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Home , Chongyi County, Laterite, Regolith

Geoderma 337 (2019) 225–237

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Geoderma

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Surficial of kaolin in a subtropical : Implications for supergene and argillization T ⁎ Qian Fanga,b, Hanlie Honga,c,d, , Harald Furnese, Jon Choroverb, Qing Luof, Lulu Zhaoa,b, ⁎⁎ Thomas J. Algeoc,d,g, a School of Sciences, University of Geosciences, Wuhan 430074, China b Department of , Water and Environmental Science, University of Arizona, Tucson, AZ 85721, USA c State Key Laboratory of Geological Processes and Resources, China University of Geosciences, Wuhan 430074, China d State Key Laboratory of Geobiology and Environmental Geology, China University of Geosciences, Wuhan 430074, China e Department of Earth Science, Centre for Geobiology, University of Bergen, Allegt. 41, 5007 Bergen, Norway f Center for Exploration of Earth Resources of Province, 330000, China g Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA

ARTICLE INFO ABSTRACT

Handling Editor: M. Vepraskas Regolith, or weathered material overlying bedrock, develops through pedogenic processes such as - “ ” Keywords: mineral formation ( argillization ). Research on products of argillization, such as kaolin, is commonly focused Regolith on its economic value rather than on an integrated understanding of the pedological, mineralogic, and geo- Chemical weathering chemical processes taking place in the regolith. Here, we analyzed three kaolin-regolith drillholes from a sub- Chemical gradient tropical climate zone in southern China using X-ray diffraction (XRD), major and trace element analyses, H and O isotopes, differential thermal-thermogravimetric analysis (DTA-TG), and scanning electron microscopy (SEM). Many kaolinite aggregates within regolith have fan-shaped stacks, a morphology that is closely associated with transformation of muscovite plates. Hypogene indicators such as a shallow level of emplacement, re- presentative structural controls on kaolinization, mineral zoning, and hypothermal cannot be observed.

Mineral assemblages and morphologies, elemental binary plots (e.g., Zr vs. TiO2 and P2O5 vs. SO3), as well as chemical profiles all show characteristics typical of surfical weathering. The δ18O and δ2H values of the clay fractions range from +16.8 to +18.7‰ and from −70 to −51‰, respectively, suggesting that supergene weathering has played a key role in forming the kaolin regolith. Dominance of kaolinite over implies a near-surface, freely draining environment. Adopting the underlying weakly altered granite (saprolith) as the , the kaolin regolith exhibits four elemental profile patterns as revealed by mass transfer coef- ficients: (1) depletion (e.g., Na), (2) depletion-enrichment (e.g., Al), (3) enrichment (e.g., Mn), and (4) biogenic (e.g., Ca). These profiles reflect a combination of chemical, geologic, and biologic processes that are typical of relatively thin, in situ regolith profiles, and that is not necessarily similar to those typically associated with deep (thick) granite weathering profiles. We propose that supergene kaolin regolith is intrinsically more similar to shallow, biologically active residual soil deposits, rather than deeply weathered granite-hosted .

1. Introduction to many of the resources that maintain our society and living standards. It is also in the upper part of the regolith that our are formed; the Regolith is defined as in situ weathered material overlying bedrock, regolith is thus the host for and agriculture (Taylor and Eggleton, and it is present to varying degrees across the Earth (Wilford et al., 2001). The distribution and nature of regolith are controlled by various 2016). It represents the entire unconsolidated and secondarily re- factors, reflecting long-term interactions among the , cemented cover that overlies relatively intact bedrock, and it is formed , lithosphere, biosphere and hydrosphere (Brantley and by , transport/deposition, and weathering of pre-existing ma- White, 2009; Buss et al., 2017). Regolith develops through a variety of terial (Anand and Paine, 2002; Keeling et al., 2003). The regolith is host processes such as argillization, i.e., the formation of clay minerals in the

⁎ Correspondence to: H. Hong, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China. ⁎⁎ Correspondence to: T.J. Algeo, Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA E-mail addresses: [email protected] (H. Hong), [email protected] (T.J. Algeo). https://doi.org/10.1016/j.geoderma.2018.09.020 Received 15 July 2018; Received in revised form 5 September 2018; Accepted 9 September 2018 Available online 21 September 2018 0016-7061/ © 2018 Elsevier B.V. All rights reserved. Q. Fang et al. Geoderma 337 (2019) 225–237 soil environment (Gilkes et al., 1973), and clay minerals commonly southeast (~500 masl), with rivers thus flowing in a southeasterly di- provide insights into the nature of these processes (Dill, 2017; Lopez rection. The study area is within a subtropical monsoonal climate zone et al., 2018). that is characterized by large seasonal variations in air temperature, One type of argillization process is kaolinization, which generates wind direction, and rainfall, with > 80% of precipitation falling during the mineral kaolin (Ekosse, 2001; Anand and Paine, 2002; Dill, 2017). the period from May to September. Mean annual temperature (MAT) is Kaolin deposits can be formed in situ by hydrothermal processes during 17.6 °C, and mean annual precipitation (MAP) is 1600 mm. The late-stage pluton cooling (hypogene), by (near-)surface weathering re- Chongyi kaolin regolith is located within the Nanling metallogenic belt, actions (supergene), or by a combination of these processes (Dudoignon which was generated during the Jurassic- (~200–134 Ma) et al., 1988; Murray, 1988; Dill et al., 1997; Galán et al., 2016). Hy- Yanshanian magmatic event. The Yanshanian event resulted in NE–SW- pogene processes play a prominent role in alteration of granitic rocks trending faults and associated pluton emplacement in the study area intruded at relatively shallow depths, and it is commonly associated (Zhou et al., 2006; Wang et al., 2018). The bedrock of the kaolin re- with high acidity and high temperatures (Marfil et al., 2005). In su- golith is a muscovite granite of early Yanshanian age (~200–170 Ma), pergene deposits, chemical weathering is the dominant process of for- and the original argillization process may have been driven by mag- mation of kaolin minerals (e.g., kaolinite, halloysite, and dickite) from a matic heat. variety of parent rocks (e.g., granite, , and slate; Chorover and According to layering of typical regolith profiles by Taylor and Sposito, 1995; Scott and Bristow, 2002; Keeling, 2015; Hong et al., Eggleton (2001) and Anand and Paine (2002), the Chongyi regolith can 2016). With time, regolith composition diverges progressively from that be divided into a lower saprolith layer and an upper pedolith layer of the parent material owing to the cumulative influences of vegetation, (Fig. 2): (1) the saprolith contains saprorock below and above; biota, topography, and, in particular, climate (Thanachit et al., 2006; and (2) the pedolith contains a plasmic zone, a mottled zone, and an Zhao et al., 2017). In the present study, “kaolin regolith” is defined as a overlying surface layer. The regolith can be further vertically sub- residual kaolin deposit that has undergone substantial chemical divided into four units based on differences in color and lithology leaching and physical weathering processes such as regolith creep/flow (Fig. 2; listed from top to bottom): the surface layer, the mottled kaolin and erosion by water (Taylor and Eggleton, 2001). layer, the homogenous kaolin layer, and the saprolith. Kaolin genesis has a direct relation to its industrial applications, with supergene kaolins generally being more concentrated and having higher economic value than hypogene ones (Ekosse, 2001; Njoya et al., 2.2. Field sampling 2006; Fernandez-Caliani et al., 2010). As a raw material for chinaware, kaolin has substantial economic value, but its economic significance is Geological field work included mapping in outcrop and description broader in that kaolin is found in many products for daily use, including and sampling of drillcore and drill-cuttings. In each of three drillcores, paper coating, ceramics, plastics, rubber, cosmetics, and pharmaceu- five samples were collected: one in each regolith layer (surface layer, ticals (Zhou and Keeling, 2013; Lu et al., 2016; Pruett, 2016). Relatively mottled kaolin, homogenous kaolin, and saprolith) with a second systematic work on kaolin argillization has been undertaken in Aus- sample taken in the homogenous kaolin layer (the two samples of the tralia (Gilkes et al., 1973; Anand and Paine, 2002), USA (Schroeder homogeneous layer are designated “upper kaolin” and “lower kaolin”). et al., 2004), and Europe (Galán et al., 2016). China represents one of This yielded a total of 15 samples for geochemical analyses (Fig. 2). the most important producers of kaolin (especially for ceramics, sani- Clay fractions (< 2 μm) were extracted from 12 samples (all but the tary ware and porcelain; Wilson, 2004). Indeed, the first use of kaolin in three saprolith samples) by conventional sedimentation and cen- pottery manufacture emerged in , Jiangxi Province, China trifugation methods. The < 45 μm fractions were extracted from three over 2000 years ago (Schroeder and Erickson, 2014), and the first kaolin samples (i.e., CY 1–3, CY 2–3, and CY 3–3). The grain-size mining site for gaoling tu (kaolin clay) was the Gaoling area, located fraction of 63–600 μm was used for analysis of heavy minerals. 50 km northeast of Jingdezhen. Despite its cultural importance, only limited research concerning the formation of kaolin regolith particu- larly related surficial weathering process has been conducted in China. 2.3. X-ray diffraction (XRD) A detailed study of kaolin deposits in China has the potential to improve our understanding of mineral alteration and elemental mi- The samples of bulk kaolin (including mottled kaolin and homo- gration during supergene processes and kaolin accumulation world- geneous kaolin), fine fractions (< 45 μm and < 2 μm), and heavy mi- wide. The Chongyi kaolin regolith in Jiangxi Province, southern China, nerals were mineralogically analyzed by XRD. These samples were is located in a subtropical climate zone with warm temperatures and pretreated to remove organic matter with 30% H2O2 at 60 °C. Air-dried high rainfall, contributing to its intensely weathered condition. Here, oriented clay mounts for the < 2 μm fractions were prepared by dis- we investigate the Chongyi kaolin regolith using an integrated ap- persing the clay slurry onto glass slides. XRD patterns were recorded proach based on XRD, SEM, XRF, ICP-MS, DTA-TG, and H and O iso- using a D/max-3B X-ray diffractometer with Cu Kα radiation at 40 kV, topic analyses. The goals of this study were to: (1) systematically 35 mA (resolution ratio = 0.02°2θ; scan rate = 4° 2θ/min; scan characterize the kaolin regolith using a variety of mineralogic and range = 3–65°2θ). The formamide test (Churchman et al., 1984) was geochemical techniques; (2) gain insights into the geochemical beha- carried out for the < 2 μm fractions to distinguish halloysite from vior of major and trace elements during supergene kaolin argillization; kaolinite. Bulk- mineral contents were determined on randomly (3) determine the genetic relationships between kaolin regolith and the oriented using reference intensity ratio(RIR) together with the typical residual soil deposit; and (4) propose a model for kaolin argil- so-called 100% approach (Hillier, 2000). RIR is also known as I/Icor, lization in Earth's surface environments. which represents the ratio of intensities (areas) of specific XRD peaks (mineral: corundum = 1:1). The term 100% approach means that the 2. Materials and methods sum of all mineral phases identified is 100%. Five minerals (muscovite, , , kaolinite, and ) that were involved in content 2.1. Study location and climatic setting calculation have RIR values (refer to corundum 113 peak) of 0.23 (~10.0 Å), 2.07 (~3.20 Å), 0.89 (~4.25 Å), 0.53 (~3.57 Å), 1.91 The Chongyi kaolin regolith is located in Chongyi County, south- (~4.85 Å), respectively. Considering this is a semi-quantitative method western Jiangxi Province, southern China (Fig. 1). The landscape con- with a calculation error of 5–10%, we used ‘—’ (not detected), ‘*’ sists of low mountains and hilly terrane. Within Chongyi County, ele- (< 15%), ‘**’ (15–40%), and ‘***’ (> 40%) to describe different mi- vations fall from northwest (~1000 meters above sea level - masl) to neral contents in order to present more reasonable data (Table S1).

226 Q. Fang et al. Geoderma 337 (2019) 225–237

Fig. 1. Location and regional geology of Chongyi kaolin. The study area is located in Jiangxi Province, southern China. The derived from Yanshanian magmatic event are widely distributed in this area. The study section is hosted in albitization muscovite granite, located in Nanling metallogenic belt.

Fig. 2. Simplified kaolin profiles and the photo of the studied kaolin. The study samples are collected from three drill cores in three representative kaolin-regolith drillholes (CY1, CY2, and CY3), and the detailed sampling positions are marked beside the kaolin profiles.

2.4. X-ray fluorescence (XRF) Agilent ICP-MS, with an analytical precision of better than 4% for REEs and 4–10% for other trace elements. Prior to ICP-MS analysis, each All collected samples were analyzed for major-, trace-, and rare- sample was leached for ~20 min in 1-N HCl, after which the residue earth element (REE) concentrations. Major-element concentrations and leachate were separated by continuous centrifugation, and the re- were measured by XRF spectrometry using a Shimadzu XRF sequential sidue was rinsed with milli-Q water. spectrometer. The analytical precision for major-element concentra- tions was < 1% and the detection limit was 0.01 wt%. 2.6. H and O isotopic analyses

2.5. Inductively coupled plasma-mass spectrometry (ICP-MS) The < 2 μm fractions of nine samples (i.e., the six homogenous kaolin plus three mottled kaolin samples) were analyzed for H and O Trace-element and REE concentrations were determined using an isotope compositions. For analysis of δ18O values, oxygen in the clay

227 Q. Fang et al. Geoderma 337 (2019) 225–237

fractions was extracted through reaction with BrF5 at 600 °C in a chamber for ~24 h and then converted to CO2 over a graphite furnace (Clauer et al., 2015). For H isotope analysis, the clay fractions were heated at 1500 °C, and the released water was converted to H2 at

900 °C. The converted CO2 and H2 were measured for oxygen and hy- drogen isotopes, respectively, using a Finnigan-MAT253 mass spectro- meter. Isotopic ratios were reported by δ notation as deviations in per mille versus the international Vienna standard mean ocean water (VSMOW) standard, with reproducibility of ± 0.5‰ for δ18O and ± 1‰ for δ2H values.

2.7. Differential thermal and thermogravimetric analysis (DTA-TG)

Differential thermal-thermogravimetric analysis (DTA-TG) was performed using a Setaram Labsys thermal analyzer, operating under a nitrogen flow of 60 ml/min. Both the bulk kaolin and < 45 μm kaolin samples were selected for DTA-TG analysis. The DTA-TG curves were obtained using 10 mg of powdered sample, heated from room tem- − perature to 1400 °C at rates of 10, 20, 30, or 40 °C min 1. Aluminum was used as a reference material.

2.8. Scanning electronic microscopy (SEM)

Both clay fractions and heavy minerals were selected for SEM analysis. These heavy minerals were hand-picked under binocular mi- croscope after heavy-liquid (solution of lithium heteropolytungstates in water) and magnetic separation of 63–600-μm fractions. An EDS- equipped FEG-SEM (HITACHI-SU8010) was used to observe the mi- cromorphology of the clay minerals and heavy minerals of re- presentative samples and analyze their chemical compositions. These Fig. 3. Representative X-ray diffraction patterns of (A) bulk kaolin, (B) finer samples were sputter-coated with thin films of platinum prior to ana- fractions (< 45 μm), (C) clay fractions (< 2 μm), and (D) heavy minerals. lysis. The SEM instrument was operated at a voltage of 10–15 kV, a current of 3–5 nA, and a 15 mm working distance, with observations performed in secondary electron mode. (mean 66.6%) and low CaO (mean 0.09%) and MnO (mean 0.10%). SiO2 (45.7–75.0%) and Al2O3 (15.0–38.4%) are the major constituents fi μ 3. Results of both the bulk kaolin samples and ner fractions (< 45 m). Fe2O3 varies from 0.63 to 2.89%, K2O varies from 1.83 to 4.81%, and the 3.1. Mineralogic characteristics remaining major-element show concentrations of < 1%. The finer fractions (< 45 μm) in all three sections show increases in Al2O3 The mineralogic compositions of representative samples are shown (up to 38.4%) and decreases in SiO2 (mean 46.2%) compared to the in Fig. 3, and data for all samples (including bulk kaolin, < 45 μm bulk kaolin samples (Table S2). The major element concentrations of fraction, and < 2 μm fraction) are given in Table S1. The mineralogic the samples correspond well to the mineralogic compositions de- composition varies with particle size: the clay (< 2 μm) fractions are termined by XRD. dominated by kaolinite with subordinate amounts of muscovite, gibb- Trace elements such as Zn, Zr, and Ba exhibit high concentrations – site, and quartz, whereas bulk kaolin samples contain larger amounts of (~100 3600 ppm), V, Ga, Sr, Nb, Ta, and Pb have intermediate con- – quartz than kaolinite. Kaolinite is identified by diagnostic narrow peaks centrations (~10 100 ppm), and Sc, Y, Sn, Th, and U show low con- at 7.16 Å and 3.58 Å. The non-basal sharp and less intense reflections of centrations (< 10 ppm). The UCC-normalized trace-element and REE doublets at 4.48/4.38 Å and 2.57/2.50 Å and triplets at 2.38/2.34/ distributions of the kaolin samples show similar features for all samples 2.30 Å are linked to well-crystallized kaolinite (Fig. 3B, C; Erkoyun and (Fig. 4), including depletions of many elements (e.g., Y, Zr, Th, and Kadir, 2011). Kaolinite is prevalent in the fine fractions (< 45 μm medium-REE Dy) that are usually considered to be resistant to re- and < 2 μm), in which quartz content is significantly lower. mobilization during weathering, enrichments of some relatively mobile Comparing the XRD patterns of the air-dried oriented clay mounts elements (e.g., Ba and Ga), and negative Ce anomalies. These features with the formamide-treated mounts, there is no change in intensity of should be mainly inherited from parent granite rocks, rather than as a the 7 Å/10 Å peaks after formamide saturation, suggesting that negli- result of chemical weathering, because the distribution patterns are gible halloysite is present in the Chongyi kaolin, even though this mi- similar for weakly weathered saprolith and strongly weathered kaolin neral is detectable by SEM (Churchman et al., 1984). Muscovite in- samples. These features are typical of trace-element and REE distribu- herited from the parent granite has concentrations of < 15% in the bulk tions of Mesozoic granitoids in South China (Zhou et al., 2006). Actu- kaolin samples (Table S1). A ~4.86 Å peak is observed in the XRD ally it has been widely suggested that chemical weathering can often patterns of bulk and fine-fraction samples, suggesting the presence of exert a limited influence on REE distributions of weathered profiles gibbsite (Fig. 3). (e.g., Galán et al., 2016; Hong et al., 2016).

3.2. Major- and trace-element compositions 3.3. H and O isotopic compositions

The major-element concentrations of the analyzed samples are Before evaluating the H and O isotopic results, it is necessary to given in Table S2. The three study sections are similar in their overall estimate the contributions of impurity minerals to the measured iso- chemical compositions. The saprolith is characterized by high SiO2 topic data. As pointed out by Savin and Lee (1988), the isotope

228 Q. Fang et al. Geoderma 337 (2019) 225–237

Fig. 4. Upper continental crust (UCC) - normalized trace element diagrams for mottled kaolin and surface layer (A), bulk homogeneous kaolin and < 45 μm homogeneous kaolin fractions (B), and saprolith (C). UCC-normalized REE distributions for mottled kaolin and surface layer (D), bulk kaolin and < 45 μm kaolin fractions (E), and saprolith (F). fractionation factors of weathering-formed (secondary) muscovite and Supergene kaolin samples of Ramon-Fazouro (Clauer et al., 2015) and gibbsite are similar to and slightly smaller than those of kaolinite, re- Chubut/San Cruz (Cravero et al., 2001) and hypogene samples of Seiliz spectively. However, at least a small part of the muscovite in our and Kemmliz (Gilg et al., 2003) are plotted for comparison (Fig. 5). samples may be residual primary muscovite from the parent granite, which formed at high crystallization temperatures and has an unknown isotopic composition. As noted before, the halloysite content is negli- 3.4. SEM observations gible and should have contributed little to the measured isotopic data. An empirical approach by Clauer et al. (2015) suggests that the po- 3.4.1. Kaolin minerals tential influence of contaminant quartz can be disregarded. Assessment The kaolin minerals occur either as discrete plates or as composite of the contributions of impurity minerals to the H isotopic results yields stacks, and they exhibit one of two morphologies: relatively porous similar conclusions as for O isotopes (cf. Clauer et al., 2015). In con- aggregates and well-developed crystals (Fig. 6). The porous aggregates clusion, the minor amounts of impurity minerals in our samples are consist of pseudo-hexagonal plates with poorly developed lateral sur- likely to have only a minor influence, at most, on measured isotopic faces (010) and (110), yielding irregularly ragged outlines (Fig. 6A). values. The clay particles range in size from 1 to 5 μm. The second morphology The δ18O and δ2H values of the clay (< 2 μm) fractions range from consists of well-developed euhedral pseudo-hexagonal crystals, usually +16.8 to +18.7‰ and from −70 to −51‰, respectively (Fig. 5). In a arranged face-to-face in elongated book-like stacks (Fig. 6B). The lateral plot of δ18O vs. δ2H, the Chongyi kaolin samples mostly lie close to the surfaces (010) and (110) are flat and clearly defined, and the particle kaolinite weathering line (representing the isotopic composition of sizes are larger, ranging from 3 to 10 μm. Many of the kaolin aggregates kaolinite in equilibrium with meteoric water at 20 °C; Fig. 5). are vermiform or fan-shaped (Fig. 6C–E), and crystal elongation parallel to the c-axis (to 1.5 mm) was retained within the composite stacks. This

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3.5. DTA-TG analysis

The DTA-TG curves for bulk kaolin samples (Fig. 8A) and < 45 μm fractions (Fig. 8B) show similar thermal reactions and are consistent with the results of XRD, SEM, and geochemical analyses. The samples show an endothermal peak at a temperature of ~90–100 °C attributed to the loss of physically adsorbed water in pores, on mineral surfaces (Kakali et al., 2001). Weight loss over the temperature range of 90–350 °C (−0.93% for bulk samples and −2.07% for the < 45 μm fractions) is associated with a pre-dehydration process, which occurs as a result of reorganization of the octahedral sheet of kaolinite (Balek and Murat, 1996; Manju et al., 2001). Weight loss over the temperature range of 400–650 °C and an endothermal peak at ~525 °C are evidence of endothermic reactions linked to dehydroxylation of kaolinite and formation of metakaolinite (Cravero et al., 2016). Weight loss over the temperature range of ~600–800 °C may represent complete dehydrox- ylation of kaolinite and conversion to metakaolinite, or possibly de-

composition of alunite. However, SO3 concentrations are low (Table S2) and, assuming that all SO3 is present in alunite, the maximum alunite content would be < 0.1% and, thus, unlikely to be a factor in sample weight loss. An exothermic reaction at ~970–990 °C is due to trans- formation of metakaolinite to spinel and amorphous silica, and exo- thermic peaks at 1100–1400 °C are attributed to transformation of Fig. 5. Plot of δ18O vs. δ2H values showing results for Chongyi kaolin and metakaolinite to mullite and cristobalite. mottled kaolin samples relative to the supergene/hypogene line and the line marking isotopic composition of kaolinite in equilibrium with meteoric water at 4. Discussion 20 °C (Savin and Epstein, 1970). Reference data for Seiliz/Kemmlitz, Ramon- Fazouro, and Chubut/San Cruz kaolin deposits are from Gilg et al. (2003), 4.1. Supergene kaolinization and associated characteristics of the regolith Clauer et al. (2015), and Cravero et al. (2001), respectively.

None of hypogene indictors (hydrothermal mineral assemblages, suggests that growth of the composite stacks was initiated from one side shallow igneous activity, mineral zoning, and typical structures such as of a muscovite layer and then propagated inward to the other side of the funnel-shaped, pipelike or elongated zones of kaolinization) are ob- layer (Chen et al., 1997). Fine tubular crystals of halloysite (an elongate served in the present study units, and, thus, hypogene kaolinization can – μ kaolin mineral; 0.3 3 m long), which are common in weathered kaolin be ruled out. Instead, the Chongyi kaolin regolith shows a variety of deposits (Churchman et al., 2010; Keeling, 2015), cover the surfaces of mineralogic, elemental, and isotopic characteristics typical of surficial blocky and book-like kaolinite aggregates (Fig. 6). However, halloysite weathering, as discussed below. is not abundant in our samples, as shown by both SEM and XRD results. Kaolinization in the Chongyi regolith is usually associated with minerals produced through low-temperature chemical weathering, e.g., gibbsite, , and -oxides. For example, gibbsite, which is ty- 3.4.2. Heavy minerals pically found in highly weathered soils/, is generated through Ti-oxides are abundant in the heavy-mineral fraction and diverse in alteration of aluminosilicate or aluminous minerals under intense their morphology and composition (Fig. 7A, B). The TiO2 content of the weathering conditions (Ramasamy et al., 2014).In addition, tungsten bulk samples varies from 0.10 to 0.13 wt%. Ti- minerals always mineralization (here as wolframite) can also be observed. The presence contain a certain amount of niobium (Nb) (Dill, 2017), which can be of tourmaline is indicative of high-temperature fractionation within the seen in the EDS spectrum (Fig. 7A) and is an indication (along with an granite, inconsistent with conditions for kaolinite formation. These XRD peak at 3.51 Å) that the grain consists of anatase. Some Ti-oxide minerals can be a weathering from a primary high-tempera- grains show indentations and notches along their crystal boundaries, ture igneous intrusive body. fl which may re ect intergrowth of rutile with other minerals such as SEM observations show that Chongyi kaolinite aggregates are quartz (Fig. 7B). Iron-oxides are rare in the homogenous kaolin layer characterized by a variety of morphological features typical of kaoli- but relatively abundant in the mottled kaolin layer. Altered magnetite is nites produced through supergene weathering processes (Chen et al., present as dense massive aggregates, and it contains a certain amount of 1997; Churchman and Lowe, 2012). Book-like pseudohexagonal kaoli- (Cr) (Fig. 7C). Altered magnetite may also be present as nite crystals are commonly due to retention of the original morphology nodular/earthy aggregates (Fig. 7D). Both types of magnetite have re- of feldspar crystals (Ekosse, 2001). latively high Cl concentrations (Fig. 7C, D), consistent with its inter- Normalized element-distribution patterns do not reveal much about action with the volatile component of a magmatic system, e.g., during chemical weathering but rather about the parent granite rocks (Fig. 4). magma cooling and crystallization (Wang et al., 2014). Tourmaline is However, elemental compositions of the Chongyi kaolin show features present as prisms, irregular fragments, and sub-round to round grains typical of surfical weathering. Although Ti and Zr can be released from (e.g., Fig. 7E). The presence of iron and and the dark color primary minerals during alteration, they are relatively immobile ele- are consistent with Fe, Mg-tourmaline (Henry and Guidotti, 1985). ments under Earth-surface conditions, and are concentrated more effi- Wolframite, which generally occurs as tabular grains, is a solid solution ciently through chemical weathering (Marfil et al., 2005). In a Zr vs. between the endmembers huebnerite (MnWO4) and ferberite (FeWO4) TiO2 plot, for example, the homogenous kaolin and mottled kaolin and shows widely varying chemical compositions. In our samples, samples of the Chongyi deposit fall very close to the supergene field, ≫ Mn Fe suggests a composition close to the huebnerite endmember suggesting a significant surficial weathering contribution (Fig. 9A). (Fig. 7F). Alunite and some aluminum-phosphate-sulfate minerals are common in hypogene deposits, thus these deposits are usually enriched in P and S

(Dill et al., 1997). As a result, the rather low P2O5 and SO3

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Fig. 6. SEM images of: (A) poorly crystallized kaolinite (Kaol) with irregular outlines typical of chemical weathering; (B) face-to-face in elongated book-like stacks; (C–E) kaolinite aggregates with vermiform or fan-shaped morphologies; (F) tubular crystals of halloysite (Ha); (G) a sketched diagram shows the progressive transformation of muscovite to kaolinite stacks, involving fan-shaped composite stacks (after Chen et al., 1997). Note that halloysite attached on kaolinite is much less abundant and more sporadic than kaolinite under SEM. concentrations of the Chongyi deposit further demonstrate few influ- kaolinite weathering line, whose equation is δ2H = 4.85 × δ18O − 152, ences of hydrothermal alteration (Fig. 9B), consistent with the field represents the isotopic composition of kaolinite in equilibrium with observations and XRD analysis. Besides, vertical elemental profiles can meteoric water at 20 °C (Savin and Epstein, 1970). Most of the Chongyi also well present supergene chemical characteristics of the Chongyi samples plot very close to the global kaolinite weathering line (Fig. 5), kaolin regolith (see Section 4.3). indicating the influence of meteoric waters (Ma et al., 2017). The re- The H and O isotopic compositions of kaolinite provide constraints lationship of δ18Otoδ2H in from weathered profiles may on the geologic conditions of its formation, assuming no post-deposi- permit calculation of a linear equation and isotope fractionation factor tional alteration has occurred (Savin and Lee, 1988; Gilg et al., 2003; (Savin and Lee, 1988; Gilg et al., 2003). The best-fit line for the Chongyi Baioumy, 2013). The oxygen isotopic composition of the kaolin sug- samples is δ2H = 4.85 × δ18O − 150. Although the δ18O and δ2H va- gests the moderate depth of weathering. In δ18O vs. δ2H crossplots, the lues data of the full sample set are poorly correlated (r = 0.124, n = 9, Chongyi samples plot entirely within the supergene field, suggesting p > 0.1), the samples with δ2H<−60‰ (which were used to define significant contribution from chemical weathering (Fig. 5). The global the best-fit line) exhibit a strong correlation (r = 0.940, n = 5,

231 Q. Fang et al. Geoderma 337 (2019) 225–237

Fig. 7. SEM images with EDS spectra of various heavy minerals: (A) anatase grain with major Ti and minor Nb; (B) rutile with some “indentations” and notches observed along grain boundaries; (C) altered magnetite is present as dense massive aggregates; (D) altered magnetite is present as nodular/earthy aggregates, and these magnetite grains have relatively high Cl concentrations; (E) a prismatic crystal of Fe, Mg-tourmaline; (F) tabular grain of huebnerite with Mn ≫ Fe. p < 0.01). homogeneous protolith (i.e., muscovite granite) without significant These observations may suggest that minor residual mineral im- inputs. The study area has a tropical monsoonal climate with purities (e.g., quartz and muscovite) with higher δ2H values warm temperatures and high precipitation. Therefore, the Chongyi (e.g., > −60‰) have exerted a measurable influence on the isotopic kaolin deposit can be considered as a “kaolin regolith” (i.e., a residual compositions of some Chongyi samples. soil deposit) formed under intense weathering conditions. Deviations from the global kaolinite weathering line are potentially Before further discussion of mineral transformations and chemical attributable to post-depositional modifications in weathered profiles gradients within the Chongyi regolith, it is necessary to vertically (Baioumy, 2013). Such modifications can result from the interactions subdivide it and compare its layering with that of typical deeply between earlier-formed kaolinite and downward-percolating meteoric weathered profiles, as given in Taylor and Eggleton (2001) and Anand waters (Baioumy, 2013). Another possible cause of deviations is con- and Paine (2002). Regolith consists of a lower saprolith layer and an tributions from high-temperature volatiles. Although evidence for such upper pedolith layer (Fig. 2). The saprolith consists of two layers, sa- influences is limited in the Chongyi deposit, the presence of Cl-enriched prorock below and saprolite above. As suggested by Taylor and magnetite may be significant in this regard (Fig. 7C, D). Eggleton (2001), the pedolith is more strongly modified through soil- forming processes and is commonly composed (from bottom to top) of a plasmic zone (also referred to as a pallid zone because of its white 4.2. Kaolin regolith subdivision and mineral weathering color), a mottled zone, and a zone (the terms ferruginous dur- icrust and lateritic residuum may also be used). The pedolith of the The Chongyi kaolin was developed on a monolithologic and

232 Q. Fang et al. Geoderma 337 (2019) 225–237

Fig. 8. Representative differential thermal (green) and thermogravimetric (blue) curves of (A) bulk kaolin and (B) < 45 μm kaolin samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Chongyi profile contains a plasmic zone, a mottled zone, and an over- plasmic zone) have fan-shaped stacks (Fig. 6C–E), a morphology that is lying surface layer (Fig. 2), but the laterite zone that is commonly above closely associated with transformation of muscovite plates. Feldspar the mottled zone is absent. This vertical layering pattern is similar to may not directly transform into kaolinite but rather first into muscovite that of the Yalanbee profile reported by Gilkes et al. (1973). Below this as an intermediate stage under certain geologic conditions (Chen et al., paragraph, we try describing the mottled zone and plasmic zone in the 1997; Lu et al., 2016), which may have been the case for the Chongyi Chongyi regolith in more details with emphasis on some related im- kaolin. portant mineralogic and geochemical reactions. As illustrated in Fig. 6G, the transformation process involves the The mottled zone, which is found in the middle-upper part of the parallel growth of kaolinite originating from the edges of muscovite pedolith (Fig. 2), has cemented segregations of subdominant color grains and subsequent topotactic growth, with the development of fan- different from the surrounding matrix within the Chongyi regolith. The shape composite stacks or coalesced stacks. The edges of the kaolinite mottles in the Chongyi kaolin have sharp and distinct boundaries, stacks may split apart into discrete plates (Fig. 6G; Chen et al., ranging in size from ~5 to ~100 mm. Mottles frequently form due to 1997).Muscovite of the parent granite can be another important source migration of Fe2+ in soil waters and precipitation as Fe3+ upon contact of kaolinite (Gilkes et al., 1973). with an oxidizing region within the regolith (depending on the depth), Thermodynamically, the Gibbs free energy of kaolinite formation is which are a common influence on the development of local redox slightly lower than that of halloysite formation, so halloysite is less variation within soils and, hence, soil mottling (Taylor and Eggleton, stable than kaolinite (Jeong, 2000; Keeling, 2015). For this reason, if 2001). Based on Taylor and Eggleton (2001), long-term seasonal rise meta-stable halloysite forms, it should do so in advance of formation of and fall of the table can to reductive mobilization of the more stable kaolinite phase, due to its lower activation energy of Fe from the saprolite zone and subsequent precipitation higher in the formation (Stumm, 1992). In the Chongyi deposit, however, halloysite soil profile upon oxidation at the groundwater table. As a result, the is observed to have crystallized after the formation of kaolinite. This saprolite zone is a major source of Fe for mottle formation. apparently contradictory relationship can be ascribed to the presence of Below the mottled zone is the homogeneous kaolin zone (Fig. 2), weathered mica grains, the surfaces of which provide favorable nu- which corresponds to the plasmic zone and is transitional to the un- cleation sites for kaolinite by significantly reducing its activation en- derlying saprolite (Gilkes et al., 1973; Anand and Paine, 2002). This ergy (Jeong, 2000; Keeling, 2015). zone, which consists mainly of kaolinite, is mesoscopically homogenous Water-saturated soils in tropical/subtropical can lead to owing to destruction of the original sediment fabric (Taylor and abundant halloysite formation (Joussein et al., 2005). Dominance of Eggleton, 2001). Kaolinization of feldspar by chemical weathering is kaolinite over halloysite in the Chongyi deposit implies a near-surface, the main source of kaolin (e.g., Gilg et al., 1999; Churchman et al., freely draining environment (Churchman and Gilkes, 1989; Churchman 2010). Many kaolinite aggregates within regolith (and especially in the et al., 2010). However, it is possible that the tubular halloysite crystals

233 Q. Fang et al. Geoderma 337 (2019) 225–237

Cjw, Cip, τi,j =−1 Cjp, Ciw, (1)

where C represents concentration of mobile (j) or immobile (i) elements

in parent (p) or weathered (w) material. Negative τi,j values suggest depletion of element j, positive values mean enrichment, and when

τi,j = 0, the concentration of an element is identical to that of the parent material. We used the underlying weakly altered granite (saprolith; Anand and Paine, 2002) as the parent material in calculating the ele- mental profiles of Fig. 10. Elemental profiles are one of the most important lines of evidence for assessing pedogenic processes in regoliths. The patterns of elemental changes with depth can be assigned to specific endmember types (Brantley et al., 2008; Brantley and White, 2009), of which four are recognized: (1) Depletion profiles develop for elements that are strongly depleted in the pedolith (mostly in the upper levels of the regolith), usually due to mineral dissolution, and without reprecipitation at depth (Fig. 10A). For example, Na commonly exhibits a depletion profile owing to weathering of feldspar (White, 1995) and a lack of reprecipitation in illuviated oxides or clay minerals. (2) Depletion-en- richment profiles develop for elements that are leached in an upper re- golith layer but reprecipitated at depth (Fig. 10B). For example, Al is commonly mobilized as colloids and organic complexes in the surface layer and then redeposited deeper in a regolith profile (e.g., mottled kaolin zone), where concentrations of organic ligands are lower (Brantley and White, 2009). (3) Addition profiles develop for elements that are added to the regolith surface layer through atmospheric de- position, and which are redistributed downward through leaching or

Fig. 9. (A) A Zr vs. TiO2 plot suggests a supergene origin of the Chongyi kaolin, mixing processes (Fig. 10C). Addition profiles are characterized by peak given samples fall more close to those supergene samples with relatively high Zr concentrations at the top of the regolith profile and declining con- and Ti concentrations; (B) rather low P2O5 and SO3 concentrations in the centrations at depth. For example, Mn profiles can be dominated by wet Chongyi kaolin also point to supergene kaolinization processes. Due to the or dry deposition from the atmosphere (Herndon et al., 2011). (4) common presence of alunite and some aluminum-phosphate-sulfate minerals in Biogenic profiles exhibit trends that are opposite to depletion-enrichment hypogene deposits, they are usually enriched in P and S (Dill et al., 1997). profiles (Fig. 10D). They show peak enrichments at or close to the top of Reference plots of supergene, hypogene, and mixed-type kaolin samples are fi ff from Kitagawa and Koster (1991), Dill et al. (1997), and Dill (2015). the regolith pro le owing to the e ects of organic acid secretion by or fungi and the consequent dissolution and uptake of nutrient elements (Brantley and White, 2009). If the released nutrients (e.g., Ca) within the weathered kaolin directly crystallized from soil waters are taken up by roots, or recycled in the upper layer of soil, a biogenic fi during periodic waterlogging of microcavities and micro ssures, when profile develops (Jobbagy and Jackson, 2001). the activity of aqueous Al and Si did not lead to nucleation of kaolinite These four types of elemental profiles tend to be distinctly devel- on existing mineral surfaces (Chen et al., 1997; Joussein et al., 2005). oped in thin in situ regoliths, whereas deeply weathered (thick) re- goliths formed on granite host rocks may show different chemical profiles (e.g., Gilkes et al., 1973; Davy and El-Ansary, 1986; Anand and 4.3. Chemical gradients in the kaolin regolith Paine, 2002; Scott and Pain, 2008). The weathering behavior of ele- ments in thick granite-derived regoliths has been summarized by Davy The weathering of rock-forming minerals (e.g., feldspar and mus- and El-Ansary (1986) and Anand and Paine (2002). Here, we compare covite) of igneous parent rocks to kaolin is mainly controlled by the four elements (i.e., Na, Al, Mn, and Ca) representing different types of chemical composition of the and the climatic and hydro- elemental profiles in the Chongyi regolith with those from published logic conditions of the supergene environment (Fang et al., 2017). sources (Fig. 10). Elemental distributions are related to retention and leaching of various Elemental profiles of mobile elements such as Na exhibit similar elements and to mineral transformations in the principal regolith layers patterns in both the Chongyi kaolin and deeply weathered profiles (Tardy et al., 1973; Anand and Paine, 2002; Scott and Pain, 2008). (reference profiles; Fig. 10A). These elements are characterized by Here, we examine chemical gradients in the Chongyi regolith to in- leaching and depletion throughout the regolith, without reprecipitation fl vestigate how supergene processes in uenced and transformed the at depth. Elemental profiles of elements like Al show different patterns original kaolin deposit, and to compare them to typical regolith models. in the two types of regolith: they are leached from upper levels (e.g., fi Elemental concentrations in weathered pro les are typically nor- mottled kaolin zone) and reprecipitated at depth in the Chongyi kaolin, malized to the concentration of an immobile element such as Ti, Nb, or whereas they are generally residually enriched towards the surface Zr (White, 1995). In the present case, Ti and Nb are present in rutile layer in the reference profiles (Fig. 10B). Elements such as Mn are fl and/or anatase, so their concentrations may be in uenced by variations leached and depleted at deeper levels (e.g., in the saprolith) in both in the relative content of these minerals in the host rock. Zr is present types of regolith, and they are enriched in the upper part of the Chongyi almost exclusively in , making it the most suitable element for regolith (e.g., mottled kaolin zone); but they are residually enriched normalization. The relative gains and losses of elements were calculated throughout the reference profiles (Fig. 10C). Elemental profiles of ele- based on the assumption of immobility of Zr. ments like Ca exhibit similar patterns in both types of regolith, and ffi τ fi The mass transfer coe cient ( i,j) quanti es elemental changes in these elements are reprecipitated/enriched at or close to the top of the fi regolith pro les, accounting for both volume changes and relative gains profile (Fig. 10D). or losses of elements (Anderson et al., 2002):

234 Q. Fang et al. Geoderma 337 (2019) 225–237

Fig. 10. Elemental profiles (orange solid lines) of the Chongyi regolith can be assigned to four specific endmember types, i.e., (A) depletion, (B) depletion-en- richment, (C) addition, and (D) biogenic profiles. These profiles tend to be well-developed in thin in situ regoliths. Elemental profiles (black dotted lines) commonly observed in deeply weathered (thick) profiles developed on granite are also shown for comparison (Davy and El-Ansary, 1986; Anand and Paine, 2002).

4.4. Implications for formation of kaolin regolith and supergene pedogenesis chemical weathering alone in such settings are usually unable to initiate supergene kaolinization. The initial processes of weathering could be Previous studies on various argillization, in particular kaolin, focus physical, for example, the development of physical openings along more commonly on its economic value, compared to systematic studies joints, which may produce small fragments moving locally (Fig. 11; regarding pedological, mineralogic, and geochemical processes taking Taylor and Eggleton, 2001). Strong weathering conditions typical of place in kaolin regoliths (Ekosse, 2001; Anand and Paine, 2002; Scott humid subtropical/tropical climate zone with high MAT and MAP (e.g., and Pain, 2008). This would limit our understanding of kaolin argilli- the Chongyi region) favor supergene argillization. As chemical weath- zation linked to supergene processes in the Earth's surface environ- ering occurs, solutes are moved by solutions passing through the ments. In this study, we demonstrated that the kaolin regolith from granite. The exposed kaolin experienced enhanced weathering under subtropical China was derived from supergene weathering processes such climatic conditions, resulting in new regolith formation (Fig. 11). using integrated mineralogic and geochemical analyses. While the kaolin regolith can be subdivided into several layers with reference to typical subdivision of the deeply weathered profile, their 5. Conclusions elemental profiles are not necessarily uniform with those of typical deep granite weathering profiles. Instead, they are generally consistent Three kaolin-regolith drillholes from a subtropical climate zone in with those of relative thin in situ soil systems as widely reported pre- southern China were investigated using an integrated mineralogic, viously. The four types of elemental profiles reflect chemical gradients elemental, and isotopic geochemical approach. Neither shallow level of in the kaolin regolith as well as some biological effects (depending on granite emplacement, representative structural controls on kaoliniza- the depth).These elemental patterns are affected by a combination of tion, mineral zoning, nor hypothermal minerals can be observed. The chemical, geologic, and biologic processes that control soil formation mineral assemblages display typical supergene characteristics, and and evolution (Brantley and White, 2009). That is to say, the supergene kaolinite mineral aggregates are characterized by a variety of mor- kaolin regolith is intrinsically more similar to shallow, biologically phological features suggestive of supergene weathering. Geochemical δ18 δ2 active residual soil deposits, rather than the deep granite weathering binary plots (e.g., Zr vs. TiO2,P2O5 vs. SO3, and O vs. H) also point profiles. to a supergene origin for the kaolin regolith. Numerous kaolinite ag- Based on the discussion above, we propose a model to illustrate gregates within regolith have fan-shaped stacks, a morphology that is geological evolution, mineralogic transformation, and elemental mi- tightly associated with transformation of muscovite plates. Dominance gration within the kaolin regolith in particular linked to supergene of kaolinite over halloysite implies a near-surface, freely draining en- processes (Fig. 11). Kaolin may form in semi-humid to humid mid-la- vironment. fi fi titude climate zones with moderate to intense weathering intensity, but Four types of regolith elemental pro les can be identi ed in the kaolin regolith: depletion (e.g., Na), depletion-enrichment (e.g., Al),

235 Q. Fang et al. Geoderma 337 (2019) 225–237

Fig. 11. A sketched model for geological evolution of the kaolin regolith (after Anand and Paine, 2002). At first, the granite was uplifted by regional tectonic movement, and the erosion base level was reaching the surface of the granite. Then primary minerals of granite were weathered to secondary minerals with the formation of saprolith and pedolith. Further weathering occurred with migration and accumulation of Fe as mottles in mottled kaolin zone. With the supergene process going on, the primary granite was gradually changed to a kaolin regolith. Also shown is typical elemental leaching and precipitation (e.g., Na, Al, Mn, and Ca) along the Chongyi regolith. addition (e.g., Mn), and biogenic (e.g., Ca) profiles. These profiles re- References flect chemical gradients in the kaolin regolith as well as some biological effects that are typical of relative thin in situ soil systems. However, the Anand, R.R., Paine, M., 2002. 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