Kaolin: Soil, rock and ore: From the mineral to the magmatic, sedimentary and metamorphic environments

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• 作者：Harald G. Dill ^{haralddill@web.de" class="auth_mail" title="E-mail the corresponding author}
• 关键词：Kaolin ; Soil ; Rock ; Ore ; Climate ; Mineral ; Environment analysis ; Magmatic/primary ; Sedimentary/secondary ; Metamorphic/tertiary ; Classification ; Field-laboratory methods ; Economic geology
• 刊名：Earth Science Reviews
• 出版年：2016
• 出版时间：October 2016
• 年：2016
• 卷：161
• 期：Complete
• 页码：16-129
• 全文大小：33617 K

文摘

The kaolinite-group minerals kaolinite, dickite, nacrite, halloysite including its metaform and their associates allophane and imogolite are phyllosilicates characterized by a rather simple chemical composition of Si, Al, O, and H. These elements contribute for the most part to the built-up of the continental earth crust, which down to a depth of approx. 1.5 km, consists of 75% of sedimentary rocks. In a sense, kaolinite-group minerals accommodating these elements in their structure chemically reflect the uppermost part of the crust. It is not a surprise that kaolin is very widespread particularly in those sedimentary rocks which came into existence under near-ambient conditions and, as a further consequence, is a mirror image of those processes taking place in the topmost parts of the crust. In a tripartite subdivision (primary: magmatic/structure bound, secondary: sedimentary, tertiary: metamorphic), the following environments bearing kaolin exist:

Primary environments of kaolinization: (1) Vein-type deposits, (2) (sub) volcanic and pyroclastic deposits, (3) skarn to epithermal deposits, (4) granitic rocks and their affiliated rocks (pegmatites and greisen).

Secondary environments of kaolinization:(1) kaolin and soil (ferralsols, plinthosols, nitisols, podzols, vertisols, andosols), (2) layered residual kaolin deposits (mixed-type residual kaolin-bauxite deposits, exposed residual kaolin, hidden residual kaolin), (3) vein-like kaolin, (4) alluvial-fluvial environments (alluvial fans, fluvial braided streams, fluvial meandering streams),(5) prograding fluvial deltas (prograding into a playa (dry delta), prograding into a marine or lacustrine basin (wet delta)),(6) lakes and ponds (permanent and ephemeral lakes), (7) coal-bearing environments (suspended load deposits in coal swamps, underclays, composite residual and hydrothermal kaolinization in coal swamps, kaolin tonsteins), (8) marine terrigenous shoreline deposits (open - tide-dominated estuary, blind - wave-dominated estuary, sealed-off lagoon).

Tertiary environments of kaolinization: (1) burial diagenesis, (2) very-low grade regional dynamo metamorphism, (3) contact metamorphism.

The above tripartite subdivision of kaolin has been established so as to be in accordance with other lithologies which formed through magmatic, sedimentary, and metamorphic processes and to link the present classification scheme directly with the “Chessboard classification scheme of mineral deposits” (Dill, 2010b). While in many classification schemes of mineral deposits kaolin and its minerals were only considered as an “ore” in the category non-metallic deposits and industrial minerals, in the current review the barriers between economic geology and its neighboring disciplines like sedimentology, pedology, geomorphology, petrography and palaeoclimatology have been torn down and the kaolin looked at from different angles, as soil, rock and ore.

Kaolin and kaolinitic clays are taken as the type-lithology of the near-surface continental environments. Together with bentonites, bentonitic clays and a varied spectrum of argillites they form part of a group of lithologies, encompassing besides bauxite, ferrites (ferricretes) and laterite, all of which developed close to the interface between atmosphere, pedosphere, hydrosphere, and lithosphere.

To accentuate the intimate relation between the various lithologies mentioned in the previous paragraph, a classification scheme has been designed. It makes use in part of pre-existing ternary plots to take also account of these interferences with the different geoscientific disciplines such as sedimentology or pedology. In addition to that, an overview of the various field and laboratory methods to identify and quantify kaolin/kaolinite-group minerals is given.

The succeeding parameters, features and settings are crucial as to the kaolinization in the magmatic, sedimentary and metamorphic environments.

Geodynamic setting

The sites most favorable to develop large (economic) kaolin deposits are located along the passive continental margin and in epicontinental basins. Almost all of the kaolin deposits have to be attributed to the secondary deposits.

Rate of uplift and weathering

Cratonic crustal sections stable over a long period of time with little vertical displacement are crustal sections favorable for kaolin formation and preservation. During slow uplift, chemical weathering operative in the peneplained hinterland and on the sedimentary bodies in the foreland helped to decompose labile constituents from the parent material and enhance the quality and increase the thickness of the kaolin. Reducing the slope angle or the paleogradient, i.e., moving from the alluvial-colluvial fan system towards deltaic and swampy environments raises the likelihood of kaolin concentration of economic grade.

The drainage pattern and hydrography

The fluvial drainages system most proximal to the residual kaolin and most favorable for kaolin is the braided-stream drainage system. In the meandering-river system two different types “bar sand kaolin” and the “overbank kaolin deposits” occur. Kaolin accumulation may be tracked down to the coastal marine deposits under humid climatic conditions. Rivers are accountable for a steady supply of suspended load, the tidal processes is held most effective in concentrating the fine-grained raw material and wave action in combination with the transgression and regression of strandlines act as a seal and preserve the kaolin deposit. The tidal analogues developing under arid climatic conditions, also known as coastal sabkha, are of no relevance for kaolin concentration. The most well-balanced state between concentration and preservation of kaolin is achieved in the blind estuary under a mesotidal regime.

Organic matter and the redox regime

The organic matter has no effect on the formation of kaolin in the primary deposits. In the secondary kaolin deposits the mediating effect increases from the residual kaolin deposits towards the coastal marine deposits. It shows “local positive anomalies” in “wet systems”. Near the basin edge and the hinterland kaolin forms in the upper oxidized vadose zone and more basinward be it lacustrine or marine it prefers the lower reduced part, also called the phreatic zone, of the hydrosphere. Organic matter in metamorphosed black shales has not only a “buffering” effect on the kinematic processes but also a preserving or stimulating effect on kaolinite to form as long as the critical temperature between low- and very-low grade stage metamorphism is not crossed. The variation in the redox conditions is well represented by the valence state of iron in its minerals, e.g., siderite vs. goethite and by the REE. Among the chemical methods, REE anomalies (cerium) can assist in constraining the redox conditions.

Temperature and pressure control

Only in magmatic kaolin deposits a correlation between the type of kaolinite-group minerals and the temperature is evident. The temperature of formation has been demonstrated to have an influence on the crystallinity of kaolinite and as a “rule of thumbs” the following succession can be given provided a T-controlled kaolinization can be confirmed: Allophane + imogolite ⇒ halloysite ⇒ kaolinite ⇒ dickite ⇒ nacrite. Frequently only part of the sequence is realized in nature. To distinguish hypogene from supergene kaolin, O and H isotopes have proved to be a successful tool, in that these isotopes assist to constrain the temperature of kaolinization. In the course of deep-burial diagenesis, kaolinite is not indicative of a mineralogical depth-response reaction but rather a marker for breaks or hiatuses during basin subsidence. As the intraformational solutions got spoiled by the immigration of fluids from whatever per ascensum and per descensum processes the resultant disequilibrium within the reservoir rocks has to be offset in case of a drop in pH by the formation of kaolinite-group minerals. Lithostatic pressure has been demonstrated to have no effect on kaolinization. Shearing stress is detrimental to the formation and preservation of kaolinite-group minerals. Structure bound kaolin has always been found to be post-kinematic and no shear zone has been mapped where kaolin is pre- or synkinematic. The presence of kaolinite in zones of structural disturbances or elevated stress and strain is due to infiltrations of meteoric and low-salinity waters.

Chemical composition of fluids

The chemical composition, as far as the content of alkaline earth, alkaline elements and the pH value are concerned, plays a significant role in the course of kaolinization. These fluids are accountable for the transformation parent rock ⇒ kaolin ⇔ bauxite/ferricretes/laterites (with the option of resilicification) and in pedological terms for the “cold kaolin” (humid mid-latitude climatic zone) and “warm kaolin” (humid tropical climatic zone). Ferralsols, plinthosols, nitisols, podzols (also present under these tropical climatic regime), andosols and vertisol are typical of the climatic zones near the equator. These fluids do not only facilitate the formation of kaolinite but also enhance the quality of kaolin by depleting the argillaceous saprolite from impurities such as Fe and Ti. As a result of pH changes the distribution of labile and intermediately stable heavy minerals negatively correlates to the clay mineral variation. In the proximal parts of the basin, where the amount of kaolin is high, the prevailing heavy minerals belong to the group of ultrastable minerals such as rutile, tourmaline, and zircon, whereas more basinward where the kaolinite contents decrease in a vadose system, apatite and labile ones become the most important heavy minerals. It provides a clear proof for an increase of the alkalinity of the basinal fluids more basinward. The opposite trend is observed in the phreatic zone as the redox-sensitive heavy minerals of the Fe-Ti system are considered. They show a positive trend along with kaolin from the basin edge to the basin center, particularly in terms of the valence state of Fe, which changes from the trivalent to the bivalent state.

Parent material

Kaolin can evolve on almost all lithologies. The likelihood increases with an increase in Al and Si, but even lithologies depleted in these elements such as carbonates form, in places, a good substrate to develop kaolin, provided enough time was given to the process of alteration and the climatic regime and geodynamic setting work hand in hand. The only lithology and environments which fail to bring about kaolin deposits are salt deposits and zones of extreme evaporation. The climatic and hydrological conditions are more crucial as to the formation of secondary kaolin deposits than the composition of the parent material.

Kaolin is a non-metallic raw material which has taken a position covering the entire spectrum from high-place value commodities when used for ceramics (e.g. bricks, earthenware) to the high-unit-value commodities in use for the body of the finest chinaware or even pharmaceuticals. Due to its wide range of final applications, methods and geophysical techniques (PIMA) are currently enhanced to preselect high quality kaolin and get rid of ingredients detrimental to the (high) end product. Requirements of kaolin are discussed in this review as to its use for different final products. Attention has been paid to the formation of geogenic dioxin in sedimentary/secondary kaolin which is closely related to the environment.

Kaolin is not only a raw material for one of the finest products, porcelain, but also a type mineral for a varied spectrum of environments of deposition characterized by a rather sharp upper limit of formation of 390 ± 10 °C at a pressure of 2000 bar. For all geoscientists dealing with processes below this physical-chemical boundary, the role of kaolin and its mineralogical constituents as marker cannot be overestimated.