Grandaite
Grandaite is an IMA-recognized mineral species in the arsenate class, with the chemical formula Sr₂Al(AsO₄)₂(OH).
It crystallizes in the monoclinic crystal system (space group P2_1/m). It is a member of the Brackebuschite structural group.
Key properties: Mohs hardness 6–6.5; specific gravity 4.378; Reddish brown streak; Waxy to vitreous luster; optically biaxial.
IMA symbol Grd, IMA status: approved, named in 2013.
From a small Fe-Mn-As deposit in metamorphosed quartzites (high-P, low-T blueschist facies).
It is found associated with Aegirine, adelite, barite, braunite, hematite, quartz, tilasite.
The name refers to the informal appellation of the Province where the Valletta mine is located - called ‘‘la Granda’’ (the Big One) for its considerable areal extent, one of the biggest in Italy.