ZINNWALDITE

    Class : Silicates
    Subclass : Phyllosilicates
    Crystal system : Monoclinic
    Chemistry : K(Li,Fe,Al)3(AlSi3O8)O10(F,OH)2
    Rarity : Uncommon

Zinnwaldite is a fairly rare phyllosilicate belonging to the mica group. It is characteristic of pneumatolytic deposits, occurring in sodolithic pegmatites and greisens of tin deposits where it accompanies other lithium minerals, as well as cassiterite, wolframite, topaz and scheelite. Its fluorine content (over 7%) underlines this genesis in pneumatolytic conditions. It owes its name to its discovery location : Zinnwald (Czech Republic). Unlike lepidolite, another lithium mica whose pink color is decisive, zinnwaldite generally has a dark gray to brownish gray color, sometimes brownish or yellowish which can lead to confusion with other micas. Pseudohexagonal crystals are often perfectly automorphic. It is an ancillary ore of lithium.

Main photo : Zinnwaldite from Poudrette Quarry, Mont St-Hilaire, Quebec, Canada © Alex Venzke

Zinnwaldite in the World

Superb crystals of zinnwaldite come from the pegmatites of Virgem da Lapa in Minas Gerais (Brazil). Very good specimens have also been extracted from American pegmatites in California (Pala), Colorado (Pikes Peak), South Dakota (Keystone), etc... But the most beautiful specimens in the world are the magnificent crystals discovered in the tin veins of Cinovec (ex-Zinnwald) in the Czech Republic. High-quality samples have also been produced by the tin veins of English Cornwall (St-Just). Finally, let us mention Narssaq (Greenland), where zinnwaldite occurs in the original setting of an alkaline complex.

Photo on the right : Zinnwaldite and quartz from Zinnwald-Cínovec, Czech Republic © Jakub Jirásek

Zinnwaldite in France

In France, zinnwaldite is known in Echassières (Allier) as well as in Salsigne (Aude).

Twinning

No twinning is known for this mineral species.

Fakes and treatments

No fakes listed for this mineral species.


Hardness : 2 to 3
Density : 2.58 to 2.82
Fracture : Micaceous
Streak : White

TP : Translucent to transparent
RI : 1.530 to 1.559
Birefringence : 0.025 to 0.029
Optical character : Biaxial -
Pleochroism : None
Fluorescence : Yellow

Solubility : -

Magnetism : NoneRadioactivity : None