Definition of buchite

An uncommon metamorphic rock type composed mostly of glass, formed by the melting of a sedimentary rock or soil by heat from an adjacent lava flow or scoria eruption, or friction along a fault or landslide, or the burning of an underground coal bed. Buchite stand for a product of a special type of metamorphism, called pyrometamorphism (contact type, high to very high temperature, low pressure, often connected with coal fires). Buchites are often confused with obsidian, which is a volcanic glass rather than metamorphic glass. The so-called para-obsidian is a buchite with small amounts of mullite and tridymite forming microlites. Rarely, buchites can be colorful and transparent, and faceted as gemstones, as for example a few blue-green gems from the Eifel hills in Germany.

Buchite is named for German mineralogist Baron Christian L. von Buch, who died in 1853. The primary description of buchite is a transformed or glassy sandstones associated with basalts (in Germany). Now the term concerns also other pelitic rocks. The first described buchite contained relic, cracked quartz with tridymite overgrowths and inclusions, feathery or needle-like clinopyroxene, magnetite trychites, rectangular and hexagonal sections of cordierite crystals, tiny crystals of a spinel, small voids, and pores interfilled with goethite. The glass of the rock is brown.

According to Grapes (2006), buchite occurs as xenoliths and within contact aureoles.

Ref.:

Grapes (2006): Pyrometamorphism. Springer-Verlag, Berlin, Heidelberg

A typical coal-fire buchite from a burning coal-mining dump: