Hanlie Hong

Kaipeng Ji

Hetang Hei

Chaowen Wang

Chen Liu

Lulu Zhao

Bruno Lanson

Chenlei Zhao

Qian Fang

Thomas J Algeo

Ji Kaipeng

Clay mineral evolution and formation of intermediate phases during pedogenesis on picrite basalt bedrock under temperate

Keywords: kaolinite/smectite (K/S), illite/smectite (I/S), desilication, weathering, soil formation, saprolite

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Introduction

During weathering and pedogenesis, the initial alteration of basic igneous rocks usually produces smectite, allophane, and imogolite in various environments, which are then transformed into other clay phases, dominated by kaolins and mixed-layer kaolinite/smectite (K/S), as soil formation proceeds [START_REF] Alexander | Ion-exchange properties and possible applications of weathered Antrim basalts[END_REF][START_REF] Kantor | Mineralogy and genesis of clays in red-black toposequences in Kenya[END_REF][START_REF] Curtin | Composition and origin of smectite in soils derived from basalt in Northern Ireland[END_REF][START_REF] Glassmann | Alteration of basalt in soils of Western Oregon[END_REF][START_REF] Righi | Pedogenic formation of kaolinite-smectite mixed layers in a soil toposequence developed from basaltic parent material in Sardinia (Italy)[END_REF][START_REF] Vingiani | Mixed-layer kaolinite-smectite minerals in a red-black soil sequence from basalt in Sardinia (Italy)[END_REF][START_REF] Rasmussen | Basalt weathering and pedogenesis across an environmental gradient in the southern Cascade Range, California, USA[END_REF][START_REF] Perez-Fodich | Organic acids and high soil CO2 drive intense chemical weathering of Hawaiian basalts: Insights from reactive transport models[END_REF]. Neoformed smectite has a dioctahedral structure typically of a ferriferous member of the montmorillonite-beidellite series, occurring in the lower soil horizon, while illite and poorly-crystalline kaolinite predominate in the upper soil profile [START_REF] Kantor | Mineralogy and genesis of clays in red-black toposequences in Kenya[END_REF][START_REF] Curtin | Composition and origin of smectite in soils derived from basalt in Northern Ireland[END_REF].

Key weathering processes, such as acid hydrolysis and oxidation of bedrock as well as the formation of secondary clay phases, are dependent on bedrock lithology and other soil-forming factors such as climate, topography, and drainage conditions [START_REF] Chesworth | The parent rock effect in the genesis of soil[END_REF][START_REF] Curi | Effect of parent rocks on chemical and mineralogical properties of some Oxisols in Brazil[END_REF][START_REF] Vingiani | Mixed-layer kaolinite-smectite minerals in a red-black soil sequence from basalt in Sardinia (Italy)[END_REF][START_REF] Deepthy | Climatic control on clay mineral formation: evidence from weathering profiles developed on either side of the Western Gnats[END_REF][START_REF] Li | Temperature dependence of basalt weathering[END_REF][START_REF] Wilson | Soil genesis and mineralogy across a volcanic lithosequence[END_REF][START_REF] Perez-Fodich | Organic acids and high soil CO2 drive intense chemical weathering of Hawaiian basalts: Insights from reactive transport models[END_REF]. Cool and moist climatic conditions favor the formation of short-range order phases, whereas warm and dry conditions produce smectite, kaolins, and crystalline Fe-oxyhydroxides [START_REF] Dekayir | Mineralogy and geochemistry of supergene alteration of an alkali basalt from the Middle Atlas, Morocco[END_REF][START_REF] Rasmussen | Basalt weathering and pedogenesis across an environmental gradient in the southern Cascade Range, California, USA[END_REF][START_REF] Jiang | Element mobilization and redistribution under extreme tropical weathering of basalts from the Hainan Island, South China[END_REF]. Under humid temperate conditions at Killundine, Morvern, Scotland, weathering of fresh basalt initially formed a trioctahedral clay phase that transformed through a mixture of di-and trioctahedral species to a dioctahedral interstratified illite/smectite (I/S) phase with minor hematite and anatase in highly weathered soils [START_REF] Bain | Geochemistry and mineralogy of weathered basalt from Morvern, Scotland[END_REF]. In the temperate Middle Atlas, Morocco, weathering of alkali basalt was initially dominated by dissolution of glass and deposition of poorly-crystalline Si-Al products, which then transformed into halloysite, kaolinite, and hematite during enhanced weathering, while illite and vermiculite were regarded as introduced phases that did not originate from the basalt bedrock [START_REF] Dekayir | Mineralogy and geochemistry of supergene alteration of an alkali basalt from the Middle Atlas, Morocco[END_REF]. A basalt weathering under seasonally wet, temperate climate conditions in Sardinia, Italy yielded mainly K/S clays, with drainage conditions controlling the proportion of kaolinite in the mixed phase [START_REF] Righi | Pedogenic formation of kaolinite-smectite mixed layers in a soil toposequence developed from basaltic parent material in Sardinia (Italy)[END_REF][START_REF] Vingiani | Mixed-layer kaolinite-smectite minerals in a red-black soil sequence from basalt in Sardinia (Italy)[END_REF]. Also, the ferric smectite layer content in the K/S decreased upward through the soil profile, and kaolinization of smectite took place by complete dissolution of preferentially ferric smectite layers and crystallization of kaolinite layers in the interlayer regions of smectite particles [START_REF] Curtin | Composition and origin of smectite in soils derived from basalt in Northern Ireland[END_REF][START_REF] Righi | Pedogenic formation of kaolinite-smectite mixed layers in a soil toposequence developed from basaltic parent material in Sardinia (Italy)[END_REF]. However, Singer and Armannsson (1999) reported that, in the weathering profile of Pleistocene basalts under moderately humid Mediterranean conditions, smectite was the only clay mineral formed in the saprolite, and the clay fractions of the regolith consisted of smectite, kaolinite, and minor halloysite without intermediate phases.

Various patterns and processes of pedogenesis in basic igneous rocks have also been proposed (e.g., [START_REF] Bain | Geochemistry and mineralogy of weathered basalt from Morvern, Scotland[END_REF][START_REF] Ma | The weathering and evolution of soil ferrallite minerals in the South China[END_REF][START_REF] Dekayir | Mineralogy and geochemistry of supergene alteration of an alkali basalt from the Middle Atlas, Morocco[END_REF][START_REF] Campodonico | Chemical weathering in subtropical basalt-derived laterites: A mass balance interpretation (Misiones, NE Argentina)[END_REF][START_REF] Sulieman | Investigation of the clay minerals composition of soils derived from basalt parent materials in the Early Miocene to Early Pleistocene on the Arabian Shield using multiple techniques: implications for paleoclimatic conditions[END_REF]. [START_REF] Orhan | Effect of toposequences on geochemical mass balance and clay mineral formation in soils developed on basalt parent material under subhumid climate condition[END_REF] showed that, in a subhumid climate, basalt-derived soils contained dominantly smectite, with minor illite, kaolinite, and vermiculite, and the content of smectite increased with depth in the soil and was controlled by local topography and drainage.

Under a temperate Mediterranean-type climate, weathering of basalts produced dominantly smectite with minor kaolin, illite, and palygorskite at lower elevations, whilst kaolin was dominant on local topographic highs [START_REF] Prudèncio | Clay mineral assemblages in weathered basalt profiles from central and southern Portugal: climatic significance[END_REF].

However, variation in the clay phases of basalt-derived soils has also been attributed to other factors such as hydrothermal alteration during lava cooling [START_REF] Bain | Geochemistry and mineralogy of weathered basalt from Morvern, Scotland[END_REF][START_REF] Pokrovsky | Basalt weathering in Central Siberia under permafrost conditions[END_REF], illitization of smectite due to plant uptake of K from deeper soil horizons [START_REF] He | Clay minerals in a soil chronosequence derived from basalt on Hainan Island, China and its implication for pedogenesis[END_REF], and influx of allochthonous aeolian material [START_REF] Dekayir | Mineralogy and geochemistry of supergene alteration of an alkali basalt from the Middle Atlas, Morocco[END_REF][START_REF] Rasmussen | Basalt weathering and pedogenesis across an environmental gradient in the southern Cascade Range, California, USA[END_REF]. Furthermore, [START_REF] Nesbitt | Recent chemical weathering of basalts[END_REF] claimed that climate conditions affect only the rates but not the mechanisms of chemical weathering of basalts, and that the diversity of clay mineral assemblages formed in basalt-derived soils under a range of climate conditions depends mainly on the steady-state conditions of the climate regime (i.e., chemical weathering vs erosion).

Although the weathering of basalt under temperate climate conditions seems to conform to a general pattern of initial formation of smectite and subsequent transformation toward kaolin [START_REF] Prudèncio | Clay mineral assemblages in weathered basalt profiles from central and southern Portugal: climatic significance[END_REF], the specific processes of basalt pedogenesis and, especially, the characterization of intermediate phases in basalt-derived soils remain a matter of debate [START_REF] Bain | Geochemistry and mineralogy of weathered basalt from Morvern, Scotland[END_REF][START_REF] Glassmann | Alteration of basalt in soils of Western Oregon[END_REF][START_REF] Dekayir | Mineralogy and geochemistry of supergene alteration of an alkali basalt from the Middle Atlas, Morocco[END_REF][START_REF] Vingiani | Mixed-layer kaolinite-smectite minerals in a red-black soil sequence from basalt in Sardinia (Italy)[END_REF][START_REF] He | Clay minerals in a soil chronosequence derived from basalt on Hainan Island, China and its implication for pedogenesis[END_REF][START_REF] Caner | Basalt and rhyo-dacite weathering and soil clay formation under subtropical climate in southern Brazil[END_REF][START_REF] Wilson | Soil genesis and mineralogy across a volcanic lithosequence[END_REF][START_REF] Jiang | Element mobilization and redistribution under extreme tropical weathering of basalts from the Hainan Island, South China[END_REF][START_REF] Perez-Fodich | Organic acids and high soil CO2 drive intense chemical weathering of Hawaiian basalts: Insights from reactive transport models[END_REF][START_REF] Sulieman | Investigation of the clay minerals composition of soils derived from basalt parent materials in the Early Miocene to Early Pleistocene on the Arabian Shield using multiple techniques: implications for paleoclimatic conditions[END_REF]. A lack of investigation of the mineralogy of picrite basalts undergoing weathering hinders a comprehensive understanding of the evolution of clay phases and the transformative processes in such pedogenic systems.

Temperate climate conditions commonly allow development of a complete, advanced sequence of intermediate weathering phases [START_REF] Bain | Geochemistry and mineralogy of weathered basalt from Morvern, Scotland[END_REF][START_REF] Vingiani | Mixed-layer kaolinite-smectite minerals in a red-black soil sequence from basalt in Sardinia (Italy)[END_REF].

In this study, analysis of the clay mineralogy and geochemical changes in a soil profile developed on the well-studied basaltic bedrock in the temperate Dali region of Yunnan Province, South China was undertaken in order to address the pattern of pedogenesis and soil mineral evolution during weathering of picrite basalt. The aims of our investigation were: (1) to provide new insights into the pedogenesis of picrite basalt under temperate climate conditions, and (2) to provide additional constraints on reaction pathways for formation of intermediate clay-mineral phases during weathering.

Study area

The study area is situated in the central zone of the Emeishan Large Igneous Province, in the southwestern part of the Yunnan-Guizhou Plateau, southwestern China (Fig. 1).

The Emeishan basaltic traps, with a cumulative thickness ranging from five kilometers (proximally) to tens of meters (distally), are part of a major large igneous province outcropping mainly in southwestern China over an area in excess of a quarter million square kilometers with a center near Dali (Yunnan) to Miyi (Sichuan) [START_REF] He | Generation and spatial distribution of the Emeishan Large Igneous Province: New evidence from stratigraphic records[END_REF]. They overlie Late Permian carbonates of the Maokou Formation and are overlain by the Triassic Wujiaping sedimentary sequence [START_REF] Courtillot | On causal links between flood basalts and continental breakup[END_REF][START_REF] He | Generation and spatial distribution of the Emeishan Large Igneous Province: New evidence from stratigraphic records[END_REF]. The lithology of the Emeishan traps changes slightly as a function of local tectonic setting, occurring as basalt, picrite basalt, alkaline basalt, tholeiite, and minor picrate, trachyandesite, and trachyte [START_REF] Hou | Emei large igneous province: characteristics and origin[END_REF]. The landscape of the Dali area consists of plateaus, mountains, and intermontane basins, including geomorphic units representing high mountain canyons, alluvial fans, lakeside slopes, broad valleys, and complex terrains, at altitudes ranging from 1340 to by a mean annual temperature of 15.1 °C (mean monthly temperatures range from 3.7 to 28.9 °C) and a mean annual precipitation of ~700 mm. The study section is located at Daying town (100°28′E, 25°48′N), Dali City, Yunnan Province, South China, at an altitude of 1722 m. Eolian sediments have not been observed in Dali, as monsoonal winds transport eolian materials only as far southward as the middle of China [START_REF] Qiao | Geochemical characteristics of the eolian deposits in southern China, and their implications for provenance and weathering intensity[END_REF]. The vegetation of the study area consists dominantly of alpine herbs and the high-altitude shrubs belonging to the genus Quercus, and human land use is constrained mainly to maize cultivation owing to the limited rainfall. Soil developed at the footslope position, where excavation for a water channel provided access to a 4.6-m-thick soil profile overlying unweathered picrite basalt bedrock.

Materials and methods

Field sampling

The topsoil (Layer 1, 0-40 cm) displays a porous and loose crumb structure and is brownish gray in color, consisting mainly of sandy clays and minor gravels, with relatively abundant organic matter and plant roots. A reddish-brown silty clay horizon (Layer 2, 40-100 cm) is present under the topsoil, exhibiting a homogeneous crumb structure and containing minor plant roots. The underlying horizon (Layer 3, 100-160 cm) is light gray in color and blocky in structure, having short white clay veins or spots. A thick grayish-brown clayey gravel saprolite horizon (Layer 4, 160-380 cm) is present beneath Layer 3. It is composed dominantly of clays and gravels of incompletely weathered bedrock, with minor amounts of Fe-Mn nodules. The bedrock is greyish-green picrite basalt that exhibits an amygdaloidal structure (Fig. 2). 10-20 cm intervals in the upper portion (0-1.6 m) and at 40 cm intervals in the saprolite (1.6-3.8 m) of the lower portion, yielding a total of 18 samples.

Selected chemical and mineralogical analyses

Soil pH measurements were performed in distilled water using a glass electrode in a 1:2.5 soil/water suspension. The cation exchange capacity (CEC) determination of soil samples was performed using the sodium chloride-ethanol method of [START_REF] Chen | Determination of cation exchange capacity of expansive soils[END_REF]. For total organic carbon (TOC) analysis, the soil samples were ground to pass through a 200-mesh sieve. The powdered samples were then reacted with hydrochloric acid (0.1 mol•L -1 ) overnight to remove carbonate. TOC measurement was performed on a Vario Macro Cube carbon analyzer. For X-ray diffraction (XRD) analysis, whole-rock samples were air-dried at 60 °C overnight and then ground manually to powder using an agate mortar and pestle. The mineral contents of the whole-rock samples were estimated using the reference intensity ratio approach with admixture of corundum as the internal standard [START_REF] Hillier | Accurate quantitative analysis of clay and other minerals in sandstones by XRD: comparison of a Rietveld and a reference intensity ratio (RIR) method and the importance of sample preparation[END_REF]. The mixing powder samples were prepared by mounting the powders into sample holders using a back-press technique. The characteristic peaks of minerals used in our calculation were 3.34 Å (quartz), 3.19 Å (plagioclase), 3.51 Å (anatase), 2.97 Å (clinopyroxene), 2.90 Å (pumpellyite), and 2.70 Å (hematite), 15 Å (smectite), and 7.40 Å (kaolinite/smectite), respectively. The relative standard deviation of the semi-quantitative method is usually < 15 %.

The clay fraction (< 2 μm) was extracted using the sedimentation method [START_REF] Jackson | Soil Chemical Analysis[END_REF], and an oriented clay-fraction sample was prepared for clay mineralogical investigation by pipetting the clay suspension, with a solid-to-liquid ratio of around 1:25, onto a glass slide that was subsequently air-dried at ambient temperature. In order to determine swelling clay components, the oriented clay samples were treated with ethylene glycol in a sealed glass desiccator heated at 65 °C for 4 h in an electronic oven. XRD analysis was performed on a Panalytical XPert PRO DY2198 diffractometer with Ni-filtered Cu Kα radiation. The instrument was operated at 40 kV tube voltage and 35 mA tube current conditions, with the slit system of 1° divergence slit, 1° anti-scatter slit, and 0.3 mm receiving slit, and the XRD patterns were collected from 3° to 65° 2θ at a scan rate of 4° 2θ min -1 and a step size of 0.02° 2θ.

For observation of the (060) peak of clay phases, the non-oriented clay samples were prepared using the back-press method. The layer content of the end member in a mixed-layer clay phase and the relative abundance of clay phases in the sample were estimated using the NEWMOD II program [START_REF] Reynolds | NEWMOD-for-Windows. The Calculation of One-dimensional X-ray Diffraction Patterns of Mixed-layered Clay Minerals[END_REF], which was performed simultaneously for the XRD patterns of both glycolated and Ca-saturated clay samples.

Major element compositions and mass balance calculations

The chemical compositions of the samples were measured using X-ray fluorescence (XRF). Prior to preparation of the fused XRF pellets, measurement of loss-on-ignition (LOI) was performed by drying a sample aliquot in an oven at 105 °C for 2 h and then heating 1 g of the dried sample to 1000 °C for 1.5 h. The LOI value was calculated from the difference in weight between the sample aliquots heated to 105 °C and to 1000 °C. Preparation of fused pellets for XRF measurement was as follows: a 5-g aliquot of sample was dried in an electronic oven at 60 °C overnight and then manually ground to a fine powder with a grain size of less than 0.074 μm (200 mesh) for generation of fused XRF pellets. Major element compositions were analyzed on an XRF-1800 SHIMADZU sequential X-ray fluorescence spectrometer. The detection limit was generally ~0.01 wt%, and the relative standard deviation was usually < 1%.

The degree of mobilization of each element in the weathering profile was estimated through a mass balance calculation using Al as an immobile reference element [START_REF] Campodonico | Chemical weathering in subtropical basalt-derived laterites: A mass balance interpretation (Misiones, NE Argentina)[END_REF] and the chemical composition of the unaltered bedrock as the starting mass. The formula for this calculation is: CGL = [CS /AlS] × AlBR -CBR, where CGL, CS, and CBR represent the gain or loss (GL) of the element of interest as well as its concentration in the soil sample (S) and the unaltered bedrock (BR), and AlS and AlBR represent the aluminum concentrations of the sample and bedrock, respectively.

SEM and HRTEM observations

Small chips (~1 cm) of whole-rock samples were selected and then gold-coated.

Scanning electron microscope (SEM) observation was undertaken on a Hitachi SU8010 field emission scanning electron microscope, equipped with an Octane Super X-ray energy-dispersive (EDS). The instrument was operated at an accelerating voltage of 20 kV and a probe current of 2 nA, with a magnification up to 8×10 5 and a resolution of secondary electron image of 1.0 nm.

For high resolution transmission electron microscope (HRTEM) observation, the air-dried clay fraction was immersed in methanol solution and then ultrasonically dispersed for 10 min, after which the clay material was collected with a copper net and dried under an infrared light. HRTEM observation was performed on a Tecnai G2 GENESIS 2000 X-ray energy-dispersive (EDS). The instrument was operated at the condition of 160 kV accelerating voltage, with a beam spot size of 1.5 nm, a point resolution of 0.24 nm, and a line resolution of 0.14 nm.

Results

The pH, TOC, and CEC values of the soil samples are listed in Table 1. The XRD patterns of representative soil samples are shown in Fig. 3a and their mineral compositions are tabulated in Table 1. The clay mineral components and their relative proportions were obtained from fitting XRD traces of the glycolated samples (Fig. 4), and were further confirmed by fitting the air-dried samples (Fig. 5), and their relative contents are listed in Table 2.

Physical and chemical properties and mineral compositions of the soil

Mineral components in the soil profile are mainly clay minerals, plagioclase, quartz, and hematite. Mineral abundances displayed only small changes between samples from the same soil horizon but varied considerably between samples from different soil layers, as reflected by intensities of their characteristic peaks of the minerals (Fig. 3a). The bedrock consists dominantly of pumpellyite (2.90, 3.79, 4.67, 2.74, 2.52 Å) and clay minerals (15.0 Å), with minor amounts of plagioclase (3.18 Å), quartz (3.34, 4.26, 1.82 Å), and hematite (2.70, 2.52, 1.485 Å) and trace clinopyroxenes (2.97 Å), suggesting that the picrite basalt bedrock has undergone low-grade burial metamorphism. In addition, the XRD profiles of selected clay-fraction samples show that clay phases from the middle to upper soil profile exhibit a uniform dioctahedral structure (1.50 Å), whereas those from the saprolite horizon consist of both di-(1.50 Å) and trioctahedral (1.54 Å) species (Fig. 3b).

Clay species in the Dali soil profile are dominantly smectite, K/S, and I/S, characterized by the 14.8 Å peak in the XRD patterns of the untreated whole-rock samples (Fig. 3), which expanded to around 17 Å after glycolation (Figs. 45).

Smectite accounts for the 17-Å peak, and the notable decrease in intensity upward in the soil profile indicates a gradual decrease in smectite content. I/S accounts for the bump peak at ~8.5-8.8° 2θ, which is evident for the upper samples but not visible for the deeper samples, indicating an upward increase of this mineral phase. The 7.4-Å peak (~12° 2θ) is ascribed to a K/S phase with no contribution from discrete kaolinite, which requires two sub-populations to be fitted properly since it is a broad reflection (Table 2). K/S occurs throughout the soil profile, forming two sub-populations marked by higher proportions and lower proportions of kaolinite layers, while I/S occurs dominantly in Layers 1, 2, and 3 but is almost absent in the Layer 4. Overall, smectite decreases concurrently with increases in I/S clays upward within the soil profile, and K/S clays are notably less abundant in the lower samples relative to the upper samples (Table 2).

Geochemical composition of the soil

The Dali soil shows marked variations in geochemical composition among its component soil layers, but only small differences between samples from the same horizon (Table 3; Fig. 6). Samples from the lower profile, especially the Layer 4, contain relatively more SiO2 (49.6-51.7%) compared to those from the upper profile (43.0-46.8%). TiO2 contents are notably high (3.5-3.6%) in the Layer 1. Al2O3 is modestly concentrated in the middle profile (Layers 2 and 3; 19.0-23.5%) relative to The MgO and CaO contents exhibit downward increases from 1.7 to 4.8% and from 1.1 to 3.3%, respectively. Moreover, the CaO content of the bedrock is extremely high (to 19.5%). The Na2O content of the soil samples shows a downward increase from 1.1 to 4.8%, whereas the K2O content exhibits a downward decrease from 0.9 to 0.1%.

However, the Na2O and K2O contents of the bedrock are markedly lower (0.02% and 0.01%, respectively). The P2O5 content of the soil samples varies from 0.1 to 0.3%, but Layer 1 and Layer 4 contain generally more P2O5 relative to the other soil layers. Fig. 6. Changes in major element contents (wt%) and CIA through the Dali picrite basalt weathering soil profile.

SEM observations

In general, clay flakes in Layer 1 display a relatively smooth and flat basal (001)

plane and exhibit a generally poorly-developed pseudo-hexagonal or plate-like outline (Fig. 7a). Also, the edges of clay mineral flakes are relatively straight, similar to the common morphologies of kaolinite and illite. Clay mineral grains in Layer 2 often exhibit relatively well-developed euhedral pseudo-hexagonal morphology with a smoother basal (001) plane and straighter edges (Fig. 7b), indicating higher crystallinity relative to those of the topsoil. In the Layer 3, although most of the clay mineral grains display a relatively smooth basal (001) plane, clay minerals usually develop curled edges and thus typically exhibit wave-shaped outlines (Fig. 7c), consistent with the morphologies of typical K/S and I/S clays [START_REF] Hong | Clay mineralogy across the P/T boundary of the Xiakou section, China: evidence of clay provenance and environment[END_REF]2017).

Clay minerals in the Layer 4 display three different morphologies: (1) Abundant clay particles that are extremely small in comparison with those in the overlying soil layers; they display ragged outlines (Fig. 7d) and often replace basalt particles. ( 2)

Clay minerals that are larger and typically develop curled edges in fissures or in the interstitial spaces among residual basalt particles, suggesting the occurrence of smectite or I/S clays (Fig. 7d).

(3) Clay minerals with a honeycombed polygonal pattern, indicative of neoformed smectite (Fig. 7e); their distinctive curled edges and interwoven structure, as well as their presence primarily in open cavities, are suggestive of smectite that crystallized from solution during weathering. The picrite basalt bedrock shows generally a solid glass structure, with only rare small vesicular pores (Fig. 7f).

HRTEM observations

Clay particles generally display a wave-shaped morphology and have a relatively thin and poorly developed (001) surface (Fig. 8a). The edges of these particles are typically curved and curled, consistent with the characteristic morphologies of smectite, K/S, and I/S clays [START_REF] Van Der Gaast | Curved smectite in soils from volcanic ash in Kenya and Tanzania: A low-angle X-ray powder diffraction study[END_REF][START_REF] Hong | Kaolinitesmectite mixed-layer clays in the Jiujiang red soils and their climate significance[END_REF]. However, in samples from the upper soil profile, some of the clay flakes exhibit relatively euhedral morphologies in long, plate-shaped crystals with straight outlines along the (001) dimension, consistent with the crystal morphology of strongly illitized I/S clays (Fig. 8b). Clay flakes with relatively well-developed euhedral pseudo-hexagonal outlines are also present in these soil samples (Fig. 8c). The basal (001) planes of clay particles are smooth and flat, and their lateral surfaces ( 110) and ( 010) are clearly defined with straight edges, similar to the morphology of kaolinite.

The margins of clay particles with thick and lath-shaped morphologies usually display straight lattice fringes with 10-Å spacing, in agreement with those of illite layers. However, the 10-Å fringes are often intercalated with 12-Å smectite layers, indicative of the presence of interstratified I/S (Fig. 8d). For clay particles with relatively thin and wave-shaped morphology, the 12-Å fringes are often irregularly interstratified with 7-Å fringes, suggesting the occurrence of K/S (Fig. 8e). In addition, the 12-Å layers are occasionally observed to be interstratified with both 7-Å kaolinite layers and 10-Å illite layers (Fig. 8f), indicating a mixed-layer smectite, illite, and kaolinite (I/S/K) stacking structure.

Discussion

Clay mineral evolution through the weathering profile

As a result of burial metamorphism, the Dali picrite basalt bedrock consists mainly of pumpellyite, smectites, hematite, quartz, and minor amounts of plagioclase, clinopyroxenes, and anatase (Fig. 3; Table 2). The Emeishan basalts commonly display amygdaloidal and vesicular structures [START_REF] Hou | Emei large igneous province: characteristics and origin[END_REF]. This volcanic rock was subjected to burial diagenesis during the Late Permian, producing mainly pumpellyite, smectite, and hematite [START_REF] Mevel | Occurrence of pumpellyite in hydrothermally altered basalts from the Vema fracture zone (mid-Atlantic ridge)[END_REF][START_REF] Inoue | Pumpellyite and related minerals from hydrothermally altered rocks at the Kalmikta area, northern Honshu, Japan[END_REF]Mirabella et al., 2005;[START_REF] De Rosa | On the origin and post-depositional history of widespread massive ash deposits: The case of Intermediate Brown Tuffs (IBT) of Lipari (Aeolian Islands, Italy)[END_REF]. The picrite basalt rock consists mainly of SiO2, FeO, MgO, CaO, Al2O3, corresponding to the general mineral compositions of olivine and plagioclase. The burial alteration process can be described as the following reaction equation: 3MgFeSiO4 (olivine) + 5CaAlSi2O8 (plagioclase

) + 12 H + → Ca4MgAl5[Si2O7]2[SiO4]2(OH)5•H2O (pumpellyite) + CaMg2[Si4O10](OH)2•1.5H2O
(smectite) + 1.5Fe2O3 (hematite) + 3SiO2. Tectonic uplift has exposed the Emeishan basalts, and supergene weathering has taken place since the Late Triassic. Initial alteration of the bedrock entailed dissolution of pumpellyite (Table 3; Fig. 6), followed by precipitation of newly formed smectite clays in small fissures and pores in the bedrock (Fig. 7e,f,g). The poorly-crystalline small clay grains are often replacements of pumpellyite aggregates as the latter alter into smectite (Fig. 7h).

Dissolution of bedrock in the first stage of weathering yielded relatively less mobile

Si-Al-rich materials in fissures and pores (Fig. 7g) and subsequently crystallized to larger smectite flakes with curled edges in honeycombed polygonal aggregates (Fig. 7e,f).

Although smectite is typically the clay-mineral phase in thermodynamic equilibrium with soil solutions during basalt weathering, microenvironmental conditions can act as kinetic influences on clay-mineral evolution that yield different products from thermodynamic predictions [START_REF] Ziegler | Halloysite as a kinetically controlled end product of arid-zone basalt weathering[END_REF]. Microenvironments such as cavities and cracks favor rapid uptake of Al released by poorly-crystalline Si-Al-rich materials (Fig. 7c) and thus facilitate smectite-to-kaolinite transformations.

At Dali, smectite is the dominant (39-64%) secondary clay species in the saprolite horizon, but it decreases in the middle to upper soil profile due to transformation of smectite to mixed-layer K/S. The limited transformation of smectite to I/S clay is probably attributable to the low K content (0.01%) of the bedrock (Tables 2 and3).

Well-drained conditions usually favor development of kaolinitic soils, whereas poorly-drained environments formation of smectitic Vertisols [START_REF] Kantor | Mineralogy and genesis of clays in red-black toposequences in Kenya[END_REF]May et al., 1986;[START_REF] Vingiani | Mixed-layer kaolinite-smectite minerals in a red-black soil sequence from basalt in Sardinia (Italy)[END_REF]. In the Dali soil profile, transformation of smectite to K/S and I/S clays was most common in pores (Fig. 7b,c,d). The clay mineral assemblage of mixed-layer K/S and I/S clays, smectite, and Fe-oxides, together with plagioclase and trace pumpellyite, suggests that smectite layers experienced desilication and transformed preferentially to kaolinitic and illitic phases [START_REF] Glassmann | Alteration of basalt in soils of Western Oregon[END_REF][START_REF] Righi | Pedogenic formation of kaolinite-smectite mixed layers in a soil toposequence developed from basaltic parent material in Sardinia (Italy)[END_REF][START_REF] Wilson | Soil genesis and mineralogy across a volcanic lithosequence[END_REF].

However, the dominance of K/S and I/S clays in association with minor discrete smectite, as well as a lack of discrete kaolinite and illite through the Dali soil profile, is probably indicative of early-stage desilication within the soil (Figs. 3 and4; Table 2). Also, the occurrence of plagioclase and absence of gibbsite at Dali confirm a low weathering intensity due to limited acid leaching during basalt alteration, consistent with its significantly low CIA values [START_REF] Bakker | A micromorphological study of pedogenic processes in an evolutionary soil sequence formed on Late Quaternary rhyolitic tephra deposits, North Island, New Zealand[END_REF][START_REF] Vingiani | Mixed-layer kaolinite-smectite minerals in a red-black soil sequence from basalt in Sardinia (Italy)[END_REF][START_REF] He | Clay minerals in a soil chronosequence derived from basalt on Hainan Island, China and its implication for pedogenesis[END_REF][START_REF] Perez-Fodich | Organic acids and high soil CO2 drive intense chemical weathering of Hawaiian basalts: Insights from reactive transport models[END_REF].

Upward within the Dali soil profile, from Layer 4 to Layer 2, I/S and K/S clays generally increase while smectite concurrently decreases, associated with increasing proportions of illitic and kaolinitic layers in I/S and K/S clays, respectively (Table 2).

The intercalation of 12-Å smectite and 10-Å illite layers in I/S clays is indicative of illitization of smectite due to K-fixation (Fig. 8d), and the random intercalation of 12-Å smectite and 7-Å kaolinite layers in clay particles suggests kaolinization of smectite due to intense weathering (Fig. 8e). The interstratification of 12-Å smectite, 10-Å illite, and 7-Å kaolinite layers reflects formation of mixed-layer I/S/K clays (Fig. 8f), probably due to simultaneous transformation of smectite to illite and kaolinite as weathering proceeded.

The upward decrease in smectite through the weathering profile is clearly due to the kaolinization and illitization of this mineral during the weathering process (cf. [START_REF] Orhan | Effect of toposequences on geochemical mass balance and clay mineral formation in soils developed on basalt parent material under subhumid climate condition[END_REF]. In wide climate conditions, smectite and smectitic interstratifications are common clay mineral assemblages in basalt-derived soils [START_REF] Bain | Geochemistry and mineralogy of weathered basalt from Morvern, Scotland[END_REF][START_REF] Eggleton | Weathering of basalt: changes in rock chemistry and mineralogy[END_REF][START_REF] Righi | Pedogenic formation of kaolinite-smectite mixed layers in a soil toposequence developed from basaltic parent material in Sardinia (Italy)[END_REF][START_REF] Vingiani | Mixed-layer kaolinite-smectite minerals in a red-black soil sequence from basalt in Sardinia (Italy)[END_REF][START_REF] Barbera | The influence of weathering processes Mediterranean on labile and stable organic matter in volcanic soils[END_REF][START_REF] Perez-Fodich | Organic acids and high soil CO2 drive intense chemical weathering of Hawaiian basalts: Insights from reactive transport models[END_REF]. In the Dali soil profile, the markedly higher K/S content of Layer 2 and the upper Layer 3 compared to other soil layers (Table 2) is consistent with their relatively greater penetrative condition in the soil profile, and the presence of smectite, K/S, I/S, and I/K/S clays is attributable to relatively weak chemical weathering under temperate, low-rainfall conditions. An increase in the kaolinitic and illitic components of a soil usually causes a decrease in cation-exchange capacity, since the CEC of a soil is dependent mainly on its clay content-especially that of expandable phases [START_REF] Bain | Geochemistry and mineralogy of weathered basalt from Morvern, Scotland[END_REF][START_REF] He | Clay minerals in a soil chronosequence derived from basalt on Hainan Island, China and its implication for pedogenesis[END_REF][START_REF] Babechuk | Quantifying chemical weathering intensity and trace element release from two contrasting basalt profiles, Deccan Traps, India[END_REF].

The high CECs of the Dali soil (24.3-54.7 cmol/kg) agree well with the presence of smectite, K/S, and I/S clays (Table 2). Unlike previously studied basalt weathering profiles (e.g., [START_REF] Curtin | Composition and origin of smectite in soils derived from basalt in Northern Ireland[END_REF][START_REF] Lessovaia | Traprock transformation into clayey materials in soil environments of the Central Siberian Plateau, Russia[END_REF][START_REF] Van Ranst | Halloysite-smectite mixed-layered clay in fluvio-volcanic soils at the southern foot of Mount Kilimanjaro, Tanzania[END_REF], clay mineral phases at Dali exhibit structural variation linked to weathering intensity: the weakly weathered saprolite horizon is characterized by a mixture of di-and trioctahedral species, whereas clays in the middle to upper profile have a more uniform dioctahedral structure due to intense weathering (Fig. 3). These changes in clay-mineral structure, which are associated with an increase of Al and a decrease of Mg, probably reflect the replacement of Mg by Al and the oxidation of Fe 2+ to Fe 3+

during kaolinization and illitization of smectite in the upper part of the soil profile.

Although the mechanism of transformation of tri-and dioctahedral clay minerals to dioctahedral mixed-layer clay minerals remains poorly understood, it is probably associated with the alteration of smectite to kaolinite in the Dali soil [START_REF] Righi | Pedogenic formation of kaolinite-smectite mixed layers in a soil toposequence developed from basaltic parent material in Sardinia (Italy)[END_REF][START_REF] Ryan | The temporal evolution of pedogenic Fe-smectite to Fe-kaolin via interstratified kaolin-smectite in a moist tropical soil chronosequence[END_REF]. The neoformed montmorillonite in the saprolite horizon is dominated by migration of Ca and uptake of Mg and Fe [START_REF] Christidis | Comparative study of the mobility of major and trace elements during alteration of an andesite and a rhyolite to bentonite, in the islands of Milos and Kimolos, Aegean, Greece[END_REF], which results in the co-existence of both di-and trioctahedral species. As weathering proceeds, the precursor of trioctahedral smectite becomes thermodynamically unstable due to reduced Mg concentrations in the pore fluid and, especially, the more acidic conditions in the middle to upper soil profile [START_REF] Kittrick | Mica-derived vermiculites as unstable intermediates[END_REF], resulting in alteration of smectite to kaolinite and/or illite. This transformation is often considered to be a solid-state process in which the tetrahedral sheets of smectite are stripped away [START_REF] Ryan | The temporal evolution of pedogenic Fe-smectite to Fe-kaolin via interstratified kaolin-smectite in a moist tropical soil chronosequence[END_REF][START_REF] Hong | Kaolinitesmectite mixed-layer clays in the Jiujiang red soils and their climate significance[END_REF]. Decomposition of the tetrahedral sheets leads to a redistribution of elements marked primarily by the loss of Si and gain of Al. In acidic conditions, Al from interlayer sites will be mobilized, and increasing Al concentrations in the pore fluid will favor the diffusion of Al between external solution and interlayer space of smectite and thus the replacement of octahedral Mg by Al [START_REF] Mavris | Clay mineral evolution along a soil chronosequence in an Alpine proglacial area[END_REF][START_REF] Cheng | New insight into biotite weathering in the subtropic Tongcheng granite regolith, Hubei Province, South China[END_REF]. In addition, Fe 2+ in dioctahedral sites of the parent mineral is likely to be oxidized to Fe 3+ during transformation of smectite to mixed-layer K/S, analogous to the oxidation of Fe 2+ during transformation of dioctahedral biotite to neoformed clay phases during early-stage weathering [START_REF] Jeong | Mineralogy, chemistry, and formation of oxidized biotite in the weathering profile of granitic rocks[END_REF][START_REF] Cheng | New insight into biotite weathering in the subtropic Tongcheng granite regolith, Hubei Province, South China[END_REF].

Given the low K content of the picrite basalt bedrock, the notably higher K concentrations in the Dali soil imply efficient retention by neoformed clay phases of K released from the basalt through weathering. The increase in K content coupled with an increase of illite in the upper soil profile suggests that K is incorporated in the interlayer of illite due to smectite illitization in response to more advanced weathering and pedogenic processes (Table 2). K enrichment of the topsoil has been widely observed in weathering soil profiles developed on basalts elsewhere (e.g., [START_REF] Nesbitt | Recent chemical weathering of basalts[END_REF][START_REF] Sheldon | Pedogenesis and geochemical alteration of the Picture Gorge subgroup, Columbia River basalt, Oregon[END_REF][START_REF] He | Clay minerals in a soil chronosequence derived from basalt on Hainan Island, China and its implication for pedogenesis[END_REF], a phenomenon that has been attributed to targeted uptake of K in the deeper soil by plants [START_REF] Barré | Dynamic role of ''illite-like'' clay minerals in temperate soils: facts and hypotheses[END_REF][START_REF] He | Clay minerals in a soil chronosequence derived from basalt on Hainan Island, China and its implication for pedogenesis[END_REF][START_REF] Bakker | Mineralogical differences in a temperate cultivated soil arising from different agronomic processes and plant K-uptake[END_REF] or to the addition of K-rich aeolian material [START_REF] Nesbitt | Recent chemical weathering of basalts[END_REF][START_REF] Sheldon | Pedogenesis and geochemical alteration of the Picture Gorge subgroup, Columbia River basalt, Oregon[END_REF]. In the Dali soil profile, the composition of the clay mineral assemblage (i.e., presence of smectite, K/S, I/S, and I/S/K but absence of a mica-like phase) strongly suggests an autochthonous weathering product. However, K-rich aeolian material from the northern Tibetan Plateau is transported via monsoonal winds only as far southward as the zone along the Yangtze River, far from the Dali area in southern China [START_REF] Qiao | Geochemical characteristics of the eolian deposits in southern China, and their implications for provenance and weathering intensity[END_REF][START_REF] Cheng | Geochemical and detrital zircon U-Pb geochronological constraints on provenance of the Xiaomei red earth sediments (Bose Basin, Guangxi Province, southern China)[END_REF]. The relative accumulation of K in the topsoil of the Dali profile may be ascribed to fertilizer in human land use of mainly maize cultivation, as indicated by the notably higher TOC value, the distinctly black color, and the sharp boundary between the underlying soil layer (Table 1; Fig. 2), and also to the recycling uptake of K by plants from deeper soil horizons. These processes result in K-fixing in illitic minerals and facilitate illitization of smectite, consistent with much more illitic clays in the topsoil [START_REF] He | Clay minerals in a soil chronosequence derived from basalt on Hainan Island, China and its implication for pedogenesis[END_REF][START_REF] Bakker | Mineralogical differences in a temperate cultivated soil arising from different agronomic processes and plant K-uptake[END_REF].

Distribution of major elements in the basalt-derived soils

Development of soil during weathering of basalts is generally accompanied by the loss of mobile elements such as Si, Ca, and Mg, and by significant accumulation of immobile elements such as Fe and Al [START_REF] Chesworth | The weathering of basalt and relative mobilities of the major elements at Belbex, France[END_REF][START_REF] Eggleton | Weathering of basalt: changes in rock chemistry and mineralogy[END_REF][START_REF] Markússon | Geothermal surface alteration of basalts, Krýsuvík Iceland-Alteration mineralogy, water chemistry and the effects of acid supply on the alteration process[END_REF][START_REF] Babechuk | Quantifying chemical weathering intensity and trace element release from two contrasting basalt profiles, Deccan Traps, India[END_REF]. Al is regarded as the most immobile element during chemical weathering of volcanic materials [START_REF] Chesworth | The weathering of basalt and relative mobilities of the major elements at Belbex, France[END_REF][START_REF] Pokrovsky | Basalt weathering in Central Siberia under permafrost conditions[END_REF][START_REF] Hong | Facies dependence of the mineralogy and geochemistry of altered volcanic ash beds: An example from Permian-Triassic transition strata in southwestern China[END_REF].

The major element oxides SiO2, MgO, and CaO were gained in the Layer 4 and lost from other soil layers at Dali. TiO2 and TFe2O3 were gained throughout the soil profile except for a few samples showing losses in Layers 2 and 3. MnO, Na2O, and K2O were mostly gained, whereas P2O5 was generally lost during weathering (Fig. 9). The notable accumulation of SiO2 in Layer 4 was likely associated with chemical weathering of altered picrite basalt bedrock, since weathering of fresh basalt is generally characterized by dissolution of silicates and glass as well as the formation of a poorly-crystalline Si-Al product [START_REF] Daux | Geochemical evolution of basaltic rocks subjected to weathering: fate of the major elements, rare earth elements, and thorium[END_REF][START_REF] Dekayir | Mineralogy and geochemistry of supergene alteration of an alkali basalt from the Middle Atlas, Morocco[END_REF].

Kaolinization and illitization of smectite is a desilication process, and the notable decreases in Mg, Ca, and Mn in the upper profile were associated with the transformation of tri-and dioctahedral clay minerals to dioctahedral clay species. The strong depletions of SiO2, CaO, and MgO in Layers 1, 2, and 3 at Dali were related to abundant formation of advanced weathering phases, as the previously formed (i.e., early) weathering products transformed over time to thermodynamically more stable phases such as K/S and I/S clays and well-crystalline Fe-oxyhydroxides (Glasmann and Simonson, 1985;May et al., 1986;[START_REF] Righi | Pedogenic formation of kaolinite-smectite mixed layers in a soil toposequence developed from basaltic parent material in Sardinia (Italy)[END_REF][START_REF] Liivamägi | Paleosols on the Ediacaran basalts of the East European Craton: A unique record of paleoweathering with minimum diagenetic overprint[END_REF].

P2O5 displays a general decrease upwards, with depletion in Layers 2 and 3 that were probably due to biogeochemical P cycling and/or leaching in view of the eluvial condition and high TOC content of the soil [START_REF] Wang | Nitrogen enrichment buffers phosphorus limitation by mobilizing mineral-bound soil phosphorus in grasslands[END_REF].

Al2O3, Fe2O3, and TiO2 show gradual increases upward through the Dali weathering profile (Table 3; Figs. 6 and9 The mineralogical index of alteration (MIA) is used to estimate the transformation ratio of a primary mineral into its equivalent alteration mineral. MIA values of <20, 20-60, and >60 reflect incipient, intermediate, and intense to extreme mineralogical transformations, respectively [START_REF] Voicu | Mineralogical norm calculations applied to tropical weathering profiles[END_REF]. Layer 4 in the lower soil profile has MIA of 8-18, consistent with the absence of K/S clays and the preservation of a certain amount of pumpellyite (Fig. 3). Layers 1, 2, and 3 have notably higher MIA of 3). The moderate weathering intensities indicate that kaolinization did not go to completion, consistent with clay mineral assemblages consisting mainly of K/S, smectite, and minor I/S species [START_REF] Caner | Basalt and rhyo-dacite weathering and soil clay formation under subtropical climate in southern Brazil[END_REF][START_REF] Van Ranst | Halloysite-smectite mixed-layered clay in fluvio-volcanic soils at the southern foot of Mount Kilimanjaro, Tanzania[END_REF]. The presence of plagioclase also indicates relatively limited chemical weathering (Table 3; Fig. 3), allowing preservation of certain labile elements (e.g., Ca, Mg, Na, and K) [START_REF] Berry | Mineralogy-Concepts, Descriptions, Determinations[END_REF][START_REF] He | Clay minerals in a soil chronosequence derived from basalt on Hainan Island, China and its implication for pedogenesis[END_REF]. Although Na is largely retained within the soil profile, its lower layers contain notably more Na2O (3.57-4.84%) than its upper layers

(1.06-2.42%), indicating a degree of Na loss. Na exhibits a gain-or-loss trend that is markedly similar to that of Ca (Fig. 6), indicating that their weathering behaviour is strongly linked. Covariation of Na and Ca reflects a common provenance in plagioclase, a mineral that is relatively well preserved in the lower soil profile where the bedrock has undergone only incipient weathering. Similar upward decreases in Na and Ca suggest their congruent removal due to chemical weathering of plagioclase in the upper soil profile. In particular, all soil layers exhibit the same clay mineral assemblage consisting of smectite and mixed-layer K/S and I/S clays, with K/S and I/S clays increasing and smectite decreasing toward the top of the profile (Table 2; Fig.

4).

Zr and Ti are also considered to be the least mobile elements during weathering of basalts [START_REF] Eggleton | Weathering of basalt: changes in rock chemistry and mineralogy[END_REF][START_REF] Nesbitt | Recent chemical weathering of basalts[END_REF][START_REF] Arslan | Origin and formation of kaolin minerals in saprolite of Tertiary alkaline volcanic rocks, Eastern Pontides, NE Turkey[END_REF]. At Dali, soil samples from Layer 4 to Layer 1 have nearly uniform Zr/TiO2 ratios of 0.008-0.011, indistinguishable from that of the basalt bedrock (0.008) (Table 3). This observation suggests that the weathering profile represents mainly or exclusively in situ weathering products, with little to no addition of allochthonous Fe-and K-rich aeolian materials. However, the soil profile contains notably more K compared to its bedrock (Fig. 9). The formation of pumpellyite and smectite in the bedrock suggests that this igneous rock has experienced a hydrothermal alteration when buried by the overlying Maokou Formation and the Wujiaping Formation [START_REF] Mevel | Occurrence of pumpellyite in hydrothermally altered basalts from the Vema fracture zone (mid-Atlantic ridge)[END_REF][START_REF] Inoue | Pumpellyite and related minerals from hydrothermally altered rocks at the Kalmikta area, northern Honshu, Japan[END_REF][START_REF] Hou | Emei large igneous province: characteristics and origin[END_REF][START_REF] He | Generation and spatial distribution of the Emeishan Large Igneous Province: New evidence from stratigraphic records[END_REF]Mirabella et al., 2005;[START_REF] De Rosa | On the origin and post-depositional history of widespread massive ash deposits: The case of Intermediate Brown Tuffs (IBT) of Lipari (Aeolian Islands, Italy)[END_REF]. The notable more K in the soil profile is probably related to incorporation of K by illite in response to more advanced weathering and pedogenic processes.

The immobile oxides TFe2O3 and TiO2 are markedly enriched in the topsoil at Dali (Fig. 9), consistent with the greater amounts of hematite and anatase in this layer (Fig. 3). The concentration of these immobile components is often attributed to the loss of mobile elements during intense weathering [START_REF] Brantley | Crossing disciplines and scales to understand the critical zone[END_REF][START_REF] Perez-Fodich | Organic acids and high soil CO2 drive intense chemical weathering of Hawaiian basalts: Insights from reactive transport models[END_REF]. However, in iron-rich duricrusts in the tropical regions of Brazil, the concentration of these immobile components may involve the direct role of microorganisms [START_REF] Levett | Microbial weathering signatures in lateritic ferruginous duricrusts[END_REF]. Microbial-induced weathering in iron-rich duricrusts promotes the dissolution of iron, aluminum, and titanium oxide minerals, allowing limited migration at a mineral scale before reprecipitation. Soluble ferrous iron readily reprecipitates in the pore spaces in proximity to microorganisms due to changes in oxidation potential of the solution [START_REF] Levett | Microbial weathering signatures in lateritic ferruginous duricrusts[END_REF]. Aluminum migrated as a soluble cation due to changes in solution pH and/or organic chemistry, resulting in the observed depletion of aluminum and the relative concentrations of titanium and iron in the topsoil [START_REF] Ramanaidou | Genesis of lateritic iron ore from banded iron-formation in the Capanema mine (Minas Gerais, Brazil)[END_REF]. The Dali topsoil has notably higher TiO2/Al2O3 (0.195-0.198) compared to Layers 2, 3, and 4 (0.103-0.132) (Table 3), suggesting that depletion of aluminum may have occurred during the weathering process.

Conclusions

The distribution of authigenic minerals in the Dali basalt-derived soil is largely defined by soil layers related to pedogenesis. Smectite decreases upward from 39-64%

in Layer 4 to 12% in the topsoil, whereas I/S and K/S increase upward from 0-2% to 24% and from 36-59% to 64%, respectively. Clay phases from the saprolite have a mixture of di-and trioctahedral species, whereas those from the middle to upper soil

Fig. 1 .

 1 Fig. 1. Location and geologic setting. (a) Climate map showing mean annual precipitation in China; (b) Regional geological map showing the distribution of the Emeishan Large Igneous Province; (c) Detail of study area.

Fig. 2 .

 2 Fig. 2. Photographs of the Dali weathering soil profile developed on picrite basalt. (a) A full view of the soil profile. (b), (c), (d), (e), and (f) are close-up photographs showing Layers 1, 2, 3, 4, and the bedrock, respectively. Scale provided by 13-cm-long marker.

Fig. 3 .

 3 Fig. 3. XRD patterns of representative samples of the Dali soil profile. (a) Whole-rock samples; (b) (060) reflections of XRD patterns of clay fractions.

Fig. 4 .

 4 Fig. 4. Fitted XRD patterns of glycolated clay fractions showing the stacking structures of mixed-layer clay species and their relative contents. Abbreviations: K/S: mixed-layer kaolinite/smectite; I/S: mixed-layer illite/smectite; S: smectite; Gt: goethite; di: dioctahedral; tri: trioctahedral.

Fig. 7 .

 7 Fig. 7. SEM images of the Dali samples. (a) Poorly-developed pseudo-hexagonal and plate-like grains indicating presence of K/S and I/S (SiO2/Al2O3 value of 1.74) clays in Layer 1 (DYX-01); (b) Relatively well-crystalline K/S (SiO2/Al2O3 value of 1.23) and I/S clay minerals in Layer 2 (DYX-05); (c) K/S clays (SiO2/Al2O3 value of 1.25) with curled edges indicating relatively weak kaolinization in the Layer 3 (DYX-09); (d) K/S and I/S (SiO2/Al2O3 value of 1.65) clays with ragged outlines confined by basalt grains showing

Fig. 8 .

 8 Fig. 8. HRTEM observations showing the morphology and interstratified lattice fringes of clay minerals of the Dali soil. (a) Wave-shaped morphology of K/S and I/S (DYX-13, Layer 4); (b) An elongate and relatively straight morphology indicative of strong illitization of I/S (DYX-13, Layer 4); (c) Euhedral pseudo-hexagonal outlines showing strong kaolinization of K/S (DYX-5, Layer 2); (d) I/S clay with interstratified 12-, 10-, and 7-Å lattice fringes (DYX-5, Layer 2); (e) K/S clay showing interstratification of 12-Å with 7-Å layer (DYX-13, Layer 4); (f) Interstratified 12-, 10-, and 7-Å lattice fringes characteristic of mixed smectite, illite, and kaolinite layers (I/S/K) (DYX-13, Layer 4).

Fig. 9 .

 9 Fig. 9. Gain or loss of major elements through the Dali soil profile.

  ). The bedrock has a CIA value of 32, and the soil profile shows a progressive change in CIA from 54 in Layer 4 to 85 in the Layer 2, indicating a general upward increase in weathering intensity. However, the gains or losses of Fe2O3, MnO, and TiO2 relative to Al2O3 are inconsistent with these CIA values. Fe2O3, MnO, and TiO2 are markedly enriched in Layer 1 and Layer 4 but relatively depleted in Layers 2 and 3 compared to Al2O3. This indicates that, although chemical weathering caused loss of mobile components and gain of immobile components, the fluxes of Fe2O3, MnO, and TiO2 and their sites of concentration or redistribution differed from those of Al2O3.

  35 to 71, although only a few values are >60, corresponding to greater amounts of K/S in those samples. This MIA pattern indicates a general upward increase in weathering intensity and an intermediate degree of mineralogical alteration for the Dali soil profile as a whole. Similarly to the MIA index, the Dali soil profile exhibits dominantly incipient (CIA = 50-60) to intermediate (CIA = 60-80) chemical weathering intensities, with only a few samples in Layers 2 and 3 showing intense weathering (CIA > 80) (Table

  profile have a uniformly dioctahedral structure. I/S clays are characterized by interstratification of 12-Å smectite with 10-Å illite layers, whereas K/S clays exhibit intercalation of 12-Å smectite and 7-Å kaolinite layers. The intercalation of 12-, 10-, and 7-Å clay layers represents mixed-layer I/S/K clays. The significantly greater abundance of K/S clay relative to I/S clay in the saprolite implies that kaolinization of smectite took place at an early stage after smectite formation, whereas illitization of smectite occurred during a later stage characterized by increased leaching. However, the processes of kaolinization and illitization of smectite overlapped during the developmental history of the soil. Desilication and K-fixation of smectite occurred with advanced weathering, but the absence of discrete kaolinite and illite and the lack of detectable gibbsite in the soils indicate that the Dali soils formed in a weathering environment of only moderate intensity. Both the immobile oxides (TFe2O3 and TiO2) and a mobile component (K2O) are markedly concentrated in the topsoil. The concentration of K in the weathering profile was potentially related to incorporation of K in the interlayer of illite due to smectite illitization in response to more advanced weathering and pedogenic processes, and the relative accumulation of K in the topsoil probably results from fertilizer in land use and recycling of K by plants.

Table 1 .

 1 Some selected morphological, physical and chemical attributes, and mineral compositions of the soil developed from Dali picrite basalt weathering

	Soil layer	Sample	Color	Texture	Structure	pH	TOC (wt%)	CEC (cmol(+)/kg)	Clays	Pl	Mineral content (wt%) Qtz Ant Cln	Hm Pum
	Layer 1	DYX-01	5 YR5/1	Sandy clay	Crumb	4.9	1.64	24.3	50	17	7	3	2	16	5
	Layer 2	DYX-05	5 YR5/6	Silty clay	Crumb	5.7	1.07	54.7	65	18	2	2	3	10	/
	Layer 3	DYX-11	7.5 YR8/1	Clay	Blocky	6.0	0.59	34.5	57	19	5	2	6	11	/
	Layer 4	DYX-16	7.5 YR6/2	Clayey gravel	Blocky	6.8	0.83	41.1	54	23	2	2	5	14	/
	Bedrock	DYX-18	Grayish green	/	/	/	/	/	13	2	24	2	7	8	44

Notes: Pl-plagioclase; Qtz-quartz; Cln-clinopyroxene; Pum-pumpellyite; Hm-hematite; Ant-anatase; "/"-not measured or not detected.

Table 2

 2

	Soil layer	Sample	Smectite	I/S (Illite layer content)	K/S (1) (Kaolinite layer content)	K/S (2) (Kaolinite layer content)	Total K/S
	Layer 1	DYX-01	12	24 (79%)	37 (75%)	27 (87%)	64
	Layer 2	DYX-03 DYX-05	16 9	17 (33%) 16 (71%)	42 (86%) 49 (82%)	25 (89%) 26 (89%)	67 75
	Layer 3	DYX-09 DYX-11	15 41	3 (90%) 11 (85%)	53 (63%) 28 (87%)	29 (90%) 20 (92%)	82 48
		DYX-13	39 (28 di+11 tri)	2 (93%)	43 (69%)	16 (87%)	59
	Layer 4	DYX-15	55 (33 di+22 tri)	/	30 (78%)	15 (84%)	45
		DYX-17	64 (22 di+42 tri)	/	23 (75%)	13 (77%)	36
	Notes: I/S: mixed-layer illite/smectite; K/S: mixed-layer kaolinite/smectite, (1) relatively weak
	kaolinization and (2) relatively strong kaolinization; "/": undetected; di: dioctahedral; tri:
	trioctahedral.

. Clay mineral compositions of representative soil samples from the Dali picrite basalt weathering profile (wt%)

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Projects 41972040, 42172045, and 42002042), the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUG170106), and the Postdoctoral Science Foundation of China (2020M672440).

The authors also wish to thank Dr. Fan Li for assistance in sample collection, and especially Dr. Fabio Scarciglia, the handling editor, and two anonymous reviewers for their insightful reviews, valuable comments, and suggestions.


Gaëlic Béchu

Antoine Beugnard

Caroline G L Cao

Quentin Perez

Christelle Urtado

Sylvain Vauttier

A software engineering point of view on digital twin architecture

Keywords: Digital Twin, Model-Driven Engineering, Architecture Deployment

Digital twins, along with Internet of Things and Artificial Intelligence, have been identified as one of the key technologies for Industry 4.0. However, the definition of Digital Twin (DT) is still abstract and context-dependent. In this paper, we present a metamodel that supports concrete and operational descriptions of digital twin deployment. This metamodel encompasses the different aspects of deployment, including the definition of hardware and software components that compose the layered cyber-physical architectures of the digital twin, along with the installation and instantiation tasks that compose deployment processes. Multiple configurations can also be defined to support the deployment of a digital twin in different execution contexts. The relevance of this metamodel was evaluated by two case studies. The first consists of deploying the digital twin of a cobot in a simulation environment. The second applies the approach in a home automation environment. In both cases, our metamodel provides complete and precise descriptions of the deployment process and thus constitutes a viable first step towards a modeldriven approach for digital twin deployment.

I. INTRODUCTION

Digital twins, along with Internet of Things and Artificial Intelligence, have been identified as one of the key technologies for Industry 4.0. However, as for many emerging concepts, the definition of Digital Twin is still abstract and context-dependent. We present in this paper a metamodel that supports concrete and operational descriptions for digital twin deployment. The remainder of this paper is organized as follows. Section II details the metamodel we proposed to handle the deployment of digital twins in a model-driven approach. Section III presents an evaluation of the completeness and adaption of our metamodel in two case studies. Section V discusses these preliminary results and provides perspectives about this on-going work.

II. A METAMODEL FOR MODEL-DRIVEN DEPLOYMENT OF DIGITAL TWINS

If there is no consensus on the definition of Digital Twins, there is even less agreement on their software architecture. The ISO23247 standard [START_REF]2021 Automation systems and integration -Digital twin framework for manufacturing -Part 2: Reference architecture[END_REF] provides a global architecture where "boxes" communicate through connecting "lines". These boxes are:

Devices

External Tools

Core Twin Data & History

Management Tools status Fig. 1: Digital Twin macro-architecture

III. MODELING THE DEPLOYMENT OF DIGITAL TWINS IN TWO INTELLIGENT ENVIRONMENTS

A. Case Study #1: Human-Robot Interaction

The first deployment case study focuses on the collaboration between a robotic arm and its operator while they are performing a collaborative task. This case study mimics the environment of a worker in a manufacturing plant and was created 1 Configuration data splits into 2 categories: technical and model. Model data must be located in the Data & History component, while technical must be in the Core Twin. Technical data collect logins, IP addresses, etc. 2 For example, a Unix symbolic link, or some kind of readme file at least. with the purpose of joining through a DT a human operator and a robotic arm. The digitalization of this interaction is done with Unity3 which is the simulator that best suits our needs. The robotic arm at our disposal is controlled on ROS 4 . We could have remained with a ROS-friendly simulator such as Gazebo, but while being adequate for robotics applications, Gazebo is not ideal for the digitalization of anything non-robotic [START_REF] De Melo | Analysis and comparison of robotics 3d simulators[END_REF]. We created the DT of the Human using gyroscopes placed on different body parts of our operator. We streamed this data between different computers to have a DT of our human operator in Unity. The factor to consider here is the communication between the various hardware and software tools. These tools were designed to work on their own for different purposes and we had to build a generalized architecture that integrated different communication means (Radio, TCP/IP, USB). We built a Composite Structure Diagram (CSD) according to the ontology proposed in Section II. As shown in Figure 4, the different hardware components support different applications and software tools. We can see the communication between the hardware components as well as the structure of the DT. The Human-Robot Interaction DT can be seen as the combination of two different DTs: the Human DT and the Robot DT.

B. Case Study #2: Home Automation System

The second case study concerns a Home Automation System (HAS). The HAS is composed of two major elements: hardware (sensors, actuators, computers, routers, etc.) and software (home automation software, operating systems, etc.) In this case, we mix real (smart light bulb, computers, router, etc.) and simulated hardware. We built a predictor of a bed pressure sensor which indicates whether a person is in bed or not. To do so, we trained a forecasting model (naive forecaster) to learn data patterns with Sktime. Data used for learning come from the dataset of Chimamiwa et al. [START_REF] Chimamiwa | Multi-sensor dataset of human activities in a smart home environment[END_REF]. We then integrated this simulated sensor in the HAS. Next, we built a Composite Structure Diagram of the HAS using our proposed ontology described in Section II. As shown in Figure 5 we are able to take into consideration all the hardware and software composing the HAS. Moreover, the ontology allows the composition of Cyber-Physical System (CPS). In Figure 5, it is the case of HomeAutomationSystem and BedPressureSystem. HomeAutomationSystem is the CPS composed of a Raspberry Pi 4 and the dedicated OS installed on it. Other components such as the zWaveDongle are also plugged in. On the Raspberry Pi OS, the home automation application (i.e., OpenHAB5 ) is installed. The ontology also allows the modeling of components and technologies to communicate between devices and the CPS. The Z-Wave protocol is used for the interaction between the light bulb and the OpenHAB software. Another example in Figure 5 is the WiFi communication between the two CPSs through the router.

A prospective work is to use this approach to model and manage DTs for the connected apartment of the HUman at home projecT (HUT)6 ) and use them as a workbench for the development and training of AI-based mechanisms.

IV. RELATED WORK

For the sake of space, we reference a survey from Minerva et al. [START_REF] Minerva | Digital twin in the iot context: A survey on technical features, scenarios, and architectural models[END_REF] which draws an interesting picture of technical features of DTs and shows that most software architectures published are tailored to specific goals. Taking a different approach, our proposal provides a generic, model-driven solution for constructing and deploying DT architectures.

V. DISCUSSION AND CONCLUSION

We have performed a preliminary validation of our metamodel defined in Section II with two different use cases. The first one is a human-robot interaction (HRI) system. The second is We are continually improving our models and verifying them with practical experiments. We aim to refine the definition of " digital twin ", initially by defining different categories according to different configurations. This categorization will allow us to identify the different shortcomings of these configurations.

Our focus on HRI digital twin brings multiple perspectives. We strive to understand the methodology needed to create a digital twin, while at the same time to create a dataset containing our HRI interactions. To date, many HRI datasets exist but are mainly focused on behavior analysis and not from a manufacturing point of view7 . Our approach allows us to contribute to a comprehensive database for constructing digital twins for the future smart factories of Industry 5.0 [START_REF] Romero | Towards the resilient operator 5.0: The future of work in smart resilient manufacturing systems[END_REF]. Our second use case of a smart environment system showcases the general applicability of our approach. Daily activity datasets are also scarce and difficult to collect. Building digital twins could be a way to leverage the data collected in the HUT project after the dismantling of its smart apartment.

Fig. 3 :

 3 Fig. 3: A DT ontology (excerpt)

Fig. 4 :

 4 Fig. 4: CSD of a Human Robot Interaction collaborative task using the DT ontology

Fig. 5 :

 5 Fig. 5: CSD of the Home Automation System using the DT ontology.

  and shared) and that usually requires configurations that are part of either the Core or the Tools boxes. The Core Twin box contains everything (hardware description and software) defining the STS. It includes sensors, actuators and drivers. It also contains all brokers (scripts, software) ensuring the connection between all parts (software and data) including status extraction and data historization.

					HEAD
	birth	origin/master	ext1 ext1		master
	Birth	AddS2	AddS3	Conf1	Conf2
			origin/var1	AddS4	ext2
	Fig. 2: A DT configuration: its birth, add some parts, two
	configurations and two variants (branches)
	be used (
					This standard is not a software architecture; it is a refer-
					ence architecture. It includes real "things" such as devices,
					humans, and applications. The software part of DTs (yet
					to be defined) is complex. Among all software, DTs may
					use OSs, drivers, controllers of hardware, databases, data
					conversion, data storage, data analysis applications, AI tools,
					3D engines, etc. Hence, defining a software architecture is
					context-dependent. This paper defines a synthetic architecture
					as a simple organization of software and data streams.
					Figure 1 illustrates the adopted organization of our DTs. The
					overall architecture is composed of four main packages. The
					content is digital (i.e., files such as electronic documents,
					spreadsheets, software, scripts, text files, etc.)
					The Devices box denotes the sociotechnical system (STS)
					itself, including humans, which are not part of the DT.
					The External Tools box refers to the external software that can

• User Entity including Human, Human-Machine Interaction, Manufacturing Execution Systems, Enterprise Resource Planning and other DTs, • Core Entity including Management Services and Digital Representation of Observable Manufacturing Elements, • Data Collection and Device Control Entities that gather elements to connect actual devices, • Cross System Entity including all communication services. 1 The Data & History box contains the status of the DT and all historical (biographical) data. It includes 3D-models, sensors values, actuators commands, etc. If, for some reason (software installation/configuration constraint), data have to be located elsewhere, a link is explicitly made 2 . The Tools box contains software such as simulation, analyzing or learning tools. It reads information from Data & History and feeds it with new information (statistics, configurations).

Dependency among tools and data have to be explicit, as in a configuration management tool such as git (see Figure

2

). The Management box contains software, scripts and data used to manage the DT. It includes introspection services (request), GUIs, controllers, etc. Beyond this organization, we define an ontology that gathers all needed concepts to describe, implement and manage a DT. An excerpt is shown in Figure

3

. For the sake of readability, we used colors to refer to the architecture of Figure

1

. The green part (Devices) is a hierarchy of RealWorldThing including Hardware and LivingBeing. The red part represents the core digital twin. The blue part describes management with Services and ConfigurationScript. The purple part denotes Data produced and consumed by Software. The white part represents Tools, whereas the yellow one denotes abstractions that organize concepts.

https://unity.com/

http://wiki.ros.org

https://www.openhab.org/

https://www.hut-occitanie.eu/

https://github.com/mjyc/awesome-hri-datasets

ACKNOWLEDGEMENTS

This work is funded in part by the CARNOT MINES and TSN Institutes, the Chair of Industry of the Future funded by the Région de Bretagne and University of South Australia, as well as GT VOCA of the HUman at home projecT (HUT) funded by FEDER and the Région Occitanie. We thank Panagiotis Papadakis, Christophe Lohr and Alireza Asvadi for their expertise.