Acuminite

Acuminite is a rare halide mineral with chemical formula: SrAlF₄(OH)·(H₂O). Its name comes from the Latin word acumen, meaning "spear point". Its Mohs scale rating is 3.5.

Acuminite
General
Category	Halide mineral
Formula	SrAlF4OH·(H2O)
IMA symbol	Acu[1]
Strunz classification	3.CC.10
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Unit cell	a = 13.223, b = 5.175 c = 14.251 [Å]; β = 11.61°; Z = 8
Identification
Color	White
Crystal habit	Clusters of acute dipyramidal Spearhead shaped crystals
Twinning	Contact on {100}
Cleavage	Perfect on {001}
Mohs scale hardness	3.5
Luster	Vitreous
Diaphaneity	Transparent
Specific gravity	3.295
Optical properties	Biaxial (+)
Refractive index	nα = 1.451 nβ = 1.453 nγ = 1.462 - 1.463
References	[2][3][4]

Acumenite has only been described from its type locality of the cryolite deposit in Ivigtut, Greenland.^[2]

See also

• List of minerals