File:Fire Clay Tonstein below the Fire Clay Coal (Hyden Formation, Middle Pennsylvanian, 312 Ma; Hazard Community College roadcut, Hazard, Kentucky, USA) 1.jpg

English: Fire Clay Tonstein & Fire Clay Coal in the Pennsylvanian of Kentucky, USA.

The dull grayish-black, resistant horizon in the upper half of the above photo is a weathered bituminous coal bed called the Fire Clay Coal. Its name is in reference to a volcanic ash bed in close proximity to the coal. At some localities, the ash bed is within the Fire Clay Coal horizon. At the site shown above, the ash bed is immediately below the coal bed. Volcanic ash beds in & adjacent to coal beds that have been diagenetically altered to kaolinite-rich claystone horizons are called "tonsteins" (a.k.a. "flint clays"). The Fire Clay Tonstein at the site shown here is the relatively thin, light gray-colored horizon just below the blackish, recessive-weathering interval that is just below the Fire Clay Coal.

The Fire Clay Tonstein has been isotopically dated to 312 million years ago. Published analyses indicate the the bed is 60% to 95% kaolinite (Al2Si2O5(OH)4 - aluminum hydroxy-silicate), a clay mineral formed by chemical alteration of primary igneous silicate minerals, such as potassium feldspar.

Stratigraphy: Fire Clay Tonstein, immediately below the Fire Clay Coal, Hyden Formation, Breathitt Group, Atokan Series (upper Duckmantian), Middle Pennsylvanian, 312 Ma

Locality: Hazard Community College outcrop - roadcut on the southwestern side of Rt. 15, due southwest of Hazard Community College, southern side of the town of Hazard, Perry County, southeastern Kentucky, USA (37° 14’ 02.07” North, 83° 10’ 37.41” West)

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Some info. derived from:

Greb et al. (1999) - Depositional history of the Fire Clay Coal bed (late Duckmantian), eastern Kentucky, USA. International Journal of Coal Geology 40: 255-280.