Prosopite

Description

Prosopite has a hardness of 4½ and a specific gravity of 2.88 – 2.894. It’s usually a “greyish, white (and sometimes) colourless in transmitted light” according to Mindat. It’s named from the Greek προσωπείον, for ‘a mask’, in allusion to the deceptive character of the mineral.

Prosopite can form as an alteration product of topaz in topaz-rich greisens (as in Altenberg, Saxony and Schlaggenwald, Bohemia), and in cryolite-bearing pegmatites, either as a primary mineral or as an alteration product of cryolite.

As a great sample of calcium, aluminum, fluorine, Hydrogen and oxygen, this mineral isn’t too expensive or valuable or used as an ore, but it’s pretty cool still!

It’s been found at 41 localities on most continents around the world, and five within Colorado alone, typically in Topaz-bearing greisens, porphyry, and in cryolite pegmatites, so keep an eye out for a cool rock like this on your next hike!