Well formed orange brown grischunite crystals associated to almost white tilasite crystals. Grischunite has only been found in this locality worldwide, so it's a real rarity. From: Falotta, Tinzen, Grischun, Switzerland. — Photo: David Hospital · CC BY-SA 4.0
Tilasite
A calcium-magnesium arsenate-fluoride named in 1895 for Swedish mining director Daniel Tilas.
Intro adapted from Wikipedia (CC BY-SA).
Tilasite is an IMA-recognized mineral species in the arsenate class, with the chemical formula CaMg(AsO₄)F.
It crystallizes in the monoclinic crystal system (space group C2/c). It is a member of the Titanite structural group.
Key properties: Mohs hardness 5; specific gravity 3.77; White streak; Resinous, vitreous on cleavages luster; optically biaxial.
IMA symbol Til, grandfathered status — known before the IMA’s founding, named in 1895.
is typically found in metamorphosed manganese or zinc deposits containing arsenic.
It is found associated with Manganoan diopside, manganberzeliite, svabite, allactite, pyroaurite, dixenite, bergslagite, hematite, calcite, barite (Långban, Sweden); spessartine.
The mineral was named to honor Daniel Tilas (1712–1772), Swedish mining engineer and mineral collector of Stockholm, Sweden.