Celsian

Celsian
Celsian (transparent/gray in photo) in sanbornite (white) and quartz matrix from Incline, Maricopa County, California (Size: 5 x 4 x 3 cm)
General
Category	Feldspar
Formula (repeating unit)	BaAl2Si2O8
IMA symbol	Cls[1]
Strunz classification	9.FA.30
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	I2/c
Unit cell	a = 8.622(4) Å, b = 13.078(6) Å, c = 14.411(8) Å; β = 115.2°; Z = 8
Identification
Color	Colorless, white, yellow
Crystal habit	Short prismatic to acicular crystals, massive
Twinning	Manebach twins on (001), baveno twins (021), rare lamellar twinning
Cleavage	Perfect on {001}, good on {010}, poor on {110}
Tenacity	Brittle
Mohs scale hardness	6–6.5
Luster	Vitreous
Diaphaneity	Transparent
Specific gravity	3.10 to 3.39
Optical properties	Biaxial (+)
Refractive index	nα = 1.580–1.584 nβ = 1.585–1.587 nγ = 1.594–1.596
Birefringence	0.014, biaxial –
2V angle	Measured: 86° to 90°
Other characteristics	[2][3][4][5]

Celsian is an uncommon feldspar mineral, barium aluminosilicate, BaAl₂Si₂O₈. The mineral occurs in contact metamorphic rocks with significant barium content. Its crystal system is monoclinic, and it is white, yellow, or transparent in appearance. In pure form, it is transparent. Synthetic barium aluminosilicate is used as a ceramic in dental fillings and other applications.

The mineral is named after Anders Celsius (1701–1744).