A new type of saidenbachite with pseudomorphs after coesite phenocrysts from the north‐western Bohemian Massif, Germany

Saidenbachite started to crystallize from melt under ultrahigh pressure (UHP). This felsic igneous rock is known from two occurrences in the world only. The here reported new type of saidenbachite, found at the type locality in the central Erzgebirge Crystalline Complex, is characterized by millimetre‐sized aggregates of coarse‐grained quartz in a matrix of significantly smaller grains of garnet, rarely containing diamond inclusions, plagioclase, K‐feldspar, white mica, some dark mica, rutile and ±kyanite. According to the shape and the regular distribution of these aggregates showing similar size, they are interpreted as former coesite phenocrysts, which grew early in a felsic magma. This interpretation is corroborated by thermodynamic modelling demonstrating that coesite is the first major phase that crystallizes from SiO2‐rich (>66 wt%) magmas at UHP.