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Doubly terminated anorthoclase crystal from Kinki Region, Honshu island, Japan (size: 1.3 x 0.9 x 0.7 cm)
(repeating unit)
IMA symbolAno[1]
Strunz classification9.FA.30
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupC1
Unit cella = 8.28, b = 12.97
c = 7.15 [Å]; α = 91.05°
β = 116.26°, γ = 90.15°; Z = 4
ColorColorless, also white, pale creamy yellow, red, green
Crystal habitShort prismatic crystals; tabular, rhombic, flattened along [010]
TwinningPolysynthetic twinning produces a grid pattern on [100]
CleavagePerfect on [001], other partings
Mohs scale hardness6
LusterVitreous to pearly on cleavage planes
Specific gravity2.57 - 2.60
Optical propertiesBiaxial (-)
Refractive indexnα = 1.519 - 1.529 nβ = 1.524 - 1.534 nγ = 1.527 - 1.536
Birefringenceδ = 0.008
2V angleMeasured: 34° to 60°

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]

An anorthoclase crystal from Mount Erebus, Antarctica

Structure and stability[edit]

Anorthoclase is an intermediate member of the high albitesanidine alkali feldspar solid solution series. Intermediate members of this series, high albite, anorthoclase and high sodium sanidine are stable at temperatures of 600 °C (1,112 °F) and above. Below 400 to 600 °C 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]


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]

See also[edit]


  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b c "Handbook of Mineralogy" (PDF).
  3. ^ a b "".
  4. ^ "Webmineral data".
  5. ^ Deere, Howie and Zussman, Rock Forming Minerals, Vol. 4 Framework Silicates, Wiley, pp. 2 - 5, Fig. 1
  6. ^ a b c Klein, Cornelis and Cornelius S. Hurlbut, Jr. Handbook of Mineralogy, Wiley, pp. 446-449 (Fig. 11-95 ISBN 0-471-80580-7