Acuminite
A rare strontium-aluminium fluoride known only from the cryolite deposit at Ivigtut, Greenland; its name refers to a spear point.
Intro adapted from Wikipedia (CC BY-SA).
Acuminite is an IMA-recognized mineral species in the halide class, with the chemical formula SrAlF₄(OH)·H₂O.
It crystallizes in the monoclinic crystal system (space group C2/c). It is a member of the Tikhonenkovite structural group.
Key properties: Mohs hardness 3.5; specific gravity 3.295; refractive index 1.457; optically biaxial.
IMA symbol Acu, IMA status: approved, named in 1986.
Found in a small cavity in a specimen from a strontium-rich portion of a cryolite deposit.
It is found associated with Fluorite, jarlite, thomsenolite, pachnolite, ralstonite, gearksutite, celestine.
The name comes from the Latin acuminis, sharp point, for spear head, the characteristic shape of the crystals.