Specific gravity: 2.88 to 2.894
Colour: Greyish, white; colourless in transmitted light
Solubility: Decomposed by sulphuric acid
Plutonic igneous 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,
kaolinite and quartz (HOM, Mindat).
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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