always remains under-saturated due to the low kinetic rate of silica dissolution: consequently dissolution will prevail during flint alteration. Groundwaters generally have a higher silica content and may be saturated with respect to the quartz varieties: dissolution of flint cortices will be limited in these environments. In clayrich host rocks, such as clay-with-flints, percolating water flow rates are reduced and the silica content of the pore waters is even higher, favouring deposition of secondary silica.

Specific flow rates and water chemistry may be ascribed to sedimentary characteristics of flint deposits. For example, colluvium on slopes has intermittent drainage. Young alluvium may be permanently drowned whereas old alluvium may have the form of emerged terraces and undergo silica leaching for long periods. Flints at the soil surface, constituting an easy supply of materials, are exposed to severe temperature changes, desiccation, overheating in the sun, and exposure to the winds, and may undergo very specific alterations. Leaching conditions may even be different between the upper and the lower faces of a flint: the upper faces may undergo leaching but may also be subject to silica precipitation when the percolation rate slows down or even stops, whereas the lower faces always experience outflows and thus leaching conditions.

Conclusions

High resolution petrographic and electron optical investigations of weathering and alteration of the various forms of silica in materials that are exposed to near-surface regolith environments provide the basis for assigning micromorphological and mineralogical features to geochemical and hydrological conditions in particular micro-environments. The materials studied over many years and on several continents include sandstones, siliceous duricrusts, and flintstones. Where the weathering and alteration of silica occurs over geological time, as in the case of siliceous duricrusts, or a millennium time scale, as in the case of the Fontainebleau Sandstone, features are generally clearly observed and comparatively easily interpreted. However, the weathering of silica can be a relatively rapid and current phenomenon. In the case of flintstones, there are examples of transformations of silica that occurred over periods of 20,000-50,000 years, thick weathering cortices have developed in less than 100,000 years, and friable cortices have formed in several decades. In addition, we describe examples of polishing and surface hardening of rock surfaces initiated by abrasion and repeated mechanical working followed by subsequent silicification that has occurred within a few decades. Details of the alteration features produced over shorter time periods are progressively less easily examined. However, the outcomes of studies of these alteration features can be used to investigate the provenance of flint tools in an archaeological context.

As well as flintstones, worked flint flakes exhibit various cortices and patinae that are due to changes in the texture and mineralogy at their surface forced by physico-chemical exchanges during working and weathering. And, as in the case of our geological and mineralogical studies in regolith environments, the textures (component particles, geometric aspects, size, mutual relations among the components, granularity) of the cortex and patinae result from changes that occurred under specific microenvironmental conditions and thus reflect evidence of these conditions. Firstly, the remnant primary cortex of flint tools may provide information that identifies the deposits from which prehistoric peoples obtained the flints. Secondly, worked flint flake cortices retain information that may aid in the reconstruction of the environment of the archaeological site and indicate if the artefacts may have been reworked or not. An inventory of flint cortex textures forms the basis for elucidating the mechanisms that triggered the formation of the textures. Three main influencing factors are the nature of the silica components, the pore water of the host sediments or soils and water flow or percolation regimes. Each flint registers its own history, and each type of environment lays down certain characteristic features on its surface, so that morphological and mineralogical changes recorded in the flint cortex can be used to reconstruct the successive environmental conditions experienced by that flint. Thus, a flint flake is not only a petrographical and archaeological entity; it also wears clues of alterations that, once deciphered, potentially reveal a chronology of sedimentary, diagenetic and archaeological events.

Figure 1 -

 1 Figure1-Rate of dissolution and asymptotic solubility of the different silica varieties at 25°C (after Siff ert 1967). The solubility equilibrium of the silica phases in water is reached slowly. Opal and chalcedony show variable solubility according to their respective crystallinity.

Figure 2

 2 Figure2-Macro-and micromorphological structures of a pedogenic silcrete profile. The siliceous matrix is the location of successive silica dissolution and deposition leading to relative accumulation of crystalline quartz.

Figure 4 -

 4 Figure 4 -Schematic relation between stability and crystallinity (size of the elementary crystallites) of the different petrographic varieties of quartz.

Figure 5 -

 5 Figure 5 -Petrography of the Fontainebleau Sandstones. a -The quartzite pans are cemented by subeuhedral quartz overgrowths. b -The quartzite rims show preferential dissolution of the overgrowth aureoles, but the primary quartz grains are not corroded. Thin sections, cross-polarized.

Figure 6 -

 6 Figure 6 -Weathering morphologies of quartzite blocks. a -Dome-like shaped boulders capped with a silica crust. b -Brown opal deposits between dislocated quartz grains in the silica crust; thin section, polarized light. c -Polygonal network on a dome flank. d -Dissolution of quartz overgrowths along the polygonal grooves; thin sections, polarized light. e -Similar as d, with quartz wedge.

Figure 7 -

 7 Figure 7 -Sketches of silica mobility during weathering of quartzite boulders. Silica dissolution is enhanced in contact with soils while silica deposition (opal) occurs at the top of the dome.

Figure 8 -

 8 Figure 8 -The voids within columnar silcretes show Si dissolution in their ceilings (together with accumulation of residual titania), where water seeps through, whereas Si accumulates in form of opal in the base of voids, where solutions concentrate during dry periods.

Figure 9 -

 9 Figure 9 -Quartzite surfaces hardened by mechanical pressures. a -Indurated crusts of the "worked" surfaces of a Neolithic polishing stone, Fäy-les-Nemours (Seine-et-Marne), Photo J.-P. Hofstetter. b -Paving stones made of Fontainebleau sandstone in an old barracks. Their abraded surface is hardened by a siliceous crust that flakes off.

Figure 10 -

 10 Figure 10 -Petrographic images of the hardened silica crusts developed on quartzite paving stones. a -Thin section perpendicular to the surface of the paving stone. Note the intense fracturing of the quartz grains at the surface and its progressive decrease with depth. b & c -Details of the fracturing of the quartz grains, showing 5 to 10 µm quartz splinters cemented by brown opal. Thin section, polarized light.Whatever the crystallo-chemical mechanisms in detail, it is obvious that the mechanical shocks have had an effect on the quartz grains and that the hardening of the exposed surfaces can

Figure 11 -

 11 Figure 11 -Mineralogical composition of flints embedded in chalk and those reworked from chalk and weathered. The reworked flints are always only composed of quartz, whereas other silica varieties coexist beside the quartz in the flints in-situ within chalk (after Trauth et al. 1978).

Figure 13

 13 Figure13-a -Selected flints showing progressive alteration and dissolution from base to top of a bleached sandstone section[START_REF] Thiry | Ground-water silicification and leaching in sands : Example of the Fontainebleau Sand (Oligocene) in the Paris Basin[END_REF]. b -Detail of an extensively altered flint from upper part of the section

Figure 14 -

 14 Figure 14 -Petrography of flint undergoing alteration in the upper part of a silcrete profile. Microspherulites become more distinctive through differential dissolution whereas they are not visible in the unweathered flint. Silica is in each case formed of very fine microcrystalline quartz.

Figure 15 -

 15 Figure15-Reworked or worked flints may undergo dissolution by seeping waters, but also be the focus of silica deposition by way of water concentration or cooling.

Figure 17 -

 17 Figure 17 -Spatial arrangement of the successive surfaces on a flint cobble. (1) Primary flint cortex; (1a) and (1b) early cupulae with neo-cortex; (2) later cupula with a more recent neo-cortex; (3) recent developing cryojoint.

Figure 18 -

 18 Figure 18 -Neo-cortex on reworked flints. a -Section from flint cortex in colluvial deposit (Cruas, Ardèche, France): (C1) Porous outer zone, microquartz with pore diameters up to 100 µm; (C2) tightly cemented zone, mainly composed of microquartz; (P) porous inner zone with dissolution of finest silica matrix and preservation of microquartz and chalcedony (bioclasts); (M) flint matrix without porosity, very fine silica phases including chalcedony and most probably poorly crystallized phases like varieties of opal and moganite. Thin section, cross polarized. b -Secondary silica coating in colluvial deposits (Cruas, Ardèche, France). Zone beneath the coating shows pores and granular texture typical of the outer zone of flint cores with dissolution and recrystallization of the more soluble silica compounds. c -Details of secondary silica coating with shrinkage cracks (arrows) resulting from loss of volume during transformation and dehydration of silica coating. Beneath is the granular and porous zone of the flint cortex. SEM picture. d -Tight silica coating on flint from alluvial deposit (Tertiary conglomerate from Naussac, Lozère, France). Thickness of coating seen in left upper corner of picture. A shock fracture affects the coating (arrow). SEM picture. After Fernandes (2012).

Figure 19 -

 19 Figure19-Neo-cortex (C) tightly cemented by secondary silica without any granulation, overlaying a porous and granular zone formed of isometric microquartz (P) that developed above the primary flint matrix in which the lengthening textures of chalcedony remain (M). Section of a neo-cortex from an artefact worked from a Lower Cretaceous (Barremian) flint (Cruas, Ardèche, France). SEM picture after[START_REF] Fernandes | Itinéraires et transformations du silex : une pétroarchéologie refondée, application au Paléolithique moyen[END_REF].

  the intermediate zone (sub-cortical zone) is the site of adjustment between the flint and its surrounding geochemical environment. Here, recrystallization and ultimately deposition of silica record the various environmental conditions experienced by the flint during its history and can be interpreted and characterised using petrographical and micromorphological techniques to provide clues to the source of the worked flint materials.

Figure 20 -

 20 Figure 20 -Conceptual sketch of changes in the sub-cortical zone of flint. These changes are consequent upon the geochemistry of the pore water in the host sediment and provide the basis for reconstructing these environments. a -Thin section of cortex on flint from colluvial deposit (Cruas, Ardèche, France). b -Conceptual sketch of the petrographic changes that occur in a cortex of a flint during alteration.

Acknowledgements: Research on prehistoric artefacts from different sites of the Massif Central and field surveys were funded by the Ministère de la Culture et de la Communication and the Région Aquitaine (project Origines).The laboratory CRP2A-IRAMT, UMR 5060 CNRS of the University Bordeaux 3, Esplanade des Antilles, F33607, is thanked for access to their SEM facilities. Christine Franke from Mines-ParisTech is acknowledged for comments of an early version of the manuscript.