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Formula: CaAl2(F,OH)8
Halide
Crystal System: Monoclinic
Specific gravity: 2.88 to 2.894 measured, 2.898 calculated
Hardness: 4½
Streak: White
Colour: Greyish, white; colourless in transmitted light
Solubility: Decomposed by sulphuric acid
Environments
Plutonic igneous environments
Pegmatites
Metamorphic environments
Prosopite occurs disseminated in fluorine-rich granite,
greisen, and cryolite-bearing
granite pegmatites; more rarely it is found in base metal deposits. In
pegmatites it may be a primary mineral or an alteration product of
cryolite
(Mindat). Associated minerals include fluorite,
cryolite, thomsenolite,
pachnolite, gearksutite,
hydrokenoralstonite, weberite,
jarlite, siderite,
hematite, cassiterite,
kaolinite and quartz (HOM, Mindat).
Localities
At the type locality, the Tin mine, Altenberg, Erzgebirge District, Saxony, Germany, prosopite can form as an
alteration product of topaz in topaz-rich
greisen (Mindat), and may be partly altered to
kaolinite and fluorite (Dana).
Associated minerals include siderite,
kaolinite, hematite and
fluorite (Mindat).
At Saint Peter's dome, El Paso county, Colorado, USA, prosopite has been found forming a druse on massive
pachnolite (AM 34.383-397).
At the Morefield mine, Amelia county, Virginia, USA, prosopite is associated with
topaz,
fluorite, albite and
kaolinite (Dana).
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