Tonstein

Tonstein (from the German "Ton", meaning clay, plus "Stein", meaning rock) is a hard, compact sedimentary rock that is composed mainly of kaolinite or, less commonly, other clay minerals such as montmorillonite and illite.

The clays often are cemented by iron oxide minerals, carbonaceous matter, or chlorite.

[3] Tonsteins occur as distinctive, thin, and laterally extensive layers in coal seams throughout the world.

The regional persistence of tonsteins and relict phenocrysts indicate that they formed as the result of the diagenetic alteration of volcanic ash falls in an acidic (low pH) and low-salinity environment, consistent with a freshwater swamp.

[3][4] In contrast, the alteration of a volcanic ashfall deposit in a marine environment typically produces a bentonite layer.