{{Short description|Type of sedimentary rock}} thumb|Sandstone-coal tonsteins in Wyoming '''Tonstein''' (from the German "Ton", meaning clay, plus "Stein", meaning stone) 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.<ref>{{cite web|url=http://www.chemikus.de/litholexikon/tonstein.htm|title=Tonstein |work=Litholexikon |language=German |publisher=Chemikus-Web }}</ref><ref>{{cite web|url=http://www.geodienst.de/tonstein.htm| title=Tonstein|work=Die Welt der Gesteine |publisher=Geowissenschaftlicher Dienst, Dr. Olaf Otto Dillmann |language=German}}</ref> Tonsteins form from volcanic ash deposited in swamps.<ref name="PotterOthers2005a"/> Tonsteins occur as distinctive, thin, and laterally extensive layers in coal seams throughout the world. They are often used as key beds to correlate the strata in which they are found. 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.<ref name="PotterOthers2005a">Potter, P. E., J. B. Maynard, and P. J. Depetris (2005) ''Mud and Mudstones'' Springer-Verlag, Heidelberg, Berlin. 297 pp. {{ISBN|3-540-22157-3}}. pp. 141–142.</ref><ref name="BohorOthers1993a">Bohor, B. F., and D. M. Triplehorn (1993) ''Tonsteins: altered volcanic ash layers in coal-bearing sequences'' Special Paper 285. Geological Society of America, Boulder, Colorado, 44 pp. {{ISBN|9780813722856}}</ref> In contrast, the alteration of a volcanic ashfall deposit in a marine environment typically produces a bentonite layer.<ref name="PotterOthers2005a"/>

The induration of tonsteins is in contrast to kaolin claystones that can be mined for kaolin clay, such as the ball clays found at Bovey Tracey which formed by the erosion of a nearby kaolinised granite. These deposits are generally softer, white, and plastic.<ref name="Selley">{{cite book|last=Selley|first=R,C|title=Applied Sedimentology|publisher=Elsevier| year=2000| edition=2| pages=337| isbn=9780126363753|url=https://books.google.com/books?id=k3PnTbjR0DMC&q=tonstein&pg=PA339|accessdate=28 December 2012}}</ref>

==See also== * {{annotated link|Claystone}} * {{annotated link|Lutite}} * {{annotated link|Mudstone}} * {{annotated link|Pelite}} * {{annotated link|Shale}}

==References== {{reflist}}

==External links== * R. A. Batchelor, [http://www.st-andrews.ac.uk/~rab/bentrefs.html Bentonite & Tonstein Bibliography], St. Andrews University

Category:Sedimentary rocks Category:Claystone

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