Keratophyres are Na-rich extrusive rocks, containing pure sodic plagioclase with or without quartz.
Keratophyre dikes and sills are found in some ophiolites, and keratophyre lava flows are present in some island arcs.
The photograph, under crossed polarizers, shows twinned albite plagioclase phenocrysts and microlites, with tiny quartz grains, in a glassy matrix.