Anorthoclase
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.[1]
Structure and stability
[edit]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).[2] Anorthoclase and high albite exhibit triclinic symmetry, whereas sanidine and the low temperature orthoclase have monoclinic symmetry.[2] If the high temperature intermediate composition alkali feldspars are allowed to cool slowly, exsolution occurs and a perthite structure results.[2]
Occurrence
[edit]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.[3]
It was first described in 1885 for an occurrence on Pantelleria Island, Trapani Province, Sicily. The name is from the Greek an- (Template:Wikt-lang), orthós (Template:Wikt-lang) and klásis (Template:Wikt-lang) ('not cleaving at right-angles'), for its oblique cleavage.[3][4]
See also
[edit]References
[edit]- ↑ Deere; Howie; Zussman. Framework Silicates. Rock Forming Minerals. 4. Wiley. pp. 2–5, Fig. 1.
- ↑ 2.0 2.1 2.2 Klein, Cornelis; Hurlbut, Cornelius S. Jr. Handbook of Mineralogy. Wiley. pp. 446–449, Fig. 11–95. ISBN 0-471-80580-7.
- ↑ 3.0 3.1 Cite error: Invalid
<ref>tag; no text was provided for refs namedHBM - ↑ Cite error: Invalid
<ref>tag; no text was provided for refs namedMindat