Antarcticite

Antarcticite is an uncommon calcium chloride hexahydrate mineral with formula CaCl₂·6H₂O. It forms colorless acicular trigonal crystals. It is hygroscopic and has a low relative density of 1.715.

Antarcticite
General
Category	Halide mineral
Formula	CaCl2·6H2O
IMA symbol	Atc[1]
Strunz classification	3.BB.30
Crystal system	Trigonal
Crystal class	Trapezohedral (32) H-M symbol: (32)
Space group	P321
Unit cell	a = 7.9, c = 3.95 [Å]; Z = 1
Identification
Color	Colorless
Crystal habit	Occurs as groups of acicular crystals
Cleavage	Perfect on {0001}, very good on {1010}
Tenacity	Brittle
Mohs scale hardness	2–3
Luster	Vitreous
Diaphaneity	Transparent
Specific gravity	1.715
Optical properties	Uniaxial (−)
Refractive index	nω = 1.550 nε = 1.490–1.500
Birefringence	δ = 0.060
Other characteristics	Deliquescent
References	[2][3][4]

As its name implies, it was first described in 1965 for an occurrence in Antarctica where it occurs as crystalline precipitate from a highly saline brine in Don Juan Pond, in the west end of Wright Valley, Victoria Land. This discovery was made by Japanese geochemists Tetsuya Torii and Joyo Ossaka.^[5] It was also reported from brine in Bristol Dry Lake, California, and stratified brine within blue holes on North Andros Island in the Bahamas. It has also been noted within fluid inclusions within quartz in pegmatite bodies in the Bushveld complex of South Africa. It occurs in association with halite, gypsum and celestine in the California dry lake.^[2]

A similar mineral, sinjarite, the dihydrate of calcium chloride, crystallizes in the tetragonal system.^[6] Sinjarite is semitransparent, with pale pink color.^[7] Hydrophilite is a now discredited calcium chloride mineral that is considered to be either antarcticite or sinjarite.^[8][9]

See also

• Geology of Sinjar Mountains, for sinjarite