Clinker is a metamorphic rock that results from natural underground fires in coal beds. The fires cook the nearby sedimentary rocks to give an unusual hard, silicified rock that can contain an unusual suite of minerals (high temperature, very low pressure). Sometimes the rock can be partially melted, resulting in a glassy appearance, and sometimes the rock is a frothy pumice-like material often called scoria by the local people, but buchite is a more formal general term for vesicular, glassy metamorphic rocks formed by near-surface processes like coal fires.
The name is from the clinking sound when the rock is struck by a hammer or smashed against another. A more general term for these rocks is paralava, and lighter colored smooth examples are sometimes called porcellanite, but that’s an ill-defined term that includes some widely variable rocks, not necessarily always originating like paralava or clinker.
More than 4000 square kilometers of southeastern Montana and northeastern Wyoming are covered in clinker formed in sediments associated with coal beds of the Cenozoic (Paleocene) Fort Union Formation deposited about 58 to 65 million years ago. The natural coal fires that created the clinker are much younger, dated to a time between about 10,000 and 1.1 million years ago (Heffern and others, 2007, Geochronology of clinker and implications for evolution of the Powder River Basin landscape, Wyoming and Montana: in Stracher, G.B., ed., Geology of Coal Fires: Case Studies from Around the World: Geological Society of America Reviews in Engineering Geology, v. XVIII.)
The specimen above, collected from a road cut in the Powder River Basin of Wyoming in 1969, has some nice white-colorless gypsum crystals (near top center) on a red substrate, which is probably silicified mudstone colored by iron oxide. The rocks are usually some shade of red to yellow to purple and blue, and when used as road metal (as on Interstate 25 in northern Wyoming, they give the highway a distinctly reddish color.
Gypsum, calcium sulfate, is expectable as a late-stage deposit because the sulfur has been mobilized by the heating of the rocks. But this specimen is also covered by tiny, semi-circular to radial and bladed white to yellow crystals.
These little clusters are radial twins of tridymite, the high-temperature, low-pressure analog of quartz. Most of these clusters and granular growths are about a third of a millimeter across. A fine example showing similar growths in tridymite from Utah can be seen here.
The metamorphism of coal-bearing sediments to make clinker also reorganizes the iron in the rocks to make abundant but irregularly distributed magnetite, which in turn gives the rock a distinctive aeromagnetic character, enough that I was able to use such data to define areas of coal burns in the Powder River Basin and Black Mesa Basin (Arizona) when I was at Gulf Oil.
Thanks for that, especially the magnified pic. I've seen this before, and I sure, hoped/wondered if i'd found something fairly exotic. At least more than a...quartz. Still, it is a relevant step in the learning process.