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Anorthoclase

Alkali feldspar mineral

Summary

Alkali feldspar mineral

Field	Value
name	Anorthoclase
image	Anorthoclase-tn23a.jpg
caption	Doubly terminated anorthoclase crystal from Kinki Region, Honshu, Japan. Size: 1.3 x
category	Tectosilicate minerals
group	Feldspar group
series	Alkali feldspar series
formula	(Na,K)AlSi3O8
IMAsymbol	Ano
strunz	9.FA.30
system	Triclinic
class	Pinacoidal ()
(same H–M symbol)
symmetry	C
unit cell	a = 8.28, b = 12.97
c = 7.15 [Å]; α = 91.05°
β = 116.26°, γ = 90.15°; Z = 4
color	Colorless, also white, pale creamy yellow, red, green
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	6
luster	Vitreous to pearly on cleavage planes
streak	White
diaphaneity	Transparent
gravity	2.57–2.60
opticalprop	Biaxial (−)
refractive	nα = 1.519–1.529
nβ = 1.524–1.534
nγ = 1.527–1.536
birefringence	δ = 0.008
2V	Measured: 34°–60°
references

(same H–M symbol) c = 7.15 [Å]; α = 91.05° β = 116.26°, γ = 90.15°; Z = 4 nβ = 1.524–1.534 nγ = 1.527–1.536 | length fast/slow =

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.

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 and above. Below 400 - only very limited solution exists (less than about 5% on both the low albite and microcline ends). Anorthoclase and high albite exhibit triclinic symmetry, whereas sanidine and the low temperature orthoclase have monoclinic symmetry. If the high temperature intermediate composition alkali feldspars are allowed to cool slowly, exsolution occurs and a perthite structure results.

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.

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.

References

Warr, L.N.. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine.
"Handbook of Mineralogy".
"Mindat.org".
"Webmineral data".
"Framework Silicates". Wiley.
"Handbook of Mineralogy". Wiley.

Wikipedia Source

This article was imported from Wikipedia and is available under the Creative Commons Attribution-ShareAlike 4.0 License. Content has been adapted to SurfDoc format. Original contributors can be found on the article history page.

alkali-feldspar-series triclinic-minerals minerals-in-space-group-2

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