Abstract Kaolin is an important industrial mineral and geological indicator. It occurs in hydrothermal, residual and sedimentary deposits. The first two are classed as primary occurrences and the third as secondary. The physical and chemical conditions under which kaolins form are at relatively low temperatures and pressures. The most common parent rocks are granites and rhyolites and the most common parent minerals are feldspars and muscovite. The kaolin minerals are kaolinite, halloysite, dickite and nacrite and by far the most common is kaolinite. The physical and chemical properties of kaolin determine its ultimate utilization. Some kaolins can be used as paper coating clays, some as filler clays in several industries, some for ceramics and refractories and some for special uses. Important kaolin deposits that are briefly described are those in Australia, Brazil, China, Czechoslovakia, England, Germany, Indonesia, Japan, Mexico, New Zealand, Spain, Ukraine, and the United States. The uses of kaolins are governed by several factors including the geological conditions under which the deposits formed, their mineralogical composition and physical, chemical and optical properties. The variable properties and uses and the differing occurrences and mineralogy explains why the title “Kaolins, Kaolins and Kaolins” was selected.

Abstract Kaolinite has a distorted 1 M sequence of layers with the octahedral vacancy said to be at the B site in every layer. The O..H vectors of the surface OH groups are quasinormal to (001), and that of the inner OH is parallel to (001) in space group C1. The major defects present in kaolinite are stacking mistakes that are related to the usual − a 1 /3 interlayer shifts by a pseudo-mirror plane along the diagonal of the unit cell base plus the presence of a few vacancies in the C sites. Single crystals are always twinned by ±120° rotations about the cleavage normal to create equal volumes of three domains of different orientations. These may form by random adoption of sites A,B,C as the vacancy during initial growth, followed by distortion of each domain to triclinic geometry. After distortion, the vacancy appears to be at the same site in each domain when using conventional triclinic axes, but with the domains rotated by 120°. The designation of B as the usual vacant site by recent authors is considered an error in nomenclature, with the vacancy really in the C site. Dickite has the same 1 M stacking sequence of layers as kaolinite, but the vacancy alternates regularly between B and C. Nacrite has the R sequence of layers. The octahedral vacancy alternates regularly between B and C, but every other octahedral sheet is rotated 180°. The pattern of vacancies reduces the symmetry to monoclinic and allows selection of an inclined Z axis along which there is a 2-layer repeat. Halloysite has mainly irregular layer stacking but with a limited tendency for 2M 1 stacking in small domains. The presence of interlayer water may be related to the presence of small amounts of Al(IV) that are balanced by exchangeable cations. Rolling of the layers into tubes as a result of lateral misfit of the component sheets is possible only if tetrahedral rotation is blocked by insertion of H 2 O or exchangeable cations into the ring openings or by dynamic disorder of the water molecules to attract the basal oxygens to rotate in different directions at the same time. Rotation may occur after dehydration and may explain the irreversibility of dehydration. Platy morphologies result when sufficient substitution of Fe 3+ for Al increases the lateral dimensions of the octahedral sheet to fit those of the tetrahedral sheet. Spheroidal morphology is due to specific growth conditions.

Abstract The industrial utility of kaolin covers a diverse range of applications which can be classified into six categories according to primary functions: kaolin in film formation, kaolin in fiber extension, kaolin in polymer extension and reinforcement, kaolin use by virtue of chemical composition, kaolin as a carrier, adsorbent, diluent, etc., and kaolin as a polishing agent. By far the most important use of kaolin is in film formation, especially paper coating, which commands the most stringent kaolin specifications. These include brightness, particle size, particle size distribution, particle shape, and rheology. Along with fine particle size, the most important property of kaolin for film formation is the thin platelet shape which for Georgia kaolins generally do not exceed about 0.15 μm in average thickness. Calcined kaolins for paper applications must be bright, fine in particle size, and have aggregate structures with high internal void volume for enhanced light scatter. Excluding rheology, the same specifications are necessary for kaolin in fiber extension such as paper filling. For polymer extension and reinforcement, such as in plastics and rubber, surface chemistry and particle size are of primary importance. The finer the particle size the better the reinforcement, but dispersion is essential to realize fully this benefit. For best dispersion, the polarity of the kaolin surface should approximate that of the polymer and can be achieved by surface treatment. In some systems silanes act as coupling agents giving a strong covalent bond between the kaolin surface and the polymer, thereby offering maximum reinforcement to the composite. Where kaolin use involves reconstitution, as in catalyst supports, cement, fiber glass, and for the production of aluminum compounds, chemical composition is most critical. As a carrier for pesticides and pharmaceuticals, catalytic activity promoted by Lewis acid and Brõnsted acid sites on kaolin surfaces can affect transformation of some organic substances. Fine particle calcined clays, subangular to subrounded, are efficient polishing agents for teeth, automobiles, and soft metals such as gold and silver. The wide utility of kaolin is a function of a broad composite of characteristics but the most important are low cost, high brightness, fine particle size, platelet shape, and hydrophilic surface chemistry.

Abstract The research of W. D. Keller was influential in resolving questions concerning the origin of the Georgia-South Carolina kaolin deposits. Keller's research on the factors controlling the formation of kaolinite is the cornerstone for our understanding of the origin of these unique kaolin deposits. The development of the ideas central to our understanding of these deposits are outlined in this paper. The present view of the origin of the soft kaolin deposits is summarized as follows. Kaolinite was formed by the alteration of feldspar and mica in coarse grained metamorphic and igneous rocks of the Piedmont Plateau and associated arkosic sands. Kaolinitic sediment from the crystalline rocks was carried southeast toward the Cretaceous shoreline. Subsequently, rapid erosion of non-altered crystalline rocks led to the deposition of arkosic sediments with partially altered feldspars. Repeated exposure to the harsh weathering conditions resulted in the alteration of most of the feldspar to kaolinite. The transportation process led to the natural separation of significant amounts of quartz, feldspar, and mica from kaolinite. The kaolin rich sediments were deposited in fresh to slightly brackish quiet water environments associated with fluvial-deltaic settings at the Cretaceous shore. Deposition of kaolinite in fresh water induced face-to-edge flocculation and an open pore structure. Continued rapid addition of fluvial sediment quickly covered the fine material in the sedimentary sequence. Subsequently, groundwater altered the remaining feldspar to kaolinite and this facilitated the formation of large vermicular crystals. The hard kaolin deposits formed in a different manner. The sea level changes during the late Cretaceous to early Tertiary eventually exposed fine grained metavolcanic rocks of east Georgia and South Carolina to intense weathering conditions. Under these conditions kaolinite formed from fine grained mica in phyllite and fine grained schist. The kaolin rich sediments were transported to the Eocene shoreline and were deposited in brackish to marine environments. Deposition in saline water caused face-to-face flocculation and resulted in a tightly packed sediment that was not easily altered by groundwater.

Abstract Three gray kaolin sites in Georgia were cored to investigate by X-ray diffraction, radiography, fluorescence and scanning electron microscopy the processes involved in kaolin deposit formation. Kaolinite crystallinity increased and mica content decreased in those regions of the cores containing abundant kaolinite vermiforms. Excess total Fe above that allocated to sulfides correlated well with K content in all reduced kaolins sampled. Allocation of the Fe in the deposit based on the K-Fe correlation suggests that much of the Fe in gray kaolins is present in an Fe-bearing mica impurity. Calculation of the Fe content of the mica using the K as a measure of mica content yields an Fe concentration of 2.9% as Fe 2 O 3 for the mica in samples from both the Cretaceous and Tertiary kaolins. This concentration corresponds to 1 octahedral site in 14 filled by Fe in muscovite. If the remaining Fe is allocated to kaolinite, it would contain 0.15% Fe 2 O 3 (i.e. 1 of every 400 octahedral sites contains Fe). The results of this investigation support a hypothesis that during the development of kaolin deposits Fe is released by the dissolution of mica and is leached or precipitated as sulfides while some of the kaolinite is altered to vermiforms of higher crystallinity.

Abstract The morphologies, textures, and compositions of minerals and amorphous materials from a saprolite located in Saline County, Arkansas were examined by scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS) supplemented by X-ray powder diffraction (XRD). As shown by Walter Keller in a landmark series of papers, SEM/EDS analysis is necessary (in conjunction with XRD) to reveal complex textural sequences and paragenetic relationships that are crucial to understanding saprolite genesis. Of particular interest are three amorphous materials identified by their distinctive textures and compositions. The three abundant amorphous textural types are likely composed of mixtures of allophane and other amorphous materials. These amorphous weathering products were precursors to halloysite, goethite, and other minerals of the saprolite. The saprolite also contains an illite-like (10 Å) mineral. The gradational boundary between the saprolite and the overlying kaolinite-rich clay is marked by a switch from a system that produced principally kaolinite to one that formed halloysite. A model of saprolite genesis is developed based on previous experimental and theoretical investigations. The observed sequence of amorphous materials and secondary minerals can be related to the coordination of aqueous aluminum and to solution saturation, both of which are pH dependent. The role of A1 coordination in the crystallization of kaolinite and halloysite is not clearly defined, but experimental evidence suggests that halloysite may form at higher aqueous A1 IV :A1 VI ratios than kaolinite. Illite only occurs in saprolite beneath a continuous layer of iron oxyhydroxide suggesting that restricted ground water flow allowed illite to precipitate.

Abstract The degree that sedimentary kaolin properties are influenced by primary source materials has rarely been fully determined. Samples of sedimentary kaolin, primary kaolin, and crystalline rock were collected from Latah County, Idaho to examine the mineralogical and geochemical relationship between sedimentary kaolins and their primary kaolin provenances. The geological setting of the Latah County kaolins establishes a connection between sediment and provenance. The parent materials for the sedimentary kaolins are saprolites developed on Idaho Batholith granodiorite and Belt Group metamorphic rocks. Kaolinitic sediments were eroded from these saprolites during the Miocene and were deposited in lakes formed in valleys dammed by flows of Columbia River flood basalt. Volcanic ash deposited within these lakes devitrified to tonsteins containing spherical halloysite. Mineralogy, crystallinity, and particle morphology were evaluated by combined X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). Elemental abundances were determined by ICP. Stable isotopic ratios of oxygen and hydrogen extracted from kaolins were obtained by mass spectrometry. The sedimentary kaolins contain tubular halloysite distinctive of the igneous saprolites and kaolinite stacks distinctive of the metamorphic saprolites. Some sedimentary kaolin strata contain halloysite spheroids emplaced from the volcanic source. Sedimentary kaolin crystallinity may correlate with content of kaolinite stacks derived from the metamorphic saprolite. The ratio of Ba, La, Sr, Th, Zn, Fe 2 O 3 , A1 2 O 3 , and alkalies concentrations are explained by mixing sediment from all three sources. Oxygen isotope values are explained by mixing sediment from the saprolites and the volcanic source. Hydrogen isotope values indicate exchange between the kaolin and pore fluids. Results show the saprolite and volcanic provenances may influence Latah Formation sedimentary kaolin mineralogy, particle morphology, chemistry, oxygen isotope ratios, and crystallinity. Therefore, provenance could significantly influence the physical properties of a sedimentary kaolin.

Abstract The Variscan granite batholith of Devon and Cornwall is host to extensive deposits of primary kaolin, known in Britain as china clay, which support an industry supplying over 3 million t.p.a. of product. This is the principal source of kaolin for W. European paper and ceramic markets. The genesis of these deposits took place in six stages. The initial stage was the intrusion of the main batholith at 290 Ma, a S-type biotite granite rich in heat producing radioelements; followed by limited Sn/W mineralisation (Stage 2). Stage 3a was the intrusion of an evolved Li-B-F rich magma at 270 Ma., low in colouring elements (Fe, Ti), which only reached the upper surface of the batholith in a significant way in the western part of the St. Austell granite intrusion, but probably underlies the batholith throughout most of its length. This was accompanied by the mainstage hydrothermal Sn/Cu mineralisation and associated greisenisation and tourmalinisation (Stage 3b). Intrusions of felsitic dykes (elvans) brought this episode to a close (Stage 3c). The ensuing Stage 4 cross-course mineralisation involved saline, lower temperature fluids without boron, and radiogenically driven convective circulation became the dominant hydrothermal mechanism. Argillation followed permeable zones established by tectonism and earlier hydrothermal activity. Because of the saline nature of the fluids, the clay mineral assemblage was dominated by smectite and illite, with only limited amounts of kaolinite. Flushing of the system by meteoric water, following the transition to a wetter climate in the Mesozoic allowed pervasive circulation of warm fresh water, which converted the clay minerals and feldspars to the kaolinite dominated assemblage we see today (Stage 5). Continuous solution and recrystallisation of the kaolinite led to leaching of colouring oxides and a steady increase in lattice order. Large authigenic curled stacks of kaolinite also formed in the matrix. Stage 5 merged into Stage 6, which is the deep weathering in Mesozoic and early Tertiary times which affected much of Europe. There is evidence that convective circulation and kaolinisation are still proceeding slowly today. Fortunately erosion has not stripped too much of the soft kaolinised granite away and the deposits, as seen today take the form of funnels or tabular (on edge) bodies with depths exceeding 200m in places. Most of the worthwhile china clay deposits are in or close to the intrusions of the Stage 3 lithium mica granite, notably in the western part of the St. Austell granite and the south-western edge of the Dartmoor granite.

Abstract Kaolinization of Hercynian granites in southwest England has resulted in the extensive development of china clay deposits. These deposits typically have a funnel-shaped form often more than 200m in diameter and extending to depths greater than 300m. They occur along major northwest - southeast trending faults and fractures, and have formed at sites where high permeability channelways control descending limbs of radiogenically-driven hydrothermal circulation cells. The presence of U in this environment is important for three reasons: U is the main source of radiogenic heat, changes in the abundance of U and its mineralogical distribution with degree of alteration provide evidence of the nature of the alteration processes, and any disequilibrium in the U decay series can be used to date the occurrence of the alteration processes. The reduction in density from about 2650 kg/m 3 for fresh granite to about 1900 kg/m 3 for highly-altered granite provides the basis for a suitable Kaolinization Index to quantify the degree of alteration. Results of gamma-ray, geochemical, autoradiographic and radioactive disequilibrium studies of U in relation to Kaolinization Index show that: an early stage of high temperature alteration preceded kaolinization, the kaolinization process is still taking place, and recystallization of kaolinite appears to be on-going. The overall picture is one of an active system which, at the present day, is of low energy. Evidence of more vigorous activity in the past suggests other geothermal and seismic phenomena have reinforced the system on several occasions.

Abstract Dickite has previously been reported in “varicolored clays” of different ages in the Central and Southern Italian Apennines and in Sicily. Recently, dickite has been observed to occur randomly in Cretaceous “varicolored clays” belonging to the Scabiazza sandstone formation in the Oltrepo Pavese area of the province of Pavia (Northern Apennines). Field observations, crystal-chemical characteristics (XRD and EDS analysis), morphological and fabric features (SEM observations) of the dickite platelets support an authigenic origin by precipitation from pore solutions. Controversial interpretations concerning the nature of such interstitial solutions, and the origin of dickite include: diagenetic squeezing in an almost closed chemical system during tectonic events; supergene origin in an open system, thermodynamically dependent on the physico-chemical characteristics of the solutions, which may be mixed with meteoric waters.

Abstract In Northland, New Zealand, several large deposits of hydrated halloysite clay have been formed by the in situ alteration of rhyolite volcanics of Pliocene or Pleistocene age. The rhyolites were erupted through pre-Pliocene sedimentary strata and locally through basalt. They are onlapped in places by more recent basalt flows. The halloysite clays have been formed from both hydrothermal alteration and sub-tropical weathering. The clays are mined by open-pit methods and processed by crushing and fractionation to a product of 98% finer than 2 μ m particle size, which is of exceptional whiteness and brightness. The unique properties of these halloysites has led to their world wide utilisation in specialised ceramic applications.

Abstract A sedimentary kaolinitic sand deposit near Maoming, Guangdong Province, China has potential as a paper coating clay. Laboratory tests of bulk samples from two major mines in the Maoming deposit, the Shange mine and the Jintang mine, show that the kaolin is composed essentially of well-crystalline, low Fe and Ti, relatively pure kaolinite. The Shange sample has a very fine particle size, high brightness, low viscosity and low abrasiveness, and therefore is suitable for use as a coating clay. The Jintang sample has the quality to be used as a filler clay and with special beneficiation treatments can be processed into coating quality.

Abstract Paramagnetic impurities, Fe 3+ , Mn 2+ and vanadyl (VO 2+ ) ions, along with paramagnetic radiation-induced defect centers, trapped holes, have been investigated with electron paramagnetic resonance (EPR) in more than 350 kaolin samples from different localities. The trace elements determined are either substituted (Fe 3+ ) or adsorbed (Mn 2+ and VO 2+ ). The thermal stability of the radiation-induced defects has been determined. The genetic significance of the EPR spectra has been considered on the basis of previous data from the literature and new informations obtained in this study. It is shown that paramagnetic centers: (1) can be used to differentiate kaolinites from the three major environments at the Earth’s surface, hydrothermal, weathering and sediments; (2) are indicators of growth conditions of kaolinite; (3) provide a basis for interpreting conditions of kaolinite formation in different environments; and (4) fingerprint accurately several stages of kaolinite growth and/or successive geochemical events.

Abstract The mineral referred to as kaolinite/smectite has been identified in soils and paleosols from Holocene to Pennsylvanian in age (and perhaps older) and most fireclays, ball clays, and other poorly crystallized kaolinites. It is a mixed-layered mineral composed of an expandable 2:1 (9.7Å, collapsed) layer and a kaolinitic 1:1 (7Å) layer. Models of bonding between layers or units of this mineral can be calculated by using stacking sequences of three structural modules-2:l layers of parent material, 1:1 layers, and 2:1 layers hydrogen bonded to a 1:1 layer. This structural modeling suggests constraints on mechanisms of kaolinitization and types of interlayers that will expand, bond, form stable crystallites, or some combination of these. It also suggests possible ordering mechanisms and limits to particle size. Layer charge on the 2:1 parent material and on the 2:1 and 1:1 alteration products ranges from the maximum of that for illite to a minimum near zero. The wide range of possible layer charge on the 2:1 layer suggests that mixed-layered kaolinite/expandables ( K/E ) would be a better name than kaolinite/smectite. The mechanism of formation of K/E is poorly understood. Our evidence supports a mechanism in which tetrahedral sheets are stripped from 2:1 layers and bonding occurs between the hydroxyl sheet of the newly formed 1:1 layer and the adjacent 2:1 layer. Inherited octahedral and tetrahedral substitutions may result in atypical 7 Å layers. Growth of K/E may be terminated by encounters between crystallites with opposite c* directions of their 1:1 layers or by strains resulting from inherited substitutions within the 7 Å layer. Pedogenic K/E occurrences are correlated with iron substitution within kaolinitic layers of K/E. Where order can be determined, we have observed ~R1.5. R1 has been reported in the literature. There appear to be two continuous genetic sequences within the kaolin group: 1) a series from allophane through halloysite, and 2) a series from 2:1 parent materials through K/E . Transformations from halloysite or K/E to well-crystallized kaolinite probably require recrystallization and therefore the last step in both sequences is discontinuous. Conceptually and structurally, we can make several useful comparisons between smectite to illite or illite to smectite transformations and the 2:1 to K/E transformation. Detection of kaolinite/expandibles is readily made by XRD studies of < 2 µ m or finer fraction samples after air drying, ethylene glycol solvation, and a heating routine (300°C, 350°C, 400°C). K/E with a composition near kaolinite-a peak near 7Å--can be distinguished from halloysite and well-crystallized kaolinite by the rapid intercalation of the latter two phases by many agents. This intercalation shifts the kaolinite and halloysite peaks to the 10Å to 12Å area of the diffractogram and leaves only the peak for K/E in the area between 7Å and 10Å. The 17Å peak of expandable-rich K/E (001 kaol /001 exp ) with ethylene glycol solvation is extremely broad even at low percentages of 7Å interlayering. This peak broadening distinguishes K/E with a low proportion of kaolinitic layers from smectite and I/S peaks near 17Å. After heating to 300°C, elevated background intensity or a peak on XRD traces between the 7Å and 10Å positions is the most sensitive diagnostic method to detect K/E. Loss of the high-angle, K/E shoulder on the 10Å peak (assuming one or more discrete 2:1 phases are present) after heating to 350-400°C can be an equally sensitive method for detecting and quantifying K/E. The difficulty of detecting K/E, especially samples with low contents of kaolinitic interlayers, suggests that K/E is much more widespread than previously thought. The A and B zones of a soil profile typically contain the most K/E and the highest proportion of kaolinitic to expandable layers. The parent 2:1 clay minerals, climate, plant community, and degree of drainage determine whether K/E, halloysite, well-crystallized kaolinite, bauxite, or a combination of these phases forms. Determination of the types and amounts of kaolin minerals in soils may offer valuable insights into the nature of soil formation and associated processes and rate of soil formation below unconformities.