A member of the mica group of 2:1 phyllosilicates (sheet silicates) with formula K2(Fe2+2–1,Li2–3,Al2)[Si6–7Al2–1O20] (F,OH)4; sp. gr. 2.9–3.2; hardness 2–3; monoclinic; grey, brown, or even dark green; vitreous to pearly lustre; forms thick, tabular crystals; cleavage well marked; occurs in granites and pegmatites in association with scheelite, wolframite, cassiterite, and fluorite. It is pneumatolytic in origin (see pneumatolysis). It is named after Zinnwald, Saxony, Germany.
Subjects: Earth Sciences and Geography.