Climatic control on clay mineral formation: Evidence from weathering profiles developed on either side of the Western Ghats

Many physico-chemical variables like rock-type, climate, topography and exposure age affect weathering environments. In the present study, an attempt is made to understand how the nature of clay minerals formed due to weathering differs in tropical regions receiving high and low rainfall. Clay mineralogy of weathering profiles in west coast of India, which receives about 3 m rainfall through two monsoons and those from the inland rain-shadow zones (<200 cm rainfall) are studied using X-ray diffraction technique. In the west coast, 1:1 clays (kaolinite) and Fe—Al oxides (gibbsite/goethite) are dominant clay minerals in the weathering profiles while 2:1 clay minerals are absent or found only in trace amounts. Weathering profiles in the rain shadow region have more complex clay mineralogy and are dominated by 2:1 clays and kaolinite. Fe—Al oxides are either less or absent in clay fraction. The kaolinite—smectite interstratified mineral in Banasandra profiles are formed due to transformation of smectites to kaolinite, which is indicative of a humid paleoclimate. In tropical regions receiving high rainfall the clay mineral assemblage remains the same irrespective of the parent rock type. Rainfall and availability of water apart from temperature, are the most important factors that determine kinetics of chemical weathering. Mineral alteration reactions proceed through different pathways in water rich and water poor environments.     

Chemical processes affecting alkalis and alkaline earths during continental weathering

The alkali and alkaline earth concentrations in the Toorongo Granodiorite weathering profile are controlled by two competing processes; leaching of cations from primary minerals during their degradation to clays; fixation, by exchange and adsorption, of the same cations onto the secondary clay minerals. Degradation and leaching dominate the early weathering stages whereas during the advanced stages, exchange and adsorption onto clays are of most influence.The alkali and alkaline earth compositional changes in the Toorongo Granodiorite weathering profile are typical of changes occurring during weathering of the continents, consequently the following generalizations apply to continental weathering. Ca, Sr and Na are most rapidly and most strongly removed (as dissolved species) during weathering of fresh continental rocks. Although large quantities of Mg are transported to the marine environment as dissolved species, appreciable amounts remain (fixed in secondary clay minerals) at the weathering site to be removed during mass wasting of continental weathering profiles. Large quantities of Rb, Cs and Ba, fixed in continental weathering profiles by exchange and adsorption onto secondary clays, are transported from the continents only during mass wasting of the continents.     

Rare earth elements in Quaternary clays of the Numedal area, southern Norway

Rare earth elements (REE) in the clay fraction (< 2μ) of the Quaternary deposits in the Numedal area, southern Norway, have been determined by a spark source mass spectrometric method. The REE content was studied in relation to weathering and sedimentological factors.

The total REE content varies from 100 to 1300 ppm. An average of the similar fraction of 16 non-marine clays, mostly tillitic, gave 527 ppm REE. An average of 38 glacial and postglacial marine clays from the lower part of the Numedal valley gave 335 ppm REE (max. 781 ppm). After removal of adsorbed ions the average total REE content of morainian and marine clays decreased to 186 ppm.

The content and distribution of the REE in the Numedal clays are strongly influenced by environmental factors. Under neutral and alkaline conditions the REE are accumulated by adsorption on clay minerals, and by increasing the hydrogen ion concentration the adsorbed ions are readily removed.     

江西龙南花岗岩稀土风化壳中粘土矿物的研究

本区燕山早期花岗岩发育的风化壳中的粘土矿物以高岭石和埃洛石(7Å)为主;蒙脱石、三水铝石及其它为新查明矿物。据粘土矿物组合特征,自风化剖面深部到地表分为三个带:含蒙脱石带,高岭石和埃洛石(7Å)带,含三水铝石带。本文探讨了矿物在风化过程中的生成演化顺序,并进行了热力学的解释。据各带粘土物质的阳离子交换量与稀土含量变化的不一致关系认为,稀土在C带中的富集是化学风化的结果,与粘土矿物组合无关。     

Some aspects of the Ca and Sr weathering cycle in the Lake Kinneret (Lake Tiberias) drainage basin

Sr/Ca ratios in the clay (< 2 μ) weathering products of basalt rocks from the Lake Kinneret drainage basin were found to be very similar to the ratios in the rocks, indicating similar depletion intensities for the two elements. In the clays from the scoriae and some lapilli tuffs the ratios were found to be somewhat higher than in the corresponding rocks, indicating preferential retention of strontium in the clays. The ratio in drainage water from these areas was much lower than that obtained by mass balance calculations.The Sr/Ca ratio in Jordan river water draining primarily limestone areas is 2.3 · 10^?3. Very high Sr/Ca ratios in Lake Kinneret water were attributed to the influence of saline sublacustrine springs. The carbonate fraction from recent lake sediments was shown to have the Sr/Ca ratios characteristic for calcite precipitated biogenically in marine environments. The relative enrichment in strontium of the lake water was explained by the intensive precipitation of carbonates with low strontium distribution coefficients.     

Shear resistance of fissured Neogene clays

Two types of fissured Neogene clays (SM and WB) were tested in the laboratory using undisturbed and reconstituted specimens. Although of similar age, the clays differ in their geological history: SM clays were deposited in a marine environment, WB are lacustrine clays with a complex tectonic history.

The index properties of both clays are similar and, accordingly, their strength parameters are mutually close. Test results of the more compact and less wet clay (WB) are affected by the sampling and trimming procedures, especially for 38 mm diameter specimens.

Although the behaviour is similar, the origin of fissures is different. For the SM clays, weathering is the main cause and fissuring reaches a depth of about 20 m. WB clays are dissected by macro- and microfissures originating as the result of tectonic processes and volumetric changes (effect of the coal formation underneath the investigated massif).

Cementation of some samples was found and the transition from clays through cemented clays to claystones (soft rocks) is speculated.     

黏土矿物古气候意义研究的现状与展望

系统地分析了利用海洋沉积物、古土壤、湖盆沉积物中黏土矿物进行古气候环境研究的现状、存在问题和发展趋势。海洋沉积物的物源范围广,影响因素复杂,其中的碎屑黏土矿物所指示的古气候参数只能用于解释母源区的气候变化,而只有自生黏土矿物才能指示沉积区的气候;古土壤形成于特定的地质背景条件下,尤其是发育于火山物质母岩之上的风化自生黏土矿物,可以准确地指示该区的古气候条件;湖盆沉积物的物源范围小,沉积物中的黏土矿物可以更有效地运用于古气候环境的分析。对于沉积物中黏土矿物来源的分析,可以借助晶体中cv空位和tv空位的精细结构特征进行判断;在风化改造的红土剖面研究中,因强烈的化学风化、淋滤和迁移,黏土矿物方法具有独特的优势。风化过程中形成的一些亚种或过渡性黏土矿物,以及同生沉积过程中形成的黏土矿物,对气候环境的变化更加敏感,应加强这方面的研究。此外,在造山带的气候环境演化研究中,自生黏土矿物稳定同位素可以更可靠地指示气候环境的变化。     

凹凸石粘土与玄武岩的成因关系

在矿物组成研究的基础上，对比研究了凹凸棒石，蒙脱石，凹凸棒石粘土，蒙脱石粘土，玄武岩，风化玄武岩的化学组成。根据化学成分对比，分析了玄武岩与凹凸棒石粘土的成因关系，解释了玄武岩对凹凸棒石粘土的时空控制，结合地质观测，提出形成苏皖地区凹凸棒石粘土的物质成分基本上来源于玄岩的化学风化，但物质成分发生了较大的分异，凹凸棒石粘土不能直接同玄武岩经地表雨水的风化淋滤形成，而是玄武岩风化形成的蒙脱石经搬运沉积，再与镁质溶液反应形成，或凹凸棒石直接从富镁碱性溶液中结晶形成，凹凸棒石粘土形成的物理化学条件为偏碱性的还原条件。     

珠江三角洲晚第四纪风化层稀土元素地球化学特征*

珠江三角洲地区上更新统与全新统之间广泛发育1层杂色黏土,其成因多认为主要是由上更新统沉积物在末次冰盛期暴露于地表风化而成。对取自珠江三角洲3条钻孔（PRD09、PRD16和PRD17）的岩心样品分析表明,受风化作用的影响,其稀土元素含量和分异特征发生了较明显的变化。杂色黏土层的稀土总量大大低于下伏沉积物,而在邻近风化层的下伏沉积物中稀土元素却表现为明显富集,尤其是重稀土元素的富集。风化作用强度较大的PRD09孔和PRD17孔下伏沉积物中的稀土富集程度高于风化作用强度相对较小的PRD16孔。珠江三角洲在末次冰盛期时普遍发育的酸性介质条件,促进了风化层的稀土元素发生溶解和迁移。在风化过程中,由于轻、重稀土元素具有不同的溶解迁移能力和吸附能力,导致杂色黏土层的REE指标值（LREE/HREE、(La/Gd)_N和(La/Yb)_N）高于下伏沉积物。风化过程对Ce、Eu异常有一定的影响,但不十分明显,杂色黏土层的Ce、Eu异常值仅略低于下伏沉积物。     

Hydrogen and Oxygen Isotopic Compositions of Sedimentary Clay in the Sea Region off the Western Antarctic Peninsula and Their Material Sources

In the geochemical study of sedimentation, hydrogen and oxygen isotope data are useful for a discussionon formation conditions and material sources of sediments. Hydrogen and oxygen isotope analyses were madeon clays separated from the sea-floor surface sediments at 9 stations in the western sea region of the AntarcticPeninsula. On the basis of these isotopic data and the clay mineral components and by use of statistical dia-grams, it may be concluded that the clay fraction in this sea region is composed mainly of terrigenous detritus:i.e. it was formed by surface weathering or supergene alteration of the rocks from the terrigenous region in thepeninsular in the presence of meteoric water. However, at least a part of montmorillonite is a marine authigenicmineral formed by alteration of submarine eruptive materials or terrigenous pyroclasts in the presence of seawater. The above result is consistent with the idea derived from the study of clay mineralogy.     

Mineral chemistry of clays associated with the Jhilmili intertrappean bed in the eastern Deccan volcanic province: Palaeoenvironmental inferences and KTB transition

Jhilmili intertrappean bed (～13 m thick) attains its significance with the recent discovery of brackish water ostrocod and planktonic foraminifera fossils (Keller et al. 2009; Khosala et al. 2009). Present XRD data revealed abundance of montmorillonite > montmorillonite/chlorite mixed layer > palygorskite in five physically distinct lithounits namely: (a) lower chocolate brown siltstone with green patches, followed by (b) brick red clayey siltstone, (c) greenish grey clay, (d) yellowish brown clay and (e) uppermost olive grey to dark brown silt layers in the successionrepresent higher weathering indices and annual precipitation, reflecting cyclic, but longer spells of weathering. Occurrence of M/C mixed layer with the smectite in Jhilmili area is suggestive of their derivation from the later, whilst montmorillonite to palygorskite transformation is ascribed to the drastic changes in the humid to arid climate, where former served as a source of Al and some of the Si and Mg ions for the later. Jhilmili and Anjar clays represent similar charge occupancies at different sites, but later contains relatively higher amount of palygorskite, formed in the arid environment. Majority of the trace elemental data plots for Jhilmili clays lie within the upper and lower limits of infra (Lametas)-/inter-trappeans clays. The continuous release of Cu throughout the succession (mainly in the palygorskite dominated clays) indicates oxidizing conditions. PAAS normalized REE data plots for these clays show progressive enrichment in the HREE contents in the lower part, but, upper part of the succession is marked by positive cerium anomaly, reflecting oxidizing conditions prevailed at the later stage of the succession. These conditions continued, but, were not conducive to HREE enrichment as evidenced by their depletion in the upper part of the succession. The Ce anomaly observed in the middle part of the succession is similar to that form by continental weathering of the basalt, ascribed to Ce precipitation in the oxic environment, thus suggestive of drastic changes in the oxidizing conditions. Thermodynamic data-sets for Jhilmili clays show I/S mixed layer and celadonite compositions, whereas, Jabalpur infra-/inter-trappean clays correspond to Mg smectites and celadonite end members, thus, representing compositional commonality with those of the other clays derived from the continental weathering of basalt protolith. Jhilmili smectites and smectite-chlorite mixtures show compositional similarity with the dioctahedral and trioctahedral smectites and the smectites formed at 250°C, having compositions between trioctahedral smectite and chlorite, thus, assigning high temperatures for their formation, where the heat required for the formation of these clays was possibly derived from the hydrothermal fluids, associated with the Deccan volcanism.     

苏皖沉积型凹凸棒石粘土矿床沉积环境探讨

本文对苏皖沉积型凹凸棒石粘土矿床形成时的沉积环境特征、水介质的ｐＨ值、氧化还原条件、温度及盐度进行了讨论。研究表明凹凸棒石粘土矿床形成于碱性、还原、低盐度的浅水湖相环境。与火山作用有关的热水溶液参与了成矿作用。     

Crystal chemistry of clay-Mn oxide associations in soils, fractures, and matrix of the Bandelier Tuff, Pajarito Mesa, New Mexico

The upper 25 m of Bandelier Tuff at Pajarito Mesa, New Mexico, include soils, shallow fractures, deeper fractures, and tuff matrices in which clays provide a record of transport and alteration. The principal pathways within this system are fractures that penetrate the tuff. Large fractures that host deep root penetration provide a setting in which clay deposits accumulate through particulate or colloidal migration from the soil zone. Clays throughout the system are predominantly expandable interstratified illite/smectites (I/S), but clays of the tuff matrix at depth are distinctly Fe-rich and are not mixed with clays transported from the surface into fractures. Chemical alteration superimposed on clay particles transported into fractures results in clays with lower Al : Si ratios, higher Na, and higher lanthanide content with increasingly negative Eu anomalies with depth. These changes are accompanied by invasion and precipitation of Mn oxides, principally birnessite, within clay bodies. Investigation of the Mn oxides by synchrotron X-ray fluorescence (SXRF) shows that Mn is associated with Ba, Ce, Ni, and Pb. In addition, synchrotron X-ray absorption near-edge structure (XANES) spectra show that Ce in Mn oxides occurs as Ce³⁺ and Ce⁴⁺, with average Ce oxidation state of ∼3.75. The Mn oxides intergrown with clays actively participate in removal of Ce from solution, accompanied by oxidation of Ce³⁺ to Ce⁴⁺. Other lanthanides are accumulated by the clays but are not concentrated along with Ce in the Mn oxides. Extraction of Ce from solution by Mn oxides is more effective than lanthanide accumulation in clay, a process that is variable and likely influenced by defects, extent of recrystallization, and particle sizes. This dichotomy in lanthanide interaction results in locally constant Ce content but either negative or positive Ce anomalies in the clay-Mn oxide system as a consequence of variability in the abundance of the other lanthanides. Nevertheless, the net lanthanide pattern for the sum of all clay-Mn oxide samples in either shallow or deep fractures has no Ce anomaly, indicating that other lanthanides segregated from Ce are not transported beyond the range of either the shallow or deep fracture systems. Evidence from Eu anomalies indicates that lanthanides accumulated in the fracture clays are acquired from the local tuff. The clay-Mn oxide assemblage is more effective than clay alone in accumulating of a wide variety of heavy metals.     

Tracing mechanisms controlling the release of dissolved silicon in forest soil solutions using Si isotopes and Ge/Si ratios

The terrestrial biogenic Si (BSi) pool in the soil-plant system is ubiquitous and substantial, likely impacting the land-ocean transfer of dissolved Si (DSi). Here, we consider the mechanisms controlling DSi in forest soil in a temperate granitic ecosystem that would differ from previous works mostly focused on tropical environments. This study aims at tracing the source of DSi in forest floor leachates and in soil solutions under various tree species at homogeneous soil and climate conditions, using stable Si isotopes and Ge/Si ratios. Relative to granitic bedrock, clays minerals were enriched in ²⁸Si and had high Ge/Si ratios, while BSi from phytoliths was also enriched in ²⁸Si, but had a low Ge/Si ratio. Such a contrast is useful to infer the relative contribution of silicate weathering and BSi dissolution in the shallow soil on the release of DSi in forest floor leachate solutions. The δ³⁰Si values in forest floor leachates (−1.38‰ to −2.05‰) are the lightest ever found in natural waters, and Ge/Si ratios are higher in forest floor leachates relative to soil solution. These results suggest dissolution of ²⁸Si and Ge-enriched secondary clay minerals incorporated by bioturbation in organic-rich horizons in combination with an isotopic fractionation releasing preferentially light Si isotopes during this dissolution process. Ge/Si ratios in soil solutions are governed by incongruent weathering of primary minerals and neoformation of secondary clays minerals. Tree species influence Si-isotopic compositions and Ge/Si ratios in forest floor leachates through differing incorporation of minerals in organic horizons by bioturbation and, to a lesser extent, through differing Si recycling.     

Radiolarites et sédiments hypersiliceux océaniques: une comparaison

The mineralogical and chemical composition of Jurassic radiolarian cherts has been studied in Morocco (Rif), Italy (Lombardy basin and Apennines), Greece (Pindus zone and Vourinos Massif), some in close association with ophiolites. We have compared these samples with Cretaceous cherts from the NW Pacific (Leg 32) and with Cenozoic diatomaceous oozes from the Sea of Japan (Leg 31). The silica in the radiolarian cherts is quartz or chalcedony. Most of these rocks also contain feldspars and hematite while the clay fraction is composed of illite and/or chlorite generally associated with swelling clays and, locally, with kaolinite. In oceanic sediments all mineralogical species of silica have been detected (from opal to quartz), the clays generally being the same as those of the radiolarian cherts, the feldspars also being present. Based on the chemical composition of the radiolarian cherts, three facies can be distinguished: massive cherts, pelitic radiolarites and ferruginous radiolarites, the latter occurring only near the contact with volcanic basement. The chemical composition of cherts and diatom oozes from the Pacific is very close to the composition of radiolarian cherts. Although the mineralogy of radiolarian cherts can be related to several models (detrital, diagenesis of pelagic clays etc.) the detrital origin of part of the clay fraction seems certain. The origin of silica and its relation to the palaeolatitudes and the relatively confined nature of the Tethys oceans as well as the influence of volcanic inputs are evaluated, Chemical and mineralogical composition of radiolarian cherts shows that the diagenesis of the clay fraction is not a significant source of silica. Accumulation of diatom oozes in the Sea of Japan and in other areas, shows that the distance from continents and very deep seas are not essential to the development of siliceous sedimentation.     

The Latnenskoe refractory clay deposit (Central Russia)

This paper reports the results of our recent studies and generalizes previously known data on the geology, mineralogy, geochemistry, and genesis of the Russia’s largest Latnenskoe refractory clay deposit. It is shown that conditions of its localization were defined by regional and local factors. The regional factors controlled the distribution of the clay raw material in the region, while the local factors were responsible for the genesis and composition of refractory clays of the Latnenskoe deposit. Our studies showed that the formation of refractory clays is not only related to terrigenous but also to authigenic processes of sedimentation. The terrigenous component of clays was formed by the erosion of kaolin weathering crusts of the Voronezh anteclise crystalline basement and Paleozoic sedimentary hydromica–kaolinite rocks. Authigenic processes were significantly contributed by organic matter, which determined the environmental pH and Eh parameters. It is established that the mineral matter of clays of the deposit is represented by three morphological modifications (crystalline, amorphous, and biomorphic), which were formed subsequently and (or) simultaneously and could be transformed into each other. Application of a complex of modern precision methods allowed us to reveal a previously unknown biomorphic modification of kaolinite, the major rock-forming mineral, as well as mixed-layer kaolinite-smectite in the clays. It is shown that the distribution of major and trace elements and the sulfur isotope composition in different technological types of clay depend mainly on the facies conditions of their formation. Technological properties of clay raw material are considered.     

Clay mineralogy, organic carbon burial, and redox evolution in Proterozoic oceans

Clay minerals formed through chemical weathering have long been implicated in the burial of organic matter (OM), but because diagenesis and metamorphism commonly obscure the signature of weathering-derived clays in Precambrian rocks, clay mineralogy and its role in OM burial through much of geologic time remains incompletely understood. Here we have analyzed the mineralogy, geochemistry and total organic carbon (TOC) of organic rich shales deposited in late Archean to early Cambrian sedimentary basins. Across all samples we have quantified the contribution of 1M and 1M_d illite polytypes, clay minerals formed by diagenetic transformation of smectite and/or kaolinite-rich weathering products. This mineralogical signal, together with corrected paleo-weathering indices, indicates that late Archean and Mesoproterozoic samples were moderately to intensely weathered. However, in late Neoproterozoic basins, 2M₁ illite/mica dominates clay mineralogy and paleo-weathering indices sharply decrease, consistent with an influx of chemically immature and relatively unweathered sediment. A late Neoproterozoic switch to micaceous clays is inconsistent with hypotheses for oxygen history that require an increased flux of weathering-derived clays (i.e., smectite or kaolinite) across the Precambrian-Cambrian boundary. Compared to previous studies, our XRD data display the same variation in Schultz Ratio across the late Neoproterozoic, but we show the cause to be micaceous clay and not pedogenic clay; paleo-weathering signals cannot be recovered from bulk mineralogy without this distinction. We find little evidence to support a link between these mineralogical variations and organic carbon in our samples and conclude that modal clay mineralogy cannot by itself explain an Ediacaran increase in atmospheric oxygen driven by enhanced OM burial.     

Structural-morphological features of kaolinite from clayey rocks subjected to different stages of lithogenesis: Evidence from the Voronezh anteclise

This paper reports the results of precision structural-morphological study of kaolinite from clayey rocks taken in various areas of the Voronezh anteclise subjected to different stages of lithogenesis: primary kaolins of the weathering crust, proluvial-talus and lacustrine secondary kaolins, as well as lacustrine-swampy fireproof and deltaic-lagoonal refractory clays. The clayey material was transported over more than 300 km. The formation of the fireproof and refractory kaolin deposits in the Voronezh anteclise was related to the Devonian and Early Cretaceous stages of the geological evolution of the region. In terms of spatiotemporal and facies features, the studied genetic series of the kaolin clay deposits is unique. It was established that the sequential structural-morphological evolution o kaolinite in the considered deposits was caused by its mechanical disintegration during transport and redeposition. Interrelation between organic and mineral matters in the fireproof clays was revealed for the first time. Experimental studies of the behavior of kaolinite during sequential grinding and heating confirmed the main reasons for its natural degradation. The formation of virtually monomineral kaolin clays was provoked by the “flow-through” diagenesis, which is similar to weathering in trend. Evolution of mineral matter of the considered genetic series in kaolinite clay deposits was accompanied by the increase of δ¹⁸O values and their dispersion. Peculiarities identified in the behavior of kaolinite and related oxygen isotope characteristics of different-aged denudation and redeposition products of the Devonian weathering crust can play an indicator role in studying different stages of the lithogenesis of clayey rocks.     

A COMPARATIVE STUDY OF GLAUCONITE AND THE ASSOCIATED CLAY FRACTION IN MODERN MARINE SEDIMENTS

The crystallinity and mineralogy of both the glauconite and the clay fraction of samples from six contemporary marine environments were investigated by X-ray diffraction. In those areas where glauconite is now forming, the mineralogy and the degree of crystallinity of both the glauconite pellets and the associated clay fraction are similar. In contrast, detrital and relic glauconites are observed to have mineralogies that are different from their clay fractions. No consistent relationship was observed between degree of crystallinity and color of the pellets. Further, only two classes of glauconite as defined by BURST (1958) were common: expandable, interlayered clays and two or more clay minerals in a mixed assemblage. Based on the clay fraction-glauconite similarities and other supporting evidence, glauconite on the Scotia Ridge is concluded to be authigenic. Glauconite from Santa Monica Bay, California and from the continental shelf off Morocco appear to be detrital. Glauconite pellets in the shelf sediments off Guinea and the southeastern Atlantic Shelf of the United States are both detrital and authigenic. The poor crystallinity exhibited by the Chatham Rise glauconite is in contrast to the well-crystallized associated clay fraction and indicates that they are not genetically related. However, the origin of this glauconite remains in doubt.     

安徽宣城红土剖面中粘土矿物过渡相及其意义

为研究长江中下游红土剖面中粘土矿物的特征及其成因意义, 对安徽宣城红土剖面中粘土矿物进行深入、系统的X射线衍射分析.结果表明, 宣城剖面各土壤层中粘土矿物成分基本一致, 主要为蛭石、伊利石、高岭石, 以及粘土矿物过渡相. 由采自剖面上部样品的X射线衍射图可知, 经乙二醇饱和后7 ?衍射峰可分解为7.15、7.60和7.92 ?三部分, 表明除了高岭石(7.15 ?)外, 还存在高岭晶层含量分别为~80%和~95%的2种高岭-蒙脱石过渡相, 并以前者为主; 剖面下部样品在乙二醇饱和后, 7 ?衍射峰可分解为7.16、7.79和8.35 ?等3个衍射峰, 其中8.35 ?峰衍射强度很小, 表明除了高岭石外, 样品中存在高岭晶层含量为~90%和~43%的高岭-蒙脱石过渡相, 后者含量甚少.甲酰胺饱和结果表明, 高岭-蒙脱石混层粘土矿物相中高岭晶层为埃洛石相.加热试验的衍射图中10 ?衍射峰强度明显增强, 证实高岭相中含有一定数量的来源于绿泥石风化的蒙脱石间层; 而10 ?衍射峰的低角度一侧没有出现拖尾现象, 则指示高岭-蒙脱石混层矿物中的蒙脱石不是简单的羟基间层蒙脱石.此外, 红土剖面中还普遍出现过渡性粘土矿物伊利石-蒙脱石混层和伊利石-蛭石混层粘土矿物.大量过渡性粘土矿物相的出现, 从成土作用的角度上说明红土沉积物经历了沉积-风化、以及多期风化作用叠加, 而且在沉积-风化成土过程中, 气候环境变化于强烈化学风化的温暖、季节性干旱和强烈风化淋滤的温暖而更加潮湿的条件.蛭石-伊利石混层粘土矿物仅发育于红土剖面上部, 表明总体上剖面上部的化学风化程度低于剖面下部.