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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 -
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