00.2.5.137.11 (Zinnwaldite)

Zinnwaldite is a very rare member of the mica group and is not well known even by mineral collectors. Zinnwaldite is darker colored than typical muscovite but lighter than phlogopite or biotite The dark color and density in the three iron containing micas increase with an increase in the iron and magnesium content. Zinnwaldite is difficult to distinguish from the other micas at least by ordinary methods and lacallity and enviroment are keys to identification. It is limited to special granites and their pegmatites. Zinnwaldite, like other micas, has a layered structure of lithium iron aluminum silicate sheets weakly bonded together by layers of potassium ions. These potassium ion layers produce the perfect cleavage. Zinnwaldite is rarely available to collectors but some nice specimens with apatite are now on the market showing nice tabular crystals.