Chemical: KAl2(Si3Al)O10(OH,F)2 A rather long equation for a potassium aluminum silicate
Crystal System: Monoclinic
Formation: In igneous rocks and some metamorphic formations.
Properties: Forms mostly in tabular sheets as shown above and below. In large sizes this interesting mineral was once used to make window panes. Large crystal formations will cleave off in perfect sheets, sometimes large enough to make windows. Before there was glass windows there were muscovite windows as shown in the photograph at left. This piece measures over 12 inches by 8 inches as shown beside this US Quarter coin.