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'Sanidine' is the high temperature form of potassium feldspar (K,Na)(Si,Al)₄O₈.^[1] Sanidine most typically occurs in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system. Orthoclase is a monoclinic polymorph stable at lower temperatures. At yet lower temperatures, microcline, a triclinic polymorph of potassium feldspar, is stable.
Due to the high temperature and rapid quenching, sanidine can contain more sodium in its structure than the two polymorphs that equilibrated at lower temperatures. Sanidine and high albite constitute a solid solution series with intermediate compositions termed anorthoclase. Exolution of an albite phase does occur; resulting cryptoperthite can best be observed in electron microprobe images.

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References

References

1. Mineral Handbook http://www.minsocam.org/msa/Handbook/Sanidine.PDF


★ Hurlbut, Cornelius S.; Klein, Cornelis, 1985, ''Manual of Mineralogy'', 20th ed., Wiley, ISBN 0-471-80580-7

★ Webmineral data

★ Mindat