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Abstract--Electron microscope microprobe analysis (EMMA) has been applied to the determination of the elemental compositions of the kaolinite particles (Mg, A1, Si, K, Ti and Fe) contained in the 0.~0-3 Izm e.s.d., the 0.9-1.0/~m e.s.d, and the 1.9-2.0/am e.s.d, fractions of an English kaolin and an American kaolin. Particles with masses as small as 10-13 g were analysed. An EMMA-4 instrument (A.E.I. Ltd.) equipped with linear fully focussing spectrometers was used. The ratio method of analysis was employed. The operating procedures used to obtain the required high experimental precision in the measurement of AI:Si atom ratio are discussed.Statistical analysis of the results gives the estimated mean and spread of the AI:Si atom ratios. In the English kaolin the mean AI: Si atom ratio differs from the ideal 1:1 at the 0.05 significance level. There is evidence for a variation in composition from kaolinite particle to kaolinite particle in the 1.9-2.0~m fraction of each kaolin. In the 0.9-l.0#m fractions, the mean Fe:Si atom ratio was close to 0.002 showing the presence of iron in the kaolinite structure. The mean K:Si ratio was about 0.002 which would be equivalent to 1 unit muscovite layer associated with a 0.175 #m thick kaolinite particle. In the American clay the Ti:Si atom ratio was 0.002 suggesting that some 12 per cent of the 'titania' found by conventional chemical analysis was associated with the kaolinite particles either as titania itself or as an isomorphous substituent.

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The main commercially important kaolin resources are the primary deposits in Cornwall and the sedimentary deposits in South Carolina and Georgia in the U.S.A. Annual world production is about 18 Mt (1 t = 10 3 kg). In 1981 the U.K. production was 2.6 x 10 6 t of which about 77 % represented paper filler and paper coating grades. About 15% of the U.K. production is used in ceramics. The kaolin extracted from the commercial deposits contains kaolinite as the major component together with ancilliary minerals. The kaolinite particles take the form of pseudo-hexagonal platelets with widths that vary from 10 to 0.1 pm and less. Surface chemical properties have been widely researched. There is no uniformity of view on the origin of the cation exchange capacity. Aqueous suspensions of kaolin are flocculated at low pH. This can be understood in terms of the positively charged edges and the negatively charged faces of the platelets. The deflocculation of kaolin under alkaline conditions is important, and is assisted by adding a polyanion. The Kubelka-Munk equations have provided a useful basis for interpreting the optical properties of kaolins in terms of an absorption coefficient ( k ) and a scattering coefficient ( s ). The parameter k ; is a bulk property and is interpreted in terms of light absorption by ancilliary minerals, by coatings of hydrated iron oxides on the kaolinite particles and by transition metals in the kaolinite structure. The parameter s depends on the size distribution of the platelets and the manner in which they are packed together. The rheological properties of deflocculated suspensions of kaolin in water are important in modern paper coating applications. The relation between the bulk physical properties of kaolin and rheology is complex. Viscosity at low rates of shear increases with increasing specific surface area of the kaolin. Viscosity at high rates of shear depends on the shapes of the particles and their packing. The extraction and production of kaolins are described for both primary and sedimentary deposits. The main use of kaolin is as a paper filler and a paper coating pigment. The various filler and coating grades are indicated, and effect of kaolin properties on coated sheet properties is discussed. Kaolin is a major component in most ceramic bodies. Examples of formulations are given. The manufacture of sanitaryware and tableware is discussed in terms of kaolin properties. Kaolin is used as an extender in aqueous based paints and as a filler in rubber and synthetic polymers.