Clay Minerals (1999) 34, 193 208

Geological shape and

controls on kaolin particle consequences for mineral

processing

A. P S Y R I L L O S * '1 , J . H . H O W E t , D . A . C . M A N N I N G . 2 AND S. D. B U R L E Y * '3

*Department of Earth Sciences, Oxford Road, University of Manchester, Manchester M13 9PL, UK, and t English China Clays International Ltd., John Keay House, St. Austell, Cornwall PL25 4D J, UK

(Received 24 April 1997; revised 22 December 1997)

A B S T R A C T : The kaolinized granites of St. Austell, England, are worked to produce a range of china clay products, for some of which the kaolin has to meet stringent particle shape and size specifications. Systematic petrographic study indicates that kaolin occurs in the form of two textural types: (i) freely crystalline kaolin (typically <5 gm in average diameter), which infills dissolution porosity of granitic feldspars, and (ii) coarsely crystalline vermiform aggregates (up to 100 gm or more in length), which are closely associated with expanded micas. The vermiform aggregates are characterized by an intergrowth of mica and kaolin crystals, which can be observed at scales of resolution offered by TEM. Textural and chemical evidence suggest that the expanded mica texture is probably the result of preferential precipitation of kaolin along mica cleavage planes and is not simply a process of chemical replacement.

Petrographic examination of kaolin slurries sampled at different points in a typical refinery circuit indicates that platy products with high aspect ratio are derived exclusively from raw materials rich in vermiform aggregates. The fine scale intergrowth of kaolin and mica within the aggregates results in circumstances where mica persists through to fine grained products. Furthermore, the absence of Fe or other chemical components in the kaolin structure suggests that any iron reported for the final products may be a consequence of the presence of Fe-bearing mica within a very free grained intergrowth.

Kaolin 4 is an important and valuable industrial mineral with a wide range of industrial applications, such as in paper, plastics, ceramics and paints

1 Venizelou 48, Nea Smyrni GR-17122, Athens, Attiki, Greece 2 Corresponding author 3 BG Technology, Ashby Road, Loughborough, Leicestershire LE11 3QU, UK 4 The term kaolin is used according to the recommenda- tions of Guggenheim et al. (1997), to refer to the kaolin subgroup within which the polytypes kaolinite, diekite and nacrite are species. The term kaolin thus carries no implications over the precise mineralogical composition of a material, The term china clay is used herein as a descriptive term that refers to kaolin-rich material in general, without any reference to the polytype species which may be present.

(Bristow, 1989). Properties of kaolin products that are particularly important in industrial applications include the crystal size distribution and shape of the clay particles and their bulk chemical composition, as well as their rheological and abrasion properties, whiteness and gloss.

Since many industrial applications use kaolin in coatings, the 'platiness' of the particle is an important characteristic. The platiness of the crystals is expressed in terms of the aspect ratio, which is defined as average diameter divided by thickness, as explained in Fig. 1. The platiness (and other geometric properties) of a kaolin product may be attributed either to characteristics of the parent rock and the kaolin it contains, to the refining procedures, or to a combination of both. The purpose of this paper is to assess the relative importance of these factors.

�9 1999 The Mineralogical Society

194 A. Psyrillos et al.

. I

Aspect ratio = /_/H

L = dmi n + d m a x

2

FIG. 1. Definition of aspect ratio for a hypothetical pseudohexagonal kaolin crystal.

The St. Austell kaolin deposits (southwest England) yield a significant percentage of world- wide kaolin production (annual production capacity of up to 3 million tonnes). No attempt is made here to address in detail the geological processes involved in kaolinization. Instead, new observations derived from techniques which extend the classic studies of Keller (1976a,b, 1977a,b, 1978) are used to describe the morphological characteristics of kaolin from the St. Austell Granite, and to determine the way in which different morphologies respond to mineral processing.

G E O L O G I C A L S E T T I N G OF T H E K A O L I N D E P O S I T S

The kaolin deposits are spatially associated with the St. Austell granite pluton, which forms an upward extension of the Cornubian granite batholith. According to geophysical investigations, the bath- olith is 200 km long and -40 km wide, elongated along an ENE WSW direction and extending from Dartmoor to the Scilly Isles (Bott & Scott, 1964; Brooks et al., 1983). The intrusion of the Cornubian batholith occurred during the latest stages of the Variscan orogeny (Late Carboniferous to Early Permian), as suggested by geological and isotope dating evidence (see Darbyshire & Shepherd, 1985; Chen et al., 1993; Chesley et al., 1993; Chen et al., 1996).

The St. Austell pluton is one of six major outcrops of the Cornubian batholith and is located approximately in the middle of the batholith. The exposed surface of the pluton is elongated along an E - W direction and occupies an area 18 km long and 1-8 km wide, containing a number of different granite types formed in distinct intrusive and metasomatic events (Fig. 2; Hill & Manning, 1987; Hill, 1988; Manning et al., 1996). The mineralogy of all parent granite types is dominated by quartz, feldspars and coarsely crystalline micas (biotite, zinnwaldite and muscovite). Accessory minerals also include tourmaline, topaz, fluorite, apatite and zircon.

The post-consolidat ion evolution of the Comubian batholith is characterized by several distinct episodes of hydrothermal mineralization, in the following sequence (from oldest to youngest; Bristow & Exley, 1994):

(l) Extensive quartz-tourmaline veining, asso- ciated with metasomatic alteration (greisenization) of the host granites and some Sn-W mineralization. The greisenization of the St. Austell granites is associated with the pervasive alteration of the feldspars (mainly plagioclase) in the parent rocks to a relatively coarsely crystalline white mica that is termed 'sericite' in the literature (Exley, 1959; Exley, 1976). In order to avoid confusion with magmatic muscovite, this mineral is referred to here as 'hydrothermal muscovite'. (2) Intrusion of quartz porphyry (rhyolite) dykes. (3) Quartz-hematite veins, the so-called 'cross-courses' and (4) Extensive kaolinization of the host granites.

Currently, there are two contrasting hypotheses regarding the genesis of the Cornish kaolin deposits. First, there is the view that the deposits formed due to weathering of the granites under subtropical conditions after the granites were unroofed and exposed to surface conditions (Hickling, 1908; Coon, 1911; Konta, 1969). This view is supported by Sheppard (1977) on the basis of kaolin stable isotope analyses. The second view argues for a hydrothermal origin of the deposits in association with late magmatic fluids, as an extension of the well-developed high-temperature Sn-W metalliferous mineralization and the greisenization of the host granites (Collins, 1878, 1909; Exley, 1959; Bristow, 1968; Bray & Spooner, 1983). In recent years the most widely accepted model for the formation of the St. Austell kaolin deposits involves a combination of the two processes, with weathering as the latest of a sequence of alteration events (Bristow, 1993).

Geological controls" on

M E T H O D O L O G Y

Samples were collected from several operating china clay mines and associated refineries in the St. Austell area, so that petrographic, mineralogical and morphological characteristics could be deter- mined at every stage in the production process. In particular, samples were taken upstream and down- stream of individual components of the refining process, to assess their influence on clay morphology. All samples were examined using optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD); selected samples were investigated using transmission electron microscopy (TEM).

Freshly broken fragments of kaolinized granite were glued to aluminium (A1) stubs, gold-coated and examined using secondary electron imaging (SEM- SEI). Other fragments were impregnated under vacuum using a mixture of blue-dye and araldite, from which polished thin-sections were prepared for

kaolin particle shape 195

petrographic work, including back-scattered electron (BSE) imaging and chemical analysis.

A Jeol JSM-6400 Scanning Microscope equipped with a Link Systems energy dispersive spectrometer (EDS) system (c.f. Dunham & Wilkinson, 1978) was used for SEM work, with the following conditions for analysis: working distance 39 mm, 15 kV acceleration voltage, 1.5 mA probe current and a 45 s livetime. The large excitation volume (6-10 pm 3) of the electron beam in the specimen prevents the representative analysis of clay particles <10 gm in size. Detection limits (elemental %wt) are estimated to be as follows: Na 0.15, Mg 0.07, A1 0.06, Si 0.05, K 0.07, Ca 0.07, Ti 0.07, Mn 0.09 and Fe 0.10.

Slurry samples from refinery circuits were filtered and dried at 110~ The XRD was conducted using an automated Philips X-ray diffractometer equipped with a PW1730 X-ray generator and Cu-K~ radiation, using randomly oriented specimens and identical experimental

58

56

54

52

SW 91 92 94 96 98 SX 00 02

Bioti te grani te [:2-=] F ine-gra ined tourmal ine granite ~ Topaz granite

[----] Li-mica gran i te ~ Tourmaline granite (globular ~ Pi t boundaries

quar tz & equ ig ranu la r facies)

FIG. 2. Geological map of the St. Austell pluton illustrating the distribution of different granite types (from Manning et al . 1996).

196 A. P6yrillos et al.

FIG. 3. (A) SEM-SE1 micrograph of a kaolin verm in a kaolinized tourmaline granite (Littlejohns pit; SW 985570). (B) SEM-SE! micrograph of finely crystalline kaolin fiom a kaolinized topaz granite (Treviscoe pit; SW 946558). (C) SEM-SEI micrograph of a partially dissolved feldspar crystal from a kaolinized globular-quartz granite (Goonbarrow pit; SX 008585). (D) Plane polarized optical micrograph of a partially dissolved K-feldspar crystal from a coarse-grained tourmaline granite (Dorothy pit; SW 975570). Elongate dissolution channels are developed, giving a high relief grain to the image. (E) SEM-BSE micrograph of expanded mica crystals (bright backscatter intensity) from an extensively kaolinized biotite granite, within a matrix of finely crystalline kaolin (Wheal Remfry pit; SW 925575). The expanded mica crystals exhibit coarse kaolin (k) plates along the exfoliated

conditions, scanning 3-40~ The results were interpreted in digital format using the Siemens Diffrac-AT v.3.2 software package, which includes a database of the JCPDS mineral XRD index cards. Textural features of dried slurry samples were examined using SEM/SEI, for which samples were

mounted on A1 stubs using double-sided adhesive tape prior to gold coating.

Transmission electron microscopy (Phillips EM430 operating at 100 kV and 15-20 nA, with EDAX energy dispersive analysis calibrated using clay mineral standards) was used to examine the

Geological controls on kaolin particle shape 197

FIG. 3. (contd.)

hydrothermal muscovite (m) cleavage planes. (F) SEM-BSE micrograph of expanded hydrothermal muscovite crystals within dissolution cavities in plagioclase from partially kaolinized lithium-mica granite (Dorothy pit; SW 975570). The expanded mica shows fine-scale interlayering of kaolin and muscovite plates. Note the gradual transition of the backscatter coefficient intensity from high in muscovite to low in kaolin. (G) SEM-BSE micrograph of two different expanded micas, biotite (highest backscatter coefficient intensity; lower half of image) and muscovite (upper part of image), in a kaolinized biotite granite (Baal pit; SX 025550). Both micas are deformed by the formation of kaolin along their cleavage planes. (H) TEM micrograph of an expanded mica crystal from a kaolinized biotite granite (Rocks pit; SX 017580), showing alternating layers of kaolin (K, beam-

damaged) and muscovite (M, mottled), as identified by EDS spectra.

intergrowth textures of mica and kaolin that are commonly observed in the St. Austell kaolinized granites. A single, carefully selected kaolinized granite sample known from SEM study to contain good examples of the texture was selected and a

polished thin-section was prepared. The chip was glued on a glass slide using thermoplastic resin. Areas of interest were selected and small sample disks, -3 mm wide, were extracted from the section with a mini-drill after heating the

198 A. Psyrillos et al.

section. The sample disk was then mounted on a copper ring, ion thinned and carbon-coated on both sides.

R E S U L T S

Petrography o f the kaolinized granites

Throughout the St. Austell pluton, irrespective of the granite variety, the clay fraction of the kaolinized granites has similar petrographic and textural characteristics, and can be divided into two textural types: (a) Vermiform kaolin aggregates or kaolin stacks occur in the form of books of kaolin crystals or occasionally coiled crystal aggregates (Fig. 3a). This textural form is characterized by several subparallel kaolin crystals. The aggregates sometimes reach sizes of up to 100 [am in composite length. Kaolin crystals have subhedral to euhedral pseudohexagonal crystal shapes, occa- sionally with irregular crystal terminations. Individual crystals are consistently >5 [am in average diameter (Fig. 1); the thickness of indivi- dual crystals was never observed to exceed 0.5 [am. (b) Fine kaolin crystals differ substantially from the vermiform aggregates (Fig. 3b), occuring as single crystals with random textural arrangement. Crystal shapes are platy pseudohexagonal with subhedral to euhedral outlines. Average diameters are typically <5 [am, although individual crystals are similar in thickness to those which form vermiform aggre- gates (i.e. up to 0.5 [am). This finely crystalline kaolin is consequently more blocky, and so has a relatively low aspect ratio when compared with individual kaolin plates in vermiform aggregates from the same parent rocks.

Relationship to feldspar dissolution. Fine grained kaolinite (<5 [am) shows a close association with dissolution of feldspar. Dissolution features (Fig. 3c) include channels and pits similar to those described by Berner & Holdren (1977, 1979) for experimentally dissolved feldspars. Channels develop along cleavage planes, tbrming a nearly orthogonal network, while pits form at cleavage intersections. In relatively little altered samples, the feldspars display shallow dissolution pits with rounded or oval shapes, which become elongate and prismatic in more altered samples. Optical microscopy and SEM-BSE imaging confirms Exley's (1976) observations concerning the relative stability of K-feldspar and plagioclase. K-feldspar is often present in samples where

plagioclase has dissolved completely. Alteration of K-feldspar has mainly affected microperthitic exsolution lamellae and other plagioclase inclu- sions. These are dissolved first, thus producing irregular rod-like void spaces within the K-feldspar. More pronounced dissolution is expressed as widened dissolution channels, which form irregular patterns and produce skeletal K-feldspar crystals (Fig. 3d). In completely kaolinized granites both K-feldspar and plagioclase are absent. The feldspar dissolution porosity is subsequently infilled with finely crystalline kaolin that exhibits textural features identical to those outlined previously (Fig. 3b).

Relationship to mica alteration. Although feld- spar dissolution is generally regarded as the main kaolinization mechanism (Bristow, 1993; Bristow & Exley, 1994), the St. Austell granites also contain a number of primary micaceous minerals such as magmatic biotite, muscovite and zinnwaldite, as well as secondary hydrothermal muscovite produced during greisenization. All of these mica species are affected by kaolinization, manifested as gradually weakening mica birefringence under the optical microscope, or reduction of the backscatter coefficient when using the SEM-BSE. These observations are identical to those described as 'expanded mica textures' in diagenetically modified sandstones (e.g. Bjorlykke et al., 1979; Irwin & Hurst, 1983; Burley, 1984; Huggett, 1984, 1986; Kantorowicz, 1984; Bjorlykke & Brendstal, 1986; Warren, 1987; Crowley, 1991) and weathered granites (e.g. Gilkes & Sudbiprakarn, 1979; Robertson & Eggleton, 1991). The expanded mica texture is extremely abundant in all kaolinized granites and is readily identified in thin-section or by using SEM-BSE (Fig. 3e,I'). The altered micas characteristically show exfoliation along the (001) cleavage planes, producing fan-shaped textures. Discrete kaolin crystals occur as elongated thin plates, orientated parallel to the mica cleavage, increasing in proportion with increasing kaoliniza- tion to produce the vermiform kaolin aggregates ('verms'; Fig. 3a). The average diameters (see Fig. 1) of individual kaolin crystals are similar to the average diameters of the relict expanded micas, while the kaolin crystals mimic the expanded mica crystal morphology and size. Kaolin verms associated with expanded hydrothermal muscovite contain individual crystals with average diameters typically in the order of 10 15 [am. However, verms associated with expanded, coarsely crystal-

Geological controls on kaolin particle shape 199

line magmatic micas (biotite, muscovite and zinnwaldite) sometimes exhibit single kaolin plates with average diameters as large as 100 gm (Fig. 3g).

Under the TEM, expanded micas comprise alternating parallel layers of mica (mottled) and kaolin (beam damaged) with variable thickness (Fig. 3h). The example featured in Fig. 3h illus- trates the intergrowth of kaolin with a hydrothermal muscovite crystal. The two minerals are arranged in an identical manner to that observed in BSE images, showing that the intergrowth persists at a much smaller scale than can be resolved using scanning electron microscopy.

M R ~

80 20 ~ adon i t e

.. covi 4 2

2 R 3 80 60 40 20 3 R 2

Chemistry o f hydrothermal muscovite and expanded micas

The intimacy of the intergrowth between kaolin and mica has implications for the chemical character is t ics o f ve rmi fo rm kaol in stacks. Analysis of unaltered muscovite and muscovite lamelli within verms (Table 1) yields compositions close to those expected for muscovite (Fig. 4), extending towards celadonite. Aluminium is the dominant octahedral atom, with up to 0.85 atoms per formula unit (p.f.u.). The interlayer site is dominated by K, with minor amounts of Na and total interlayer site occupancy between 1.7 and 1.98 atoms p.f.u. Calcium is at or below the detection limit of 0.02 atoms p.f.u., and Mg is present at low

FIG. 4. Velde temary plot of hydrothermal muscovite compositions obtained with SEM/EDS analyses. The data points plot along a trend parallel to the phengite

substitution line of Velde (1983).

levels (up to 0.15 atoms p.f.u.). Octahedral Fe varies from 0-0 .85 atoms p.f.u. The ratio of Si to tetrahedral AI is consistently >3:1; A1TM contents vary between 1.2 and 1.9 atoms p.f.u., and show a linear correlation against Fe z+ (Fig. 5).

In the expanded mica textures, a reduction in backscatter coefficient intensity corresponds to a progressive change in chemical composition from mica to kaolin, which is supported by analytical data collected point-by-point every 2 gm in a

1.00,

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o ~ oX~Oo~ ~ ~ b~

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0.00 ~ ,

3.00 3.20 3.40 3.60 3.80 4.00 4.20

Octahedral AI 3+

FIG. 5. Plot of octahedral A13+ vs. Fe 2+ in hydrothermal muscovite. An increase in octahedral Fe results in a decrease of octahedral A1, indicating that Fe 2+ substitutes for A1 in octahedral sites.

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A. Psyrillos et al.

~ N ~ I ~ I ~ ~ I I M I ~

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~ 1 0 0 ~ ~ ~ 1 0 0 ~ I ~

M O ~ I o ~ ~ ~ 1 ~ 1 ~

~ 0 0 +

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~ 1 0 ~ ~ ~ 1 ~ 1 ~

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~ ~ I ~ ~ ~ ~

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+ ~ % o o ~ ~ > + + < + +

Geological controls on kaolin particle shape 201

TABLE 2. Sequence of EDS analyses of an expanded mica texture, from points approximately 2 gm apart in a linear traverse. Analyses 1 - 4 are from the unaltered hydrothermal muscovite crystal. Analyses 5-11 are expanded mica textures with intermediate backscatter coefficient intensities and analysis no. 12 is kaolin intergrown with muscovite. Formulae were calculated on the basis of 22 equivalent oxygen atoms (kaolinized

biotite granite, sample 125, Baal pit).

1 2 3 4 5 6 7 8 9 10 11 12

Si TM 6.346 6.198 6.304 6.289 6.236 6.199 6.316 6.423 6.372 6.385 6.373 6.398 A1TM 1.654 1.802 1.696 1.711 1.764 1.801 1.684 1.577 1.628 1.615 1.627 1.602 A1 vI 3.644 3.919 3.654 3.697 3.883 4.079 4.047 4.151 4.274 4.370 4.333 4.353 Fe 0.248 0.095 0.319 0.336 0.132 - 0.135 0.132 0.124 0.053 0.098 0.108 Mg 0.153 0.040 0.134 0.104 0.044 - 0.165 0.054 0.056 - 0.040 0.050 Na 0.084 0.060 0.097 0.060 - 0.095 0.102 0.180 . . . . K 1.836 1.715 1.728 1.681 1.552 1.289 0.734 0.400 0.364 0.317 0.223 0.140 Ca - - - 0.025 0.026 0.016 0.040 . . . .

traverse across an expanded hydrothermal musco- vite crystal (Table 2). As the proportion of kaolin increases, there is a progressive depletion in K (Fig. 6). Other elements (including Fe) remain virtually unchanged in the sequence. However, EDS spectra obtained independently, using TEM, indicate that the intergrown kaolin contains no detectable K, Fe or other elements (Fig. 7). Thus SEM analysis is unable to resolve chemically the f ine-scale o f the in tergrowth of kaol in and muscovite.

Petrography and mineralogy of processed kaolin

The production of kaolin from St. Austell begins with hydraulic washing of a kaolinized granite face, and generalized production sequences are presented by Bristow & Exley (1994). The flow chart shown in Fig. 8 summarises the processes which are used in refinery circuits once the kaolin slurry has been removed from the mine. The clay suspension passes th rough a series of stages of par t ic le size classification, separating relatively coarse (oversize)

2,0 .

1.8, ~ -e--Fe r ~ "-'=- Mg

1.6, ~ ~ M n

~ N a 1.4, ~ K

1.0,

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0.6,

0.4,

0.0, . . . . . . .

2 3 4 5 6 7 8 9 10 11 12

A n a l y s i s n u m b e r

Fie. 6. Variation in chemical composition shown by a traverse (~2 gm steps) across an expanded mica crystal (data from Table 2), in which the K content systematically decreases from fresh hydrothennal muscovite end-

member compositions (analyses 1-4) to the kaolin end member composition (analysis 12).

202

AI

Si

A. Psyrillos et al.

feed

classi f icat ion

undersize I G3

classi f icat ion

undersize ~ G4

FIG. 7. TEM-EDS spectrum of kaolin intimately interlayered with hydrothermal muscovite in the expanded mica texture illustrated in Fig. 3g. The spectrum only features Si and A1 peaks, with the

characteristic absence of K and Fe.

and fine (undersize) fractions, and comminution. These procedures vary in sequence from one refinery to another. At various stages the coarser fraction derived from size classification is passed though a comminution step to reduce particle size, and returned to an appropriate point in the process stream.

,~ oversize G2

oversize ~[ G5 v[ comminution }

classi f icat ion

undersize ~ G8

.~ oversize

G7

FIG. 8. Schematic illustration of part of a simplified refinery circuit that is used for the production of china

clay products with a high aspect ratio specification.

The feed to the sequence o f processing procedures summarized in Fig. 8 has already been treated to remove rock fragments and coarse sand from the kaolin slurries, and contains kaolin and mica (layered silicates), as well as quartz and feldspar (blocky particles) (sample G1, Fig. 9).

At the first stage of classification, the oversize material includes predominantly micas, quartz and feldspars (sample G2, Fig. 9), with small amounts

A

A.

A

J• . A

G8

G4

G2

3 8 13 18 23 28 33 38

02 0

FIG. 9. Randomly orientated specimen XRD patterns of the refinery samples examined (sample numbers GI to G7; see Fig. 8 for sample locations); Cu-Kc~ radiation.

Geological controls on

of kaolin (vermiform kaolin aggregates; Fig. 10a). The undersize fraction (sample G3) is enriched in kaolin, although it contains appreciable amounts of micas and minor amounts of feldspars and quartz (Fig. 9). This material is then fed into a second stage of classification, in which the undersize (sample G4) is almost entirely composed of kaolin (Fig. 9), as fine crystals as well as small vermiform aggregates and verm fragments (see Fig. 10b).

The oversize fraction from the second stage of classification (sample G5) exhibits distinct miner- alogical and textural characteristics, when compared with the undersize fraction. Kaolin and mica predominate (with basal peaks of comparable intensities; Fig. 9), as coarsely crystalline vermi- form kaolin-mica aggregates (Fig. 10c). This material is comminuted, to break down the vermi- form aggregates into their component plates of kaolin and mica. Mineralogically, samples before (G5) and after (G6) comminution are identical, but they differ in texture. After commiuntion (Fig. 10d; sample G6), the vermiform kaolin-mica aggregates have been disaggregated to give individual crystals with similar shape to those shown in Fig. 10b, but with scalloped grain edges and a larger size. A third stage of classification then removes blocky particles from the slurry. The oversize fraction at this stage (sample G7, Fig. 10e) concentrates whatever vermi- form aggregates have failed to disaggregate during the comminution stage. Mineralogically, this frac- tion is composed primarily of mica and kaolin. The undersize fraction is a kaolin-rich material (sample G8, Fig. 9) that contains relatively coarse, platy particles with anhedral crystal outlines (Fig. 10f).

D I S C U S S I O N

Origin o f kaolin textural types

The petrographic characteristics of kaolin from the St. Austell deposits distinguish two textural types of kaolin, each of which is associated with particular precursor minerals. Thus there are two different sources of kaolin, which have different textural and crystal shape properties.

Finely crystalline kaolin infills the secondary porosity produced by the dissolution of feldspars in the parent granites, and is considered to be the product of precipitation from solution in cavities produced by feldspar dissolution. The exact relationship between feldspar dissolution and the subsequent kaolin precipitation cannot be deduced

kaolin particle shape 203

from petrographic observations. Thus the time interval between dissolution and precipitation is not known, although it is assumed that the solutes responsible for the precipitation of kaolin were derived directly from the dissolution of feldspars, which is a surface controlled reaction (Holdren & Berner, 1979). The textural characteristics of fine kaolin from St. Austell closely resemble those illustrated by Keller (1976a) for kaolin precipitated within cavities from a wide range of geological environments.

Vermiform kaolin aggregates and kaolin stacks are closely associated with expanded mica textures and are characterized by kaolin and mica plates intergrown parallel to the (001) crystallographic direction of layer silicates. This intergrowth was also identified by Bray & Spooner (1983), although these authors did not associate this texture with expansion of micas, instead regarding intergrown kaolin and mica as co-genetic.

In his description of kaolins from St. Austell, Keller (1976b) shows both types of morphology, and this is a typical SEM-SEI observation for many kaolinized samples. In order to identify the link between mica and kaolin within vermiform aggregates, it is important to extend SEM-SEI observations through the use of BSE imaging on polished thin-sections.

The nature o f expanded mica textures

In the kaolinized St. Austell granites, the dominant association between kaolin and mica involves hydrothermal muscovite. Unkaolinized hydrothermal muscovite from St. Austell is chemically similar to 'sericite' from greisens at Cligga Head, Cornwall (Hall, 1971), showing Fe 2+ substitution for octahedral A13+ (Fig. 4,5) and a ratio of AllV:si >3:1. Both of these features comply with the definition of phengites given by Deer et al. (1992), although the term phengite is inappropriate because hydrothermal muscovite also contains F and small amounts of Li (Hall, 1971).

According to Bjorlykke et al. (1979), Burley (1984) and Kantorowicz (1984), expanded mica textures are the product of chemical alteration of the parent mica crystals, which undergo gradual or 'step-by-step' kaolinization. Analytical traverses across expanded mica textures demonstrate gradual loss or removal of K from the micas, in a manner similar to that suggested by Burley (1984) and Kantorowicz (1984). However, TEM imaging

204 A. Psyrillos et al.

F~G. 10. (A) SEM-SEI micrograph of a vermiform kaolin aggregate found in the oversize fraction (sample G2) of initial classification. Finely crystalline, euhedral kaolin particles are also scattered on the surface of the aggregate. (B) SEI-SEI micrograph of the second stage classification undersize fraction (sample G4), consisting almost entirely of finely crystalline kaolin, which exhibits subhedral to euhedral crystal shapes and sizes <5 gm. (C) SEM-SEI micrograph of a large vermiform kaolin aggregate found in the second stage classification oversize fraction (sample G5). The aggregate exhibits stacked euhedral crystals with lengths >10 lain. (D) SEM-SEI micrograph of a kaolin material immediately after comminution (sample G6). The vermiform aggregates observed in sample G5 have broken down and the material is composed of relatively large plates with corrugated margins. (E) SEM-SEI micrograph of a vermiform kaolin aggregate occurring in the oversize fraction from the final stage of classification (sample G7). The verm illustrated was not disaggregated during comminution and was rejected by the classifiers. (F) SEM-SEI micrograph of the undersize fraction from the final stage of classification

(sample G8). The sample is dominated by subhedral to anhedral plates with sizes >5 lam.

Geological controls' on kaolin particle shape 205

of expanded micas indicates that the kaolin-mica intergrowth persists at a scale too small to be resolved using microprobe or SEM analytical techniques, and that the contact between mica and kaolin crystals is sharp rather than gradual. This suggests that the observed chemical transition from muscovite to kaolin (Fig. 6) is an artificial consequence of the technical inability to analyse single minerals in fine grained intergrowths. Furthermore, the absence of Fe in TEM-EDS spectra of kaolin layers suggests that the Fe detected in hydrothermal muscovite by SEM-EDS analyses truly resides in the mica crystal structure and not in kaolin, in contrast to the findings of Jepson (1988).

Robertson & Eggleton (1991) used TEM imaging to describe similar muscovite-kaolin intergrowths from weathered granites, suggesting that muscovite is modified to kaolin through a 'layer-by-layer' topotactic transformation process involving the replacement of K + ions by H3 O+ ions at the mica interlayer sites. The absence of Fe in kaolin intergrown with Fe-bearing mica in St. Austell is inconsistent with this process. Also, Crowley (1991) points out that alteration of muscovite to kaolinite involves a volume decrease of 4.5%. However, the textures observed in the kaolinized granites (and in diageneticaUy modified sandstones) indicate that muscovite crystals have undergone expansive defor- mation. For these reasons, layer-by-layer chemical replacement is unlikely to be the process involved in the formation of the expanded mica texture.

Instead, a model involving precipitation from solution is preferred (Crowley, 1991), along the cleavage planes of mica crystals. This mechanism accounts for the observed volume increase of the original micas and implies that the micas remain chemically inert during kaolinization. Crowley (1991) also suggested that kaolin precipitation has been facilitated by localized increases of pH along the mica (001) cleavage surfaces, as proposed by Boles & Johnson (1984). This interpretation adequately explains the observed similarities in crystal size and shape between intergrown kaolin and mica crystals. It is also suggested that kaolin is templated and thus mimics the habit of the original mica crystals.

refining processes which are designed to generate products with particular geometrical characteristics. Each textural type will respond in its own way to the processes which are in operation.

The petrographic characteristics of the refinery samples suggest that there are two crucial aspects of the treatment of kaolin with respect to crystal shape properties: (1) classification and (2) comminution. Classification achieves the separation of the vermi- form aggregates from the finely crystalline kaolin. Comminution achieves the disaggregation of the verms to their component single crystals. The processes operating on vermiform kaolin aggregates during comminution are summarized in Fig. 11. The primary function of comminution is the disaggregation of kaolin stacks into their component individual plates, which undergo peripheral frac- turing (although some verms remain unaffected). This conclusion is supported by the irregularity of the crystal outline, which contrasts sharply with the

Vermiform kaolin aggregates composed of interlayercd mica and kaolin crystals

with euhedral shapes

R e l i c vermiform aggregates

r Small kaolin and mica plates fragments from the periphery

of the larger crystals

Kaolin and mica plates with good shape properties; "

indentations along the crystal periphery

The effect o f refining on kaolin crystal shape

Recognition of two textural types of kaolin within the St. Austell deposits has implications for

FIG. 11. Schematic illustration of the disaggregation undergone by vermiform kaolin aggregates during

comminution.

206 A. Psyrillos et al.

subhedral to euhedral shape of the original crystals as seen in the parent rock. The small fragments that are broken from around the periphery of the plates constitute a population of crystals that may be detrimental to the properties of the bulk material, having a low aspect ratio.

The final stage of the refining removes the coarser particles from the slurry. At this stage, any reduction in the high aspect ratio plates is compensated by the removal of blocky minerals (quartz and feldspar impurities) as well as verms that have not been disaggregated.

Implications for the mineralogy of kaolin products

The mineralogical composition of the china clay produced in this case study, as determined by XRD analysis, indicates the presence of appreciable amounts of a micaceous mineral (strong basal reflection at -10 A). The presence of mica in the final product is easily explained if the textural characteristics of the clay fraction are considered. The vermiform kaolin aggregates within the feed consist of interlayered mica and kaolin (expanded mica textures). A proportion of the mica evidently persists to the end of the refining process. The final classification of the slurry removes a significant proportion of the mica, although the final product is by no means mineralogically pure kaolin.

The pervasive nature of sericitization in the St. Austell Granite (Exley, 1976), means that the dominant mica within the final kaolin product is hydrothermal muscovite. This species is abundant in all parent granite samples examined, especially as inclusions in feldspar. The vermiform kaolin aggregates associated with hydrothermal muscovite are much smaller than those associated with expansion of primary micas, which are removed from the slurries during the earliest stages of treatment. The chemical characteristics of hydro- thermal muscovi te strongly suggest that the presence of mica in the final china clay product is responsible for the presence of K, Fe and F.

C O N C L U S I O N S

Petrographic examination of the St. Austell kaolinized granites has demonstrated the presence of two distinct kaolin textural types: finely crystal- line kaolin and vermiform kaolin aggregates. These textural forms of kaolin are associated with the

alteration of particular parent minerals within the host granite, irrespective of granitic rock type. Finely crystalline kaolin is associated with feldspar dissolution and precipitation of kaolin within pore space, and vermiform kaolin aggregates are derived from expansion of pre-existing micas. These textural characteristics influence the response of a raw material to refining. Kaolin products with large aspect ratio are derived exclusively from materials enriched in vermiform aggregates, which originate from expanded micas. The intimate textural association between kaolin and micas in vermiform kaolin aggregates indicates that products made from verm-enriched materials need not be mineralogi- cally pure. Finely crystalline kaolin is unlikely to yield a high aspect ratio platy product. The proportions of these two kaolin textural types within a raw material will influence its ability to yield products with particular shape characteristics.

ACKNOWLEDGMENTS

The petrographic description of the kaolinized granites and the chemical analyses of hydrothermal muscovite form parts of a PhD thesis by A. Psyrillos, who acknowledges the financial support given by the U n i v e r s i t y o f M a n c h e s t e r (Boyd Dawkins Scholarship). English China Clays International Ltd have funded the detailed petrographic characterization of kaolin products. The authors thank ECCI Ltd for allowing access to the china-clay pits and the refining plants in the St. Austell area, as well as for permission to publish this paper. The contribution and assistance of ECCI staff in the pits, the refinery plants and the labs is gratefully acknowledged, as are the anonymous referees for their comments.

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