Anorthoclase

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Anorthoclase
Anorthoclase-tn23a.jpg
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 Tectosilicate minerals, feldspar group, alkali series
Formula (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
ColorColorless, 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 hardness6
Luster Vitreous to pearly on cleavage planes
Streak White
Diaphaneity Transparent
Specific gravity 2.57–2.60
Optical propertiesBiaxial (−)
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 proportion of the sodium-aluminium silicate member exceeds the potassium-aluminium silicate member. It typically consists of between 64 and 90 percent of NaAlSi3O8 and between 10 and 36 percent of KAlSi3O8. [5]

Contents

An anorthoclase crystal from Mount Erebus, Antarctica Anorthoclase-219058.jpg
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. 1 2 3 "Handbook of Mineralogy" (PDF). Archived from the original (PDF) on 2016-03-04. Retrieved 2011-12-04.
  3. 1 2 "Mindat.org".
  4. ↑ "Webmineral data".
  5. ↑ Deere; Howie; Zussman. Framework Silicates. Rock Forming Minerals. Vol. 4. Wiley. pp. 2–5, Fig. 1.
  6. 1 2 3 Klein, Cornelis; Hurlbut, Cornelius S. Jr. Handbook of Mineralogy. Wiley. pp. 446–449, Fig. 11-95. ISBN   0-471-80580-7.
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