The Tulameen coal field is part of an Eocene nonmarine basin which received extensive volcaniclastic sediments due to its location within an active magmatic arc. Bentonite partings in the coal originally consisted of glassy rhyolitic tephra with phenocrysts of sanidine, biotite, and quartz. During the initial alteration, which took place within the swamp or shortly after burial, glass was transformed to either smectite-cristobalite-clinoptilolite or to smectite-kaolinite. The formation of kaolinite depended on the degree of leaching of silica and alkalies in the swamp environment. Some beds are nearly 100% kaolinite and can be designated as tonsteins. The smectite shows no evidence of interlayering; the kaolinite is well ordered. During alteration, sodium, originally a component of the glass, was lost from the system.

A later thermal event, which affected only the southern part of the basin, metamorphosed the smectite to a regularly interstratified illite/smectite with 55% illite layers and rectorite-type superlattice (IS-type). The source of potassium was dissolution of sanidine. Vitrinite reflectance measurements of the coal suggest that the smectite was stable to 145–160°C, at which temperature it transformed to K-rectorite.

The absence of randomly interstratified intermediates, even in beds rich in potassium, suggests that the transformation of smectite to K-rectorite was controlled by a steep thermal gradient possibly resulting from local magmatism or circulating geothermal fluids.

A kaolinite-rich bed (tonstein) and an associated bentonite in the upper part of Yegua Formation at College Station, east-central Texas, were formed by in situ weathering processes in a late Eocene swamp. X-ray powder diffraction, infrared spectroscopy, petrographic studies, and scanning and transmission electron microscopy not only show that dioctahedral smectite and coarsely crystalline kaolinite are the dominant minerals in the bentonite and tonstein, respectively, but that cryptocrystalline halloysite and kaolinite are localized along the weathering front (transitional zone) between the tonstein and the bentonite. As weathering progressed, the cryptocrystalline minerals gradually recrystallized to yield the coarse books and vermicular growths of kaolinite characteristic of the tonstein.

Small amounts of cristobalite, sanidine, and euhedral zircon crystals with liquid or gaseous inclusions accord with the formation of the bentonite by alteration of volcanic ash. Clinoptilolite in the lignitic layer and sandstone below the bentonite probably formed from ions that were released during alteration of the volcanic materials to smectite, but clinoptilolite in the tonstein and overlying strata appear to have formed after kaolinization of the bentonite.

The coarse, non-clay fraction of many flint-like kaolinitic claystones often contains mineral grains diagnostic of the claystone's origin and, in the case of tonsteins (altered volcanic ashes), may also provide minerals suitable for radiometric dating. Separation of the non-clay mineral fraction is often difficult because flint clays and flint-like clays resist slaking in water and thus are difficult to disaggregate. Chemical disaggregation of resistant kaolinitic claystones may be achieved by immersion in either hydrazine monohydrate or DMSO for periods ranging from one day to several weeks. Generally, hydrazine monohydrate works more quickly and efficiently than DMSO to disaggregate most kaolinitic claystones and flint clays.

Gray-black kaolinitic claystones of industrial value are abundant in Upper Carboniferous–Lower Permian coal-bearing strata of the Datong Coalfield of northern China. The main types are tonsteins and cryptocrystalline kaolinitic claystones, distinguished by the thinness and greater crystallinity of kaolinite in the former and by the presence of detrital illite and authigenic pyrite in the latter. In order to determine the formation history of these two types of kaolinitic claystone, the petrological, mineralogical, and geochemical characteristics of borehole samples from the Upper Carboniferous Taiyuan Formation which comprises siliciclastics and coal seams deposited in a coastal environment, were analyzed. In addition to kaolinite, the claystones contain subordinate illite, quartz, pyrite, anatase, feldspar, siderite, and calcite. The tonsteins and cryptocrystalline kaolinitic claystones have different sources, as shown by petrographic data, elemental ratios, and chondrite-normalized rare earth element (REE) patterns. The volcanic origin of the tonsteins is revealed by an abundance of volcanic quartz and vitric fragments as well as Al2O3/TiO2, Zr/Hf, and Nb/Ta ratios consistent with a felsic igneous source. Their REE fraction was derived from feldspars or micas of the parent rocks, and the fraction decreased with alteration of these minerals to kaolinite. The sedimentary origin of the cryptocrystalline kaolinitic claystones is revealed by an abundance of detrital quartz and illite grains derived from either granite or sedimentary upper crust, and by the total REE contents (ΣREE) and (La/Yb)N values which are consistent with granitic material. Their depositional environment was in a transitional (coastal) setting (as shown by intermediate Sr/Ba ratios) hosting an open acidic hydrologic system (as shown by high chemical index of alteration (CIA) values indicative of intensive chemical weathering) that was suboxic to anoxic (as shown by high U/Th ratios and trace-metal enrichment factors). The present chemistry of these claystones was thus controlled by a combination of parent rock type and diagenetic alteration.

Kaolinite-rich rocks are widespread in Chinese coal-bearing strata. Three main types of deposits are recognized. Those deposits identified as flint clays are several metres thick and show lateral variations in bed composition. The kaolinite is thought to have formed mainly on the adjacent landmass, but some crystallization of gels within the basin is not ruled out. Tonsteins, which formed from the in situ alteration of airfall volcanic ashes, are very common in the coal measures and are up to 0.5 m thick. Finally, kaolinite deposits are described where the development of kaolin is related to weathering of coals either close to or at the present land surface.