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]

Structure and stability

Anorthoclase is an intermediate member of the high albite – sanidine alkali feldspar solid solution series. Intermediate members of this series, high albite, anorthoclase and high sodium sanidine are stable at temperatures of 600 °C (1,100 °F) and above. Below 400–600 °C (800–1,100 °F) only very limited solution exists (less than about 5% on both the low albite and microcline ends).^[6] Anorthoclase and high albite exhibit triclinic symmetry, whereas sanidine and the low temperature orthoclase have monoclinic symmetry.^[6] If the high temperature intermediate composition alkali feldspars are allowed to cool slowly, exsolution occurs and a perthite structure results.^[6]

Occurrence

Anorthoclase occurs in high temperature sodium rich volcanic and hypabyssal (shallow intrusive) rocks. The mineral is typically found as a constituent of the fine grained matrix or as small phenocrysts which may occur as loose crystals in a weathered rock.^[2]

It was first described in 1885 for an occurrence on Pantelleria Island, Trapani Province, Sicily. The name is from the Greek αν-, ορθός and κλάσις (not cleaving at right-angles), for its oblique cleavage.^[2][3]