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Article

Mineralogical Characterization and Geochemical Signatures of Supergene Kaolinitic Clay Deposits: Insight of Ropp Complex Kaolins, Northcentral Nigeria

by
Adamu Yunusa
1,2,
Hanlie Hong
1,*,
Atif Salim
1,
Tarig Amam
1,
Chen Liu
1,
Yanxiao Xu
1,
Xiaochao Zuo
3 and
Zhaohui Li
4
1
School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
2
Department of Geology, Ahmadu Bello University, Zaria 810107, Nigeria
3
Engineering Research Center of Nano-Geomaterials, Ministry of Education, China University of Geosciences, Wuhan 430074, China
4
Geoscience Department, University of Wisconsin—Parkside, Kenosha, WI 53144, USA
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(9), 869; https://doi.org/10.3390/min14090869
Submission received: 15 July 2024 / Revised: 20 August 2024 / Accepted: 21 August 2024 / Published: 27 August 2024
(This article belongs to the Collection Clays and Other Industrial Mineral Materials)
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Abstract

:
This study presents the chemical and mineralogical composition of clay deposits and associated rock types within the Ropp Complex in order to assess the influence of parent lithology on the kaolinization, genesis, and utility of the deposit. Representative kaolin samples from E horizons of the weathering profiles and their bedrocks were collected from six sites in the Ropp Complex. Clay mineralogy was determined via the XRD technique, while a geochemical analysis was conducted using XRF, SEM coupled with EDS, and ICP-MS. The results showed that all kaolins dominantly contain kaolinite with a content of 77%–98% except for the AS1 kaolin with only minor kaolinite (20%) but mainly illite (65%). The notably lower crystallinity of kaolinite (HI value of 0.53–1.1) as well as its markedly small grain size is consistent with the formation of kaolinite from intensive chemical weathering of igneous rocks. The AS1 kaolin was probably formed from hydrothermal alteration in the burial stage due to the heating of groundwater by the late volcanism. Mobile trace elements (Sr, Ba, and Eu) exhibited a depletion trend, while immobile elements (Hf, Ta, Th) showed enrichment. The relatively more zirconium in kaolins implies the formation of low-temperature kaolinization. The notably high kaolinite content, accompanied by reasonable levels of Fe2O3 and TiO2, signifies a medium-grade quality. Furthermore, chondrite-normalized rare earth element (REE) patterns exhibit congruent trends in rocks and kaolin samples, indicating a relative enrichment in light rare earth elements (LREEs) alongside a discernible negative Eu anomaly. The abundant kaolinite and silicon–aluminum composition make the kaolins suitable for refractories, pharmaceutics, cosmetics, and supplementary cementitious material (SCM).

1. Introduction

Industrial minerals and clays are pivotal components within various industrial sectors [1,2,3]. Consequently, exploring and assessing these resources hold critical significance for emerging economies. The kaolinite group minerals (Al2Si2O5(OH)4) are derived from the chemical weathering of aluminum-rich silicate minerals in different types of rocks and soils [4,5,6]. This group comprises kaolinite, dickite, halloysite, and nacrite, each characterized by unique structural arrangements [7].
Kaolinite group minerals find extensive utility in diverse industrial applications, contributing significantly to many nations’ economic and technological development [8]. Global demand for kaolin continues to rise [9]. To meet local and international needs, it is imperative to accurately assess and characterize the nation’s kaolin deposits, which offer potential economic benefits in terms of GDP and foreign exchange earnings.
While kaolin suitable for industrial use is relatively scarce worldwide, notable deposits exist in the Cornwall and Devon regions of southwest England and along the Jari and Capim Rivers in the northern region of Brazil, specifically the Amazon, which are acknowledged as the world’s largest and best quality primary kaolin deposits [10]. However, such kaolin is widely spread all over the world (Turkey, India, Russia, Iran, China, and so on). In Africa, prominent kaolin occurrences are primarily found in Nigeria, Uganda, Egypt, Eritrea, and South Africa [11]. Nigeria, in particular, boasts substantial reserves of primary and secondary kaolin [9,12], with numerous deposits identified, especially in regions supported by the younger granites and sedimentary basins [13], amounting to approximately three billion metric tons, as stated by the Raw Material Research Council of Nigeria [14]. Over forty kaolin deposits were identified in Nigeria [9]. To stimulate domestic sector growth, Nigeria has imposed a ban on kaolin imports [14,15].
The genesis of kaolin deposits in Nigeria predominantly arises from the intricate interplay of geological processes, chiefly encompassing the phenomena of weathering and hydrothermal alteration acting upon the parent rocks [16]. In the Nigerian context, supergene kaolin deposits commonly form in areas characterized by tropical climatic conditions distinguished by abundant rainfall, fostering an environment conducive to profound weathering and leaching mechanisms [9]. The alteration of rock-forming minerals, such as feldspar and muscovite, in igneous bedrocks into kaolin is predominantly influenced by variables, including the chemical composition of the parent rock and the prevailing climatic and hydrological conditions within the supergene environment [17].
Numerous research endeavors have explored the widespread Nigerian clay deposits and have been thoroughly examined for various applications (Ref. [16], and references therein). Daspan et al. (2009) conducted an initial physical as well as chemical evaluation of Kuba kaolin deposits to study its industrial potential, and their findings revealed that the clay is medium grade and appropriate for use in manufacturing paper, paint, and ceramics [18]. Christopher et al. (2013) further investigated the Kuba clays and discovered that they have a low kaolinite concentration but a reasonably high montmorillonite content [19]. Odewumi et al. (2015) investigated the Kuba and Nahuta clays and concluded that Nahuta clays formed from the chemical weathering of felsic rock, and Kuba clays were derived from the weathering of mixed sources (felsic and mafic rocks) [20]. Geochemical characteristics and their implications for the paleo weathering proxy of soil profiles in Nahuta clays were also undertaken in a recent study [21].
Although a number of studies on clay deposits in Nigeria have been undertaken, a rare investigation was performed on clay deposits of the Ropp Complex. Oyawoye and Hirst (1964) reported the presence of montmorillonite in the Ropp Complex’s younger granites of Nigeria [22], whereas the mineral characteristics and formation of the clay deposits remain largely unknown. Moreover, the mineralogical and geochemical properties of clays in the clay ore deposit are essential for their quality and possible industrial uses [23]. In this study, we explore the genesis, mineralogical diversity, and geochemical signatures of kaolinitic clays within the complex. Through a detailed analysis of kaolinitic saprolites and their bedrock across various locations, we offer unprecedented insights into the formation and mineralogy of kaolin, enriching our understanding of the geological history. Our investigation focuses specifically on the saprolite horizon, which typically has the highest kaolinite content within the profile. We did not conduct a comprehensive examination of the entire profile, as commonly performed in typical regolith studies, with aims to (1) analyze the hidden lateral variations and potential source influences on studying multiple kaolin deposits within the Ropp Complex and (2) obtain a more comprehensive understanding of the formation of the kaolinite deposits.

2. Geological Settings

The Ropp Complex (Figure 1) is located in the northeastern part of Plateau State, bordered by the Barikin Ladi, Bokkos, and Mangu Local Government Area. This region forms a roughly triangular area of about 622 km2, with approximately 183 km2 underlain by units of alkaline granite [24]. The Complex is notable for its mineralization and contains economically valuable minerals, such as tin and columbite [25].
Encircling the Ropp Complex is an arcuate and polygonal ring dyke that encompasses a basement and a prominent central massif of rugged younger granite hills [27]. Some of these central hills rise over 300 m above the plateau, creating prominent landmarks. The Ropp Complex features distinct volcanic and plutonic cycles. The granitic intrusion cycle began with coarse-grained granite porphyry, akin to the Jos-Bukuru Complex, followed by a series of biotite and riebeckite granites. Later stages included recurring granite porphyry, hornblende–biotite granite, and late biotite granites. The volcanic cycle comprises quartz porphyries, rhyolites and explosion breccias, and early basic dykes [24].
The Ropp Complex predominantly consists of biotite granite along with minor albite arfvedsonite granites, which occasionally show rare metal Nb-Sn mineralization [24,28]. The biotite granite in Bukka Bakwai village holds the highest primary columbite, while the Ropp biotite granites rank second in tin production after the Jos-Bukuru biotite granites [24,29]. The Ropp alkaline A-type granites have coeval intrusive ages ranging from 161.1 ± 1.7 Ma to 162.3 ± 5.1 Ma, with chemical characteristics indicative of alkaline ferroan granites [26].
The Ropp Complex is known to be deformed by shearing, faulting, and jointing [27]. The study area features a massive, elongated intrusion of riebeckite–biotite granite porphyry. This intrusion includes a small, flat-lying sheet in the core of the complex, positioned above the older biotite granite. In the southwest, this structure appears as an eroded dome bordered by a small ring dyke, creating an elongated crescent. A notable crescentic intrusion of granite porphyry extends 9.6 km at the complex’s northern end, with a maximum width of 2.3 km.
The biotite granite is mainly found at the center of the Complex. Some of the granites, especially in Gana, are compositionally similar to the N’gell biotite granite of the Jos-Bukuru complex and exhibit lower columbite concentrations and less evidence of tin mineralization [24]. Beneath the aegirine biotite granite, multiple screens indicate its more extensive spread. The central massif is characterized by rugged boulder-strewn hills rising hundreds of meters above the surrounding lowlands. The southern core of the complex comprises the biotite granite of the Bukka Bakwai hamlet, flanked by the aegirine biotite granite. The albite arfvedsonite granite, which is greyish and coarse-grained to porphyritic, shows noticeable acicular grains of sodic amphiboles and clear replacement textures [28]. The Bukka Bakwai biotite granite contains the biggest primary columbite deposits in the Ropp Complex [24,29].

3. Samples and Methods

3.1. Sampling

The study area exhibits a unique occurrence of kaolin deposits formed through supergene processes associated with weathering tin-bearing minerals within the rocks. The deposits are found beneath the tin-bearing bedrock where active tin mining operations are ongoing. The weathering of tin-bearing minerals and other rock components is believed to be the primary process responsible for the supergene kaolin formation. Robust placer mining operations were also observed, which focused on extracting tin from alluvial deposits along the intricate network of river channels. Road banks and hillside exposures contribute to the exposure of kaolin deposits in the area, providing convenient access for sample collection (Figure 2a,b). Notably, tailings from mining pits are found to be kaolinized and left as hilly dumps. An intriguing feature observed was the presence of three distinct horizons, particularly near Manjahota and Kuba, with laterite cappings (Figure 2c).
Samples were collected from the illuvial horizon of the weathering profiles as well as the relevant bedrocks. The illuvial horizon often displays variations in height and thickness across different occurrences. About 400 g of each of the twenty raw clay samples were taken at different locations within the Ropp Complex. The representative samples collected were stored in sample bags to prevent any potential contamination. The samples were air-dried for two weeks and then crushed manually to powder at 0.07 mm in size for geochemical and mineralogical analyses. Preliminary XRD investigations of the samples showed that mineral compositions in most of samples above with the same parent bedrock were relatively uniform, and twenty kaolin samples were divided into six kinds of representative samples according to their distinct color and associated accessory mineral components. In further measurement, six representative kaolin and rock samples were selected for further geochemical and mineralogical analyses, respectively.

3.2. Methods

XRD was used to characterize the mineralogical composition of the samples, while SEM/EDS was employed to analyze the morphology and chemical composition of each mineral, respectively.
The XRD measurement was performed on a Panalytical X’Pert PRO DY2198 (PANalytical Co., Ltd., Eindhoven, The Netherlands) at the State Key Laboratory of Biogeology and Environmental Geology of the China University of Geosciences, Wuhan, China. The whole-rock sample was used to determine the mineral composition, and the clay fraction (<2 mm) obtained by the sedimentation method was used for the clay mineralogical investigation. The whole-rock powder samples were prepared by mounting the powders into sample holders using a back-press technique. The oriented clay-fraction samples were prepared by the following procedures. Carbonate minerals were removed from the samples by treating them with 10 mL of HCl acid. To remove organic matter, approximately 5 mL of hydrogen peroxide was added to the sample. A total of 400 mL of water was mixed in a quantity of sodium hexametaphosphate and was then incorporated into the mixture. The entire solution was stirred using a kitchen-type rotary mixer to facilitate the gravity settling of clays to obtain the <2 µm fraction. The mixture was placed into centrifuge tubes and centrifuged at 1000 rpm for two minutes to concentrate the clay fraction. The upper 2 cm of the suspension were selected and centrifuged at 3000 rpm for eight minutes. Finally, the concentrated clay fraction was placed onto a glass slide and allowed to dry at room temperature. Each clay-fraction sample was analyzed after being air-dried and undergoing ethylene glycol solvation in a sealed glass container at 60 °C for about eight hours.
Minerals were identified by characteristic reflections [30]. Non-clay minerals were identified using the following reflections: quartz, 4.26 Å and 3.34 Å; orthoclase, 3.24 Å; and ettringite, 9.92 Å and 3.85 Å, respectively. The mineral contents of the whole-rock samples were estimated using the reference intensity ratio approach with admixture of corundum as the internal standard [31]. The basal 001 reflection of air-dried and glycolated smectite is ~15 Å and 17 Å, and the 001 and 002 reflections of kaolinite are 7.16 Å and 3.57 Å, respectively. Illite was identified using the 10 Å, 5 Å, and 3.34 Å reflections. The ‘crystallinity’ (degree of structural order) of kaolinite was measured using the Hinckley index, which was calculated as the rate of the sum of (1ī0) and (11ī) heights from above a line drawn from the trough between the (020)–(1ī0) peaks to the total height above background of the (1ī0) reflection [32].
For SEM-EDS analysis, a small number of sample fragments were prepared by gold-coating and analyzed at the SEM laboratory of the School of Materials Science and Chemistry, China University of Geosciences, Wuhan, China, using Hitachi SU8010 Ultra High-Resolution Scanning Electron Microscope (Hitachi Ltd., Tokyo, Japan) equipped with an energy dispersive X-ray spectroscopy detector for elemental analysis.
The geochemical analysis of kaolin samples and their host rock samples was performed using X-ray Fluorescence (XRF) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) at Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. The sample pretreatment for major rock element analysis involved the melting method, using a flux mixture of lithium tetraborate, lithium metaborate, and lithium fluoride (45:10:5). Ammonium nitrate and lithium bromide were used as oxidants and release agents, respectively. The melting temperature was set at 1050 °C for 15 min. Major element analysis was conducted using a RIGAKU XRF spectrometer (Rigaku Corporation, Japan) with a 4.0 kW end window and an Rh target. The test conditions included a voltage of 50 kV and a current of 60 mA. Trace element analysis was conducted using an Agilent 7700e ICP-MS (Agilent Technologies, Inc., Santa Clara, CA, USA). Sample powder was dried, weighed, and dissolved in a Teflon bomb with nitric and hydrofluoric acids. The solution was heated, evaporated, and treated further. Finally, the solution was transferred to a polyethylene bottle and diluted for analysis.
The chemical index of weathering (CIW), index of compositional variability (ICV), and chemical index of alteration (CIA) for each sample were calculated by employing the following equations [33,34,35]:
CIW = {Al2O3/(Al2O3 + CaO + Na2O)} × 100
CIA = {Al2O3/(Al2O3 + CaO + Na2O + K2O)} × 100
ICV = {CaO + K2O + Na2O + Fe2O3(t) + MgO + MnO + TiO2}/Al2O3

4. Results

4.1. Mineralogy of the Kaolin Deposits

The mineral components in the Ropp Complex whole-rock kaolin samples are dominantly kaolinite, with only minor amounts of smectite, quartz, orthoclase, and ettringite (Figure 3a). Mineral abundances displayed only small changes between samples from the same bedrock but varied significantly between samples derived from different bedrocks, as reflected by intensities of their characteristic peaks of the minerals (Figure 3a). Kaolinite is identified by the presence of basal diffraction near 7 Å in the air-dried sample, which remain stable after the ethylene–glycol solvation, and the presence of smectite is indicated by the 15 Å peak in the air-dried sample, which expands to 17 Å after glycolated treatment (Figure 3b). Sample AS1 consists dominantly of illite and kaolinite, with minor amounts of smectite and quartz. Samples AS8, AS14, and AS16 have the same mineral assemblage of mainly kaolinite and quartz, while the content of quartz shows a decreasing trend. The AS9 sample contains mainly kaolinite, with minor amounts of smectite, quartz, and orthoclase. The AS20 sample consists of mainly kaolinite with minor quartz, ettringite, and hematite. The percentage of each mineral in the Ropp Complex kaolin samples is summarized in Table 1. Except for the presence of a large proportion of illite in sample AS1, all other samples dominantly contain kaolinite with a proportion of 77%–98%, indicating the kaolinite ore samples are notably pure with only minimal contamination from other minerals or impurities. Kaolinite in samples AS20 and AS1 have relatively higher HI values of 1.1 and 1.0, while the kaolinite of samples AS8, AS9, AS14, and AS16 have significantly lower HI values of 0.80, 0.65, 0.73, and 0.53, respectively (Table 1), consistent with kaolinite formed from the supergene weathering of igneous rocks [36].

4.2. SEM-EDS Analysis

Detailed scanning electron micrographs coupled with EDS spectra show significant differences between the samples from different locations. In general, kaolinite was characterized by its layered structure and appears as thinly platy crystals with a smooth basal surface. However, kaolinite in different samples displayed marked difference in its grain size and edge surface and thus showed clearly different morphological characteristics between the six kaolin-ore samples. Sample AS1 contains kaolinite with relatively large grain size and aggregates of small rectangular illite in pore space (Figure 4a). Kaolinite sample AS14 exhibits euhedral pseudo-hexagonal morphology, characteristic of kaolinite minerals (Figure 4b). Also, in sample AS20, kaolinite exhibits characteristic book-like aggregates of pseudo-hexagonal crystals (Figure 4c). The well-developed euhedral pseudo-hexagonal crystals usually have large grain sizes, ranging from 0.5 to 5.0 μm. Most of kaolinite crystals in samples AS8, AS9, and AS16 occur in poorly developed thin plates with a relatively smooth or flat basal (001) plane, but their lateral surfaces (110) and (010) were poorly developed and extremely small and thus exhibit irregularly ragged outlines (Figure 4d–f). The particle sizes are quite uniform and usually small, ranging generally from 0.3 to 0.6 μm. The spectra from the EDS in all the samples show that kaolinite is primarily composed of silicon, aluminum, and oxygen in significant amounts. The presence of Fe, Mg, Ti, and Ba in the kaolinite indicates the presence of impurities. EDS spectra for illite indicate the presence of potassium in addition to silicon, aluminum, and oxygen. The presence of potassium in the illite is notable and helps distinguish it from kaolinite.

4.3. Major Geochemical Composition and Degree of Weathering

The distribution of major elements indicates the origin and suitability of clays for industrial purposes [37,38,39]. The SiO2 content of the bedrocks ranges from 77.80 to 55.71%, with a mean value of 70.32%, consistent with their acidic lithology (Table 2).
The kaolin deposits of the study area have significant quantities of SiO2 (44.25%–52.62%) and Fe2O3 (1.23%–7.54%). Samples AS14 and AS16 show compositions similar to that of pure kaolinite (46.30% SiO2 and 39.80% Al2O3) [40], consistent with their mineral compositions that are overwhelmingly kaolinite. The Fe2O3 content of the deposits is higher than the permissible limits for kaolin in industrial uses (1%) [41], even though it depends on the industry. This concentration of Fe2O3 present in the deposits is likely attributed to the alteration of iron-bearing minerals within the bedrock. However, for certain types of ceramics, such as stoneware or earthenware, which have a more rustic or natural aesthetic, slightly higher concentrations of iron oxide may be acceptable. In this case, an iron oxide level of up to 2% can contribute to color variations, improving the appearance of the final product. The deposits are characterized by appreciable concentrations of Al2O3 (23.19–35.87%), which are lower than the commercial-grade ceramic value (37%–38%) [41]. The concentrations of mobile element oxides in the deposits are modest when compared to Fe2O3 {CaO (0.06%–0.34 wt%), MgO (0.02%–2.55%), P2O5 (0.05%–0.18%), and MnO (0.01%–0.07%)}, which indicates a total loss of these mobile elements during the process of kaolinization.
Sample AS1 contains markedly more K2O (4.34%), Na2O (0.42%), MgO (2.55%), CaO (0.34%), and Fe2O3 (7.54%) compared to the other kaolin-ore samples due to the presence of large amounts of illite. Conversely, other kaolin-ore samples have significantly low levels of CaO, MgO, K2O, and Na2O contents, pointing to the low level of impurities. The TiO2 value varies from 0.638 to 2.75%
With an average of 1.53%, all additional elements, including iron, have been mobilized with the exception of aluminum and titanium, which remain relatively immobile.
Figure 5 illustrates the correlation between Al2O3 and each of the following, SiO2, Fe2O3, K2O, Na2O, and CaO, showing a positive correlation between Al2O3 and TiO2, indicating the presence of titanium-bearing minerals. The negative correlations between Al2O3 and SiO2, Fe2O3, and CaO indicate the depletion or removal of silicate, iron, and calcium minerals during the alteration process. The observed positive correlation between Al2O3 and TiO2 indicates that these elements were relatively immobile during weathering, while the negative correlation observed between Al2O3 and SiO2 as well as that of CaO suggests the preferential leaching of Si and Ca during weathering, potentially due to feldspar breakdown.
The CIW, ICV, and CIA for each sample (Figure 6a,b) were contrasted with well-known kaolins. The CIA and CIW of any deposit can be used to identify the degree to which potassium feldspar and micas have been transformed into clay minerals [42]. Any kaolin deposit with a high CIW and CIA value (>80) indicates an exceptionally high level of silicate weathering and therefore shows the deposit’s maturity [43]. In this work, the clay samples are plotted within the field of ‘Extreme Silicate Weathering’, which confirms the advanced degree of weathering. The ICV is used to compare the abundance of Al2O3 to the other major oxides found in kaolins.
The ternary diagram Al2O3−(CaO + Na2O)-K2O (A-CN-K) diagram is employed to determine the weathering trend [34,44,45,46]. All of the kaolin samples show an intensive weathering history on the ACNK diagram and cluster towards Al2O3 (Figure 6c).
The kaolin samples had SiO2/Al2O3 ratios ranging from 1.32 to 2.27 (Table 3), which is more than the stoichiometric SiO2/Al2O3 ratio (1.18) [40]. Sample AS1 has the greatest SiO2/Al2O3 ratio (2.27). The ICV of the examined kaolins varies between 0.09 (AS14 and AS16) and 0.70 (AS1). The elemental composition of the kaolin is compared with industrial specifications (Table 4). The composition is quite consistent with the renowned standard kaolin found in the Cornwell deposits, Sekeharman kaolin (SKD). The high SiO2, LOI, and Al2O3 values in the analyzed kaolin match favorably with pharmaceutical, ceramic, porcelain, paper coating, and refractory brick standards.

4.4. Trace Element Concentration

Kaolin deposits, except sample AS1, have greater concentrations of a set of trace elements, including Hf, Pb, Th, Nb, Ga, Cr, Ni, Ta, and U (Table 5). Zr shows an increase in all the studied kaolin samples, reaching up to 872 ppm, while depletion, especially for Cs and Rb as compared to the parent rock, was also observed. High values of chromophore elements (V, Cr, and Cu) were also noted in the kaolins as compared to the parent rocks. The binary plot of Ba + Sr versus Ce + Y+La, TiO2 versus Zr, Cr + Nb versus TiO2 + Fe2O3, and ternary plot of Pb, Ba + Sr and Ce + Y+La have been widely used to distinguish hypogene and supergene kaolin formation [53]. All the Ropp Complex kaolinites were plotted within the field of supergene origin on both the binary and ternary diagram except sample AS1 in the field of hypogene origin (Figure 7a,b).

4.5. Rare Earth Element (REE) Concentration

Large ion lithophile elements (LILEs), such as Sr (40.2–122 ppm), Rb (0.85–674 ppm), Ba (34.6–911 ppm), and Cs (0.071–27.5 ppm) (Table 6), show depletion on the chondrite-normalized spider plot, while high field-strength elements (HFSEs), such as Zr (284–666 ppm), Hf (7.46–57.8 ppm), Nb (18.6–245 ppm), Ta (1.04–40 ppm), and rare earth elements (REE), exhibit relative enrichment. HFSEs are resistant to weathering and are often incorporated in the weathering-resistant accessory minerals (zircon, rutile, and monazite), even after extensive weathering. As such, their concentrations are not expected to be affected.
The multi-element diagram (Figure 8a) depicts the depletion of mobile HFSEs (K, Sr, and Ba) and the enrichment of less mobile HFSEs (Th, Ta, and Hf). Europium (Eu) exhibits a very strong negative anomaly (Figure 8b). In general, the parent rock samples exhibit a much more pronounced negative Eu anomaly in comparison to the kaolin-ore samples. The chondrite-normalized REE diagram reveals a depletion of LREEs over HREEs, which is typical for most felsic rocks. The crystal structure of kaolinite exhibits a stronger affinity for LREEs over HREEs and thus leads to a pronounced depletion of Eu. Ce was observed to be more than 100 ppm in the REE compositions of the kaolin-ore samples. La, Nd, and Y were greater than 25 ppm. Other REEs were less than 2 ppm (Eu, Tb, Tm, and Lu), while Sm, Dy, and Gd were usually less than 12 ppm.

5. Discussion

5.1. Alteration and Element Behavior during Kaolinization

The relatively high Fe2O3 averages (3.20%) result from the intense oxidation of iron-bearing silicate minerals, such as biotite, present in the parent rock leading to the formation of more stable iron oxides [56]. The decrease in silica content from the parent rock to the weathered material is due to the transformation of feldspars into kaolinite through weathering processes [57]. CIW (96.82–99.83) and CIA (81.96–99.77) values of the Ropp Complex kaolinites suggest an extent of alteration processes in the kaolin deposits [33,34]. This indicates that the bedrock from which the kaolin deposits were formed has undergone an extensive breakdown and alteration of their original mineral composition. Furthermore, the low ICV values suggest that the samples are likely derived from a homogeneous parent rock, while the only sample (AS1) with a high ICV originates from the similar bedrock type of riebeckite–biotite granite but experiences a different process of hypogene alteration (Figure 6). Therefore, the high estimated CIA and CIW values and the low ICV values of the kaolin samples suggest the significant weathering of the parent rocks under warm and humid climatic conditions [4,5,6,21].
The supergene kaolinization process often concentrates HFSEs in a kaolin deposit while depleting mobile LILEs significantly [58,59]. During the kaolinization process, the alteration of feldspars releases Ca, Na, K, and Ba into the altering solutions [58,60]. The decrease in CaO, Na2O, and K2O in the study area could be influenced by feldspar weathering, while the decrease in MgO and Fe2O3 could be due to the breakdown of biotite alongside the feldspars. These extremely mobile components can be rapidly leached out of the system during weathering processes. The relative enrichment in TiO2 and Al2O3 could be a result of the formation of secondary mineral phases, particularly rutile and clay minerals [4]. The resembling pattern of HFSEs and LILEs between the kaolins in the study area reflects the intensity of weathering. During the weathering process, REEs and HFSEs remain immobile [61]. As a result of the aforementioned process, it can be inferred that weathering processes during kaolinization of the deposit concentrate HFSEs and deplete LILEs in the kaolin-ore sample. The depletion in LILEs (Rb, Ba, and Sr) and the enrichment in HFSEs (Zr, Nb, and Hf) indicate that the source rock of the kaolin deposits has a felsic composition [62]. The presence of a large amount of illite suggests that the kaolin deposit (AS1) has experienced hydrothermal alteration in the burial stage due to the heating of groundwater by the late volcanism instead of surface chemical weathering. The initial alteration of igneous rocks usually produces smectite, which transforms rapidly to kaolinite, with weathering proceeding under tropical to subtropical climate conditions [58,59], as also indicated by the discrimination diagram for the supergene–hypogene alteration of kaolins (Figure 7). The loss of Na, K, and Ca can be explained by their significant mobility during weathering [63].
The weathering processes and the characteristics of the parent rock affect how trace elements are distributed in clays in sediments and soils [4,5,6,62,64,65,66]. In comparison with the parent rocks, the Ropp Complex kaolinite samples are depleted in Rb, Sr, and Ba and enriched in Zr, Cr, Ni, Cu, and Zn. The zirconium content of the kaolins is relatively high, and such a high Zr content suggests that the parent rock contained zirconium-rich minerals in significant amounts. Zirconium minerals, like zircon, have relatively high concentrations of zirconium compared to other common rock-forming minerals.
The Ropp Complex kaolinite samples are more Ba enriched (mean 225.83 ppm) than Sr enriched (mean 70.23 ppm). The relative abundance of feldspars in the parent rock and their stability contribute a substantial role in the enrichment of barium compared to strontium in the kaolin samples [67]. The values of Cr and Ni in kaolin can provide evidence to differentiate a felsic (silica-rich) source from a mafic (magnesium- and iron-rich) source. A low Cr (<100 ppm) and nickel (<50 ppm) content suggests a felsic origin, as it represents the absorption of small Cr and Ni ions to clay particles during the weathering of rocks containing Cr- and Ni-bearing minerals. High concentrations of Cr and Ni, on the other hand, are generally observed in sediments originating from ultramafic rocks [68]. The contents of Cr (mean 92.88 ppm) and Ni (mean 25.1 ppm) are low in the Ropp Complex kaolins, consistent with their felsic parent rocks. Nb and V exhibit lower mobility, aligning with established knowledge regarding their behavior during the process of weathering [69]. Great values of V, Cr, and Cu in the kaolin samples indicate the influence of chromophore elements on variations in the color of the kaolins.
REE geochemistry can provide useful evidence for a source of kaolin deposits [70,71]. The absence of a Ce anomaly and negative Eu anomaly are considered a result of feldspar alteration and the formation of clay minerals under elevated temperatures [72]. The Ropp Complex kaolin deposits show a higher concentration of LREEs relative to HREEs and display a distinct negative Eu anomaly while lacking Ce anomalies, suggesting a supergene origin. The magnitude of the negative Eu anomaly differs between the host rocks and the kaolins, with stronger anomalies observed in the rocks than in the kaolins. This variation in the extent of negative Eu anomalies may be linked to differences in the compositions of the source rocks. Our findings are consistent with the work of Odewumi et al. (2015) [20]. However, chemical weathering typically has a limited effect on the distribution pattern of rare earth elements in weathered profiles [73,74].

5.2. Genesis of the Kaolin Deposit

Kaolin deposits can be classified as hypogene (hydrothermal) or supergene (weathering) depending on the kaolinization process of the parent rock. Hypogene kaolins form in hydrothermal systems where hot, element-rich fluids circulate through the parent rock [75,76,77,78]. Hydrothermal kaolin deposits are commonly linked with high-temperature alteration minerals, including pyrophyllite, diaspore, nacrite, dickite, and topaz [53,79]. On the other hand, illite usually forms from the alteration of muscovite and feldspar in potassium-rich environments and is often associated with hydrothermal processes, particularly in low-temperature environments compared to the high-temperature minerals mentioned above [80,81]. It is also considered a significant mineral in supergene environments due to physical weathering under cold and arid climates [82]. However, under the warm and humid climate conditions in the area, the association of illite with kaolinite in the Ropp Complex kaolin (AS1) may indicate a complex alteration history involving both hydrothermal activity and supergene weathering processes [80], taking into account the presence of almost no illite observed in the weathering regolith of other locations around the area (Figure 6). In the triangular ACNK diagram, most of the kaolin samples fall close to the Al2O3 peak position, and the geochemical evolution trend clearly indicates a genetic relationship of intense weathering between the kaolin deposit and the granite bedrock (e.g., [42]). Obviously, the source of the kaolin deposits is presumed to be the biotite granite. This is also supported by field investigations, which revealed that numerous kaolin occurrences were connected with the granite bedrock. In addition, the notably high kaolinite content in kaolin-ore samples suggests an intense chemical leaching during the weathering process. It can also be inferred from the ACNK diagram that the kaolin was formed under a warm and humid climate with efficient drainage, as illustrated in Figure 9.
LILEs are mostly concentrated by hydrothermal alteration, and a common kaolin deposit formed through hydrothermal fluid exhibits a silica/alumina ratio ranging from 1.04 to 1.45 [58,83]. Kaolin can also develop through diagenetic processes within sedimentary basins [84]. The kaolin deposits in the study area show a SiO2/Al2O3 content range between 1.32 and 2.27, and this range is characteristic of a mixed origin. Binary and ternary plots of different major and trace element combinations have been used to identify the formation of hypogene and supergene kaolin formations. In almost all cases, the samples from the study area were plotted mainly within the supergene field, while sample AS1 shows some affinity to the hypogene field, as confirmed by the chemical alteration index (Figure 7). All these lend credence to the clay deposits’ supergene origin, with little impact of the hypogene origin, and also indicate intensive weathering. Furthermore, the LOI of the kaolin deposits ranges from 7.02 to 16.73% (average 12.24%), comparable to the well-known deposits in Georgia (13%) and Cornwall (13.6%) [40].

5.3. Grade, Quality, and Economic Significance

Except for the AS1 kaolin, in which kaolinite occurs as a minor phase (20%), all other Ropp Complex kaolins contain a markedly high amount of kaolinite, up to 77%–98% (Table 1). The kaolinite content significantly influences the kaolin grade [40]. High-grade deposits contain over 75% kaolinite [85]. Obviously, kaolin deposits in the Ropp Complex meet the high-grade criterion.
The quality of kaolin is dependent on the type and quantity of impurities present. Impurities are often observed adhering to the kaolinite surface or substituting a component inside the kaolinite crystal structure [40]. Kaolins containing Fe and Ti are crucial in identifying their quality because they provide the kaolin yellow, pink, or deeper hues [86]. The relatively higher contents of Fe2O3 (1.23%–7.54%) and TiO2 (0.64%–2.75%) as well as the significantly lower crystallinity (HI value of 0.53–1.1) of kaolinite of the Ropp Complex kaolin deposits do not meet the requirement for filler and paper coating industries, unlike those reported by Daspan et al. (2009) [18]. Fe2O3 in kaolin typically has a higher tolerance as a supplementary cementitious material (SCM) than other applications, like ceramics and paper.
The economic importance of kaolin deposits is determined by the mineralogical, chemical, and physical properties that govern their applicability [87]. The collected data on the Ropp Complex kaolin deposits indicate that it is well suited for manufacturing porcelain, ceramic tableware, sanitary ware, high-grade refractories, premium ceramic tiles, and specialty paper coating. The chemical constituents SiO2 (46%) and Al2O3 (38%) are the major components of kaolin [88]. Although the kaolinite content is within the range of chemical compositions (81%–93%) necessary for the ceramic industry [37], the concentrations of Fe2O3 and TiO2 require some beneficiations.
Most toxic elements, such as Hg, As, Sb, and Cd, were not detectable due to their low concentration. The low concentration of MgO, CaO, Na2O, and K2O in the kaolin deposits make them highly suitable for use in the pharmaceutical and cosmetic industries, and their elevated SiO2 and Fe2O3 levels also make them well suited for filling applications. The crucial requirement for the industrial utilization of kaolin is its chemical composition aligning closely with its theoretical composition. However, achieving this is often challenging due to the association of kaolinite with other minerals and its rarity in a pure form [9].
The grade and applications of the Ropp Complex kaolins are compared to known commercial kaolins from Cornwall deposits, deposits from English China clay (ECC), and Sekeharman kaolin (SKD) [89]. The markedly high grade of kaolinite for the Ropp Complex kaolins makes them potential wide application prospects. The only limitation is the impurities (Fe2O3 and TiO2) in the Ropp Complex kaolinitic clays, which can be removed through beneficiation (Table 4). To generate a good-grade product, Fe2O3 can vary from 0.2 to 1.0%, and TiO2 can range from 1% to 2.2% [90]. The chemical characteristics (SiO2 + Al2O3 + Fe2O3 > 70%) and kaolinite content (kaolinite > 65%) of the Ropp Complex kaolins also indicate that they can probably be utilized as a supplementary cementitious material for the cement industry, which is a potential approach for reducing CO2 emissions and improving the overall sustainability of the construction sector [91].

6. Conclusions

The mineralogical and geochemical investigation of clay fractions and bulk samples from primary kaolin deposits formed on biotite granites has provided insights into their origin. The pronounced negative Eu anomaly, along with the relative enrichment of LREEs over HREEs, and the absence of a Ce anomaly strongly indicates a supergene origin for the Ropp Complex kaolins. Particularly noteworthy is the enrichment of chemically immobile elements (Al2O3, TiO2, Zr, and Sc), coupled with the depletion of mobile elements (MgO, CaO, Na2O, and K2O) and the notably high CIA value (81.96–99.73), indicating the significant chemical weathering of the parent rock. Except for AS1 kaolin with minor kaolinite (20%), all other kaolins dominantly contain kaolinite with a content of 77%–98%. The crystallinity of kaolinite is significantly low, with an HI value ranging from 0.53 to 1.1, consistent with their notably small grain size, indicating that kaolinite formed from the intensive chemical weathering of igneous rocks. The observed chemical correlations in the kaolin samples align with the anticipated mineralogical transformations during supergene weathering and kaolin formation. Exceptionally, the AS1 kaolin with dominantly illite (65%) was probably formed from hydrothermal alteration in the burial stage due to the heating of groundwater by the late volcanism. Furthermore, a comparative analysis of the chemical composition between the Ropp Complex kaolin and a renowned deposit in Cornwall, the UK, underscores the economic potential of the Ropp Complex deposits.

Author Contributions

A.Y.: Writing original draft. H.H.: Review and editing, Supervision, Funding acquisition, and Conceptualization. C.L., Y.X. and X.Z.: Analytical works. A.S. and T.A. partially contributed with idea discussion and manuscript writing. Z.L.: Manuscript editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Projects 42172045 and 41972040).

Data Availability Statement

All data are available upon request.

Acknowledgments

The authors thank Wang Hongquan and Chen Jin for assistance in sample preparation and SEM analysis; they are also grateful to the anonymous reviewers for their insightful reviews, valuable comments, and constructive suggestions.

Conflicts of Interest

The authors declare that they have no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. A geologic map of the study area (modified from [26]). (a,b) generalized maps showing the location (the purple square) of the Ropp Complex, (c) a geological map showing lithology of the study area and sampling sites.
Figure 1. A geologic map of the study area (modified from [26]). (a,b) generalized maps showing the location (the purple square) of the Ropp Complex, (c) a geological map showing lithology of the study area and sampling sites.
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Minerals 14 00869 g002
Figure 2. Field images showing two distinct occurrences of kaolin in the weathering profiles of the Ropp Complex. (a) The overview of white kaolinite, (b) the close-up view of white kaolinite, and (c) yellow kaolinite in the E horizon.
Figure 2. Field images showing two distinct occurrences of kaolin in the weathering profiles of the Ropp Complex. (a) The overview of white kaolinite, (b) the close-up view of white kaolinite, and (c) yellow kaolinite in the E horizon.
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Minerals 14 00869 g003
Figure 3. (a) XRD patterns of representative whole-rock samples of the Ropp Complex kaolinite deposits. I: illite; S: smectite; K: kaolinite; Q: quartz; Or: orthoclase; Et: ettringite; Ht: hematite. (b) XRD patterns of the samples showing the presence of kaolinite (K), smectite (S), and illite (I); Et: ettringite.
Figure 3. (a) XRD patterns of representative whole-rock samples of the Ropp Complex kaolinite deposits. I: illite; S: smectite; K: kaolinite; Q: quartz; Or: orthoclase; Et: ettringite; Ht: hematite. (b) XRD patterns of the samples showing the presence of kaolinite (K), smectite (S), and illite (I); Et: ettringite.
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Figure 4. Scanning electron micrographs with corresponding EDS spectra illustrating the morphology and chemical composition of kaolinite and associated minerals. (a) Aggregates of small elongated-plate illite in pore space showing precipitation from the solution, kaolinite occurring in relatively large plates (AS1). (b) Hexagonal kaolinite plates from sample AS14. (c) The characteristic book-like aggregates of kaolinite (AS20). (d–f) Kaolinite flakes with irregular outlines (AS8, AS9, and AS16).
Figure 4. Scanning electron micrographs with corresponding EDS spectra illustrating the morphology and chemical composition of kaolinite and associated minerals. (a) Aggregates of small elongated-plate illite in pore space showing precipitation from the solution, kaolinite occurring in relatively large plates (AS1). (b) Hexagonal kaolinite plates from sample AS14. (c) The characteristic book-like aggregates of kaolinite (AS20). (d–f) Kaolinite flakes with irregular outlines (AS8, AS9, and AS16).
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Minerals 14 00869 g005
Figure 5. Harker diagram showing the relation between Al2O3 and other oxides.
Figure 5. Harker diagram showing the relation between Al2O3 and other oxides.
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Figure 6. The relation between CIW and CIA (a), CIA and ICV (b), and the ACNK diagram in molecular proportions (c) for the clay samples.
Figure 6. The relation between CIW and CIA (a), CIA and ICV (b), and the ACNK diagram in molecular proportions (c) for the clay samples.
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Minerals 14 00869 g007
Figure 7. Discrimination diagrams for the supergene–hypogene alteration of the Ropp Complex kaolinites (after [53]). (a) The log(Ce+La+Y) vs. log(Ba+Sr) plot; (b) The (La+Ce+Y)-(Ba+Sr)-Pb ternary plot.
Figure 7. Discrimination diagrams for the supergene–hypogene alteration of the Ropp Complex kaolinites (after [53]). (a) The log(Ce+La+Y) vs. log(Ba+Sr) plot; (b) The (La+Ce+Y)-(Ba+Sr)-Pb ternary plot.
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Minerals 14 00869 g008
Figure 8. (a) Chondrite-normalized trace element diagram (after [54]); (b) Chondrite-normalized REE diagram (after [55]). Kaolin-ore samples in blue and bedrock samples in red.
Figure 8. (a) Chondrite-normalized trace element diagram (after [54]); (b) Chondrite-normalized REE diagram (after [55]). Kaolin-ore samples in blue and bedrock samples in red.
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Minerals 14 00869 g009
Figure 9. A proposed model of formation of the Ropp Complex kaolins.
Figure 9. A proposed model of formation of the Ropp Complex kaolins.
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Table 1. Mineral compositions of whole-rock samples from the Ropp Complex clay deposits (wt%).
Table 1. Mineral compositions of whole-rock samples from the Ropp Complex clay deposits (wt%).
SampleAS1AS8AS9AS14AS16AS20
ColorGrayish YellowLight GreyDark GreyGrey WhiteLight PinkYellow
kaolinite207977949884
illite65/////
smectite9512///
quartz6168627
orthoclase//3///
ettringite/////5
hematite/////4
HI1.00.800.650.730.531.1
“/”: under detection limit.
Table 2. Major oxide compositions of whole-rock clays (AS) and the relative bedrocks (AY) (wt%).
Table 2. Major oxide compositions of whole-rock clays (AS) and the relative bedrocks (AY) (wt%).
SamplesRock TypeSiO2TiO2Al2O3Fe2O3MnOMgOCaONa2OK2OP2O5LOISUM
AS1RBG52.620.9523.197.540.072.550.340.424.340.187.0299.22
AS8BGM53.772.0129.831.690.010.090.150.010.100.0511.8099.51
AS9CAB44.252.7532.462.970.020.140.190.030.130.0716.7399.73
AS14BGM47.261.1435.861.730.010.030.070.010.080.0513.2299.47
AS16B47.631.7335.871.230.010.020.060.000.030.0613.1199.75
AS20CAB51.880.6430.954.040.020.080.090.020.260.1311.5899.68
AY3RBG76.020.0412.981.180.020.050.334.284.450.010.4699.81
AY4RBG69.050.5714.923.610.060.822.234.313.640.150.5599.91
AY6ABG77.800.1211.911.030.000.050.073.684.860.000.4199.93
AY8BGM67.820.2816.232.530.050.811.763.505.800.370.7899.93
AY10BGU75.530.0612.911.170.020.040.484.004.780.000.3899.38
AY14CAB55.710.9215.948.500.154.796.873.432.150.280.6199.36
RBG: Riebeckite–biotite granite; ABG: aegirine biotite granite; BGM: biotite granite (mineralized); BGU: biotite granite (unmineralized); CAB: calc-alkaline basement complex; B: basalt; LOI: loss on ignition.
Table 3. Elemental ratios and weathering indices of the Ropp Complex kaolins.
Table 3. Elemental ratios and weathering indices of the Ropp Complex kaolins.
SamplesRock TypeSiO2/Al2O3CIWCIAICV
AS1RBG2.2796.8281.960.70
AS8BGM1.8099.4699.140.14
AS9CAB1.3699.3498.950.19
AS14BGM1.3299.7799.560.09
AS16BST1.3399.8399.730.09
AS20CAB1.6899.6698.820.17
Table 4. The comparative chemical composition of the kaolin deposits against established standards and specifications (wt%) [37,47,48,49,50,51,52].
Table 4. The comparative chemical composition of the kaolin deposits against established standards and specifications (wt%) [37,47,48,49,50,51,52].
SamplesSiO2TiO2Al2O3Fe2O3MgOCaONa2OK2OLOI
Ropp49.571.5431.363.200.490.150.080.8212.24
ECC47.000.0338.000.390.100.030.150.8013.00
SKD50.001.0032.901.200.300.200.201.6012.60
Pharmaceuticals48.000.0236.000.100.200.010.101.1011.90
Ceramics48.000.3037.000.600.300.100.101.6012.40
Filling agent48.700.0536.000.820.250.600.102.1011.90
Fertilizer46.070.5038.070.330.010.380.270.4313.47
Porcelain48.000.0337.000.600.300.100.101.6012.40
Refractory bricks 51–701.00–2.8025–440.50–2.400.20–0.700.10–0.200.80–3.50--
Table 5. Trace element concentrations of whole-rock clays (AS) and the relative bedrocks (AY) (ppm).
Table 5. Trace element concentrations of whole-rock clays (AS) and the relative bedrocks (AY) (ppm).
SamplesAS1AS8AS9AS14AS16AS20AY3AY4AY6AY8AY10AY14
Rock TypeRBGBGMCABBGMBSTCABRBGRBGABGBGMBGUCAB
Sr12262.868.140.248.779.63.62042.52456.3513
Zr28466646387278457912736850799.515460.3
Nb18.624580.223974.832.914021.81348.918711.5
Sn17743.910.546.065.526.18.421.210.94.712.23.6
Cs27.52.11.71.60.070.96.27.72.514.68.814.2
Ba91110910934.682.41097.443714.1164620.9386
Hf7.525.312.057.823.316.811.410.318.52.89.02.0
Ta1.040.04.932.96.06.337.41.810.11.124.80.8
Tl5.00.30.80.20.00.22.91.71.11.02.30.7
Pb48.996.130.270.860.688.633.639.620.245.830.319.3
Th9.346.224.662.834.324.948.555.526.61.844.04.7
U3.214.54.913.63.64.65.77.05.91.612.72.2
Sc17.111.429.28.814.99.41.15.00.55.11.224.0
Co15.12.27.31.62.17.70.16.70.45.30.226.1
Li23927.075.717.414.117.637120.833.736.816968.1
Be13010.34.53.11.75.28.316.65.55.69.53.6
V96.563.220841.063.484.51.134.11.136.61.0194
Cr60.163.022365.870.475.00.79.81.16.20.887.7
Ni17.018.347.519.728.819.30.35.40.44.90.233.3
Cu12443.483.821.915.025.30.611.92.23.21.023.4
Zn154011597.310859.158.281.617585.951.250.1104
Ga36.487.754.712254.850.463.830.548.520.747.721.9
Rb67411.98.927.80.923.6804200269183557127
Table 6. Rare earth element concentrations of whole-rock clays (AS) and the relative bedrocks (AY) (ppm).
Table 6. Rare earth element concentrations of whole-rock clays (AS) and the relative bedrocks (AY) (ppm).
SamplesAS1AS8AS9AS14AS16AS20AY3AY4AY6AY8AY10AY14
Rock typeRBGBGMCABBGMBSTCABRBGRBGABGBGMBGUCAB
La44.668.267.114162.793.441.485.91229.980.019.2
Ce8014314917813418590.915915724.614942.7
Pr10.214.716.426.814.522.511.817.233.03.220.45.5
Nd37.952.459.477.249.379.736.158.512813.972.122.0
Sm8.59.710.911.18.112.09.69.429.44.317.94.9
Eu1.71.61.81.21.62.40.11.21.51.10.51.3
Gd7.810.48.88.76.67.58.86.727.04.417.84.4
Tb1.31.81.41.51.11.02.00.94.00.83.70.7
Dy6.611.77.89.56.34.713.74.823.05.425.44.1
Ho1.22.51.52.01.20.83.00.94.21.15.30.8
Er3.17.53.86.53.42.210.42.311.43.216.72.3
Tm0.41.20.61.20.50.32.00.31.60.52.80.3
Yb2.68.83.79.63.52.215.31.910.23.318.92.2
Lu0.41.30.51.40.50.42.20.31.40.52.60.3
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Yunusa, A.; Hong, H.; Salim, A.; Amam, T.; Liu, C.; Xu, Y.; Zuo, X.; Li, Z. Mineralogical Characterization and Geochemical Signatures of Supergene Kaolinitic Clay Deposits: Insight of Ropp Complex Kaolins, Northcentral Nigeria. Minerals 2024, 14, 869. https://doi.org/10.3390/min14090869

AMA Style

Yunusa A, Hong H, Salim A, Amam T, Liu C, Xu Y, Zuo X, Li Z. Mineralogical Characterization and Geochemical Signatures of Supergene Kaolinitic Clay Deposits: Insight of Ropp Complex Kaolins, Northcentral Nigeria. Minerals. 2024; 14(9):869. https://doi.org/10.3390/min14090869

Chicago/Turabian Style

Yunusa, Adamu, Hanlie Hong, Atif Salim, Tarig Amam, Chen Liu, Yanxiao Xu, Xiaochao Zuo, and Zhaohui Li. 2024. "Mineralogical Characterization and Geochemical Signatures of Supergene Kaolinitic Clay Deposits: Insight of Ropp Complex Kaolins, Northcentral Nigeria" Minerals 14, no. 9: 869. https://doi.org/10.3390/min14090869

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