Dacostaite

dacostaite

antimony

stibnite

gypsum

Images

Formula: K(Mg2Al)[Mg(H2O)6]2(AsO4)2F6.2H2O
Hydrated arsenate
Crystal system: Monoclinic
Specific gravity: 2.162 calculated for the empirical formula
Streak: White
Colour: Colourless to white
Luminescence: Not fluorescent under long wave or short wave UV
Environments

Sedimentary environments
Hydrothermal environments

Localities

At the type locality, Le Cetine di Cotorniano Mine, Chiusdino, Siena Province, Tuscany, Italy, the antimony ore deposit formerly exploited at this locality is probably related to the circulation of hydrothermal fluids. Antimony ores, represented almost exclusively by stibnite, are associated with jasperoids and vuggy silica masses replacing host rocks at the contact with the overlying argillic formations.
In the type material, dacostaite occurs as aggregates of thin pseudo-hexagonal micaceous, colourless to white flakes up to 0.5 mm in size. The streak is white, and the lustre is silky. The dacostaite is associated with quartz, sulphur, gypsum and a pharmacosiderite-like mineral in a small cavity of silicified limestone (EJM 37.39–52).
Dacostaite from Le Cetine di Cotorniano Mine - Image

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