Anorthoclase

Anorthoclase
Doubly terminated anorthoclase crystal from Kinki Region, Honshu, Japan. Size: 1.3 cm × 0.9 cm × 0.7 cm (0.5 in × 0.4 in × 0.3 in)
General
Category	Feldspar
Formula (repeating unit)	(Na,K)AlSi3O8
IMA symbol	Ano[1]
Strunz classification	9.FA.30
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H–M symbol)
Space group	C1
Unit cell	a = 8.28, b = 12.97 c = 7.15 [Å]; α = 91.05° β = 116.26°, γ = 90.15°; Z = 4
Identification
Color	Colorless, also white, pale creamy yellow, red, green
Crystal habit	Short prismatic crystals; tabular, rhombic, flattened along [010]
Twinning	Polysynthetic twinning produces a grid pattern on [100]
Cleavage	Perfect on [001], other partings
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	6
Luster	Vitreous to pearly on cleavage planes
Streak	White
Diaphaneity	Transparent
Specific gravity	2.57–2.60
Optical properties	Biaxial (−)
Refractive index	nα = 1.519–1.529 nβ = 1.524–1.534 nγ = 1.527–1.536
Birefringence	δ = 0.008
2V angle	Measured: 34°–60°
References	[2][3][4]

The mineral anorthoclase ((Na,K)AlSi₃O₈) is a crystalline solid solution in the alkali feldspar series, in which the sodium-aluminium silicate member exists in larger proportion. It typically consists of between 10 and 36 percent of KAlSi₃O₈ and between 64 and 90 percent of NaAlSi₃O₈.^[5]