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Tonstein (from theGerman "Ton", meaning clay, plus "Stein", meaning rock) is a hard, compactsedimentary rock that is composed mainly ofkaolinite or, less commonly, otherclay minerals such asmontmorillonite andillite. The clays often are cemented byiron oxide minerals,carbonaceous matter, orchlorite.[1][2] Tonsteins form fromvolcanic ash deposited in swamps.[3]Tonsteins occur as distinctive, thin, and laterally extensive layers incoal seams throughout the world. They are often used askey beds to correlate thestrata in which they are found. The regional persistence of tonsteins and relictphenocrysts indicate that they formed as the result of thediagenetic alteration ofvolcanic ash falls in an acidic (lowpH) and low-salinity environment, consistent with a freshwaterswamp.[3][4] In contrast, the alteration of a volcanic ashfall deposit in a marine environment typically produces abentonite layer.[3]
Theinduration of tonsteins is in contrast to kaolinclaystones that can be mined for kaolin clay, such as theball clays found atBovey Tracey which formed by the erosion of a nearby kaolinisedgranite. These deposits are generally softer, white, and plastic.[5]
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