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
CategoryFeldspar
Formula
(repeating unit)		(Na,K)AlSi3O8
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
Identification
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
FractureUneven
TenacityBrittle
Mohs scale hardness6
LusterVitreous to pearly on cleavage planes
StreakWhite
DiaphaneityTransparent
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°–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]

An anorthoclase crystal from Mount Erebus, Antarctica

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]

See also

References

  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). Archived from the original (PDF) on 2016-03-04. Retrieved 2011-12-04.
  3. ^ a b "Mindat.org".
  4. ^ "Webmineral data".
  5. ^ Deere; Howie; Zussman. Framework Silicates. Rock Forming Minerals. Vol. 4. Wiley. pp. 2–5, Fig. 1.
  6. ^ a b c Klein, Cornelis; Hurlbut, Cornelius S. Jr. Handbook of Mineralogy. Wiley. pp. 446–449, Fig. 11-95. ISBN 0-471-80580-7.