Tonstein (from the

German German(s) may refer to: * Germany (of or related to) ** Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ge ...

"Ton", meaning clay, plus "Stein", meaning rock) is a hard, compact

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

that is composed mainly of kaolinite or, less commonly, other clay minerals such as

montmorillonite Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic crystals, known as clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite gro ...

and

illite Illite is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – alumina ...

. The clays often are cemented by iron oxide minerals,

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...

aceous matter, or chlorite. Tonsteins form from

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...

deposited in swamps. Tonsteins occur as distinctive, thin, and laterally extensive layers in

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...

seams throughout the world. They are often used as

key bed Marker horizons (also referred to as chronohorizons, key beds or marker beds) are stratigraphic units of the same age and of such distinctive composition and appearance, that, despite their presence in separate geographic locations, there is no do ...

s to correlate the strata in which they are found. The regional persistence of tonsteins and relict

phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...

s indicate that they formed as the result of the diagenetic alteration of

falls in an acidic (low pH) and low- salinity environment, consistent with a freshwater swamp.Potter, P. E., J. B. Maynard, and P. J. Depetris (2005) ''Mud and Mudstones'' Springer-Verlag, Heidelberg, Berlin. 297 pp. . pp. 141–142.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. In contrast, the alteration of a volcanic ashfall deposit in a marine environment typically produces a

bentonite Bentonite () is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-m ...

layer. The

induration A skin condition, also known as cutaneous condition, is any medical condition that affects the integumentary system—the organ system that encloses the body and includes skin, nails, and related muscle and glands. The major function of this ...

of tonsteins is in contrast to kaolin claystones that can be mined for kaolin clay, such as the

ball clay Ball clays are kaolinitic sedimentary clays that commonly consist of 20–80% kaolinite, 10–25% mica, 6–65% quartz. Localized seams in the same deposit have variations in composition, including the quantity of the major minerals, accessory ...

s found at

Bovey Tracey Bovey Tracey () is a small town and civil parish in Devon, England, on the edge of Dartmoor, its proximity to which gives rise to the "slogan" used on the town's boundary signs, "The Gateway to the Moor". It is often known locally as "Bovey". ...

which formed by the erosion of a nearby kaolinised

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...

. These deposits are generally softer, white, and plastic.

References

External links

* R. A. Batchelor
Bentonite & Tonstein Bibliography
St. Andrews University Sedimentary rocks Claystone {{petrology-stub

See also

References

External links