zinnwaldite

Zinnwaldite is a lithium-iron mica, a member of the phyllosilicate mica group. It is a lithium-bearing variety closely related to lepidolite and muscovite, and it is interpreted as a potassium lithium iron mica in which lithium and iron substitute for aluminum in the octahedral sheet while lithium partially replaces interlayer potassium. It crystallizes in the monoclinic system and forms tabular, plate-like crystals or fine-grained masses with the characteristic perfect basal cleavage of micas.

Physically, zinnwaldite is typically brown to gray or yellowish-brown and is transparent to translucent in thin splittings. It has a Mohs hardness of about 2.5 to 3 and a relatively low density for a mica, with cleavage that is easily observed along the basal planes.

Occurrence and associations: zinnwaldite occurs in lithium-rich pegmatites and hydrothermal veins, often in granitic settings. The type locality is the Zinnwald deposit in the Erzgebirge (Ore Mountains) on the border between Germany and the Czech Republic. It is commonly associated with other lithium minerals such as spodumene and lepidolite, as well as quartz, topaz, beryl, and other mica species.

Uses and significance: zinnwaldite has been mined as a source of lithium in some deposits, particularly where pegmatites are rich in lithium minerals. In modern lithium production, spodumene and other lithium-bearing minerals are more economically important, but zinnwaldite remains of interest in research and for historical ore records. The mineral is named after its type locality, the Zinnwald area.