This diagram shows how feldspar minerals are classified on the basis of their chemical composition. The sequence of minerals along the base of the triangle represents the solid solution series of plagioclase between albite and anorthite.
Sanidine (K, Na) (AlSi 3 O 8) is the monoclinic high-temperature alkaline K-Na feldspar, which usually contains about 30%, but sometimes up to 62% sodium (Ab) component. It is found only in young volcanic discharges or volcanic rocks (rhyolite, trachyte, and dacite).
This diagram shows how feldspar minerals are classified on the basis of their chemical composition. The sequence of minerals along the base of the triangle represents the solid solution series of plagioclase between albite and anorthite.
Albite (0-10% anorthite) is found mostly in very Na-rich environments. It forms from more anorthite-rich plagioclase by ion-exchange with sea water, and as an authigenic mineral in marine sedimentary rocks.
Its chemistry and atomic arrangement are variable, so optical properties vary as well. The three principle varieties of K-feldspar are microcline (low temperature), orthoclase (medium temperature), sanidine (high temperature). Other, rare, related feldspar varieties include adularia and anorthoclase.
4Sanidine and genesis of magmas. 5References. Toggle the table of contents. Sanidine. 29 languages. Afrikaans. العربية.
1Structure and stability. 2Occurrence. 3See also. 4References. Toggle the table of contents. Anorthoclase. 14 languages. Afrikaans. Català. Deutsch.
There are two main series, alkali feldspar (K–Na) and plagioclase (Na–Ca), and the group includes orthoclase, microcline, albite, anorthite and the gem varieties moonstone, sunstone, amazonite and labradorite.
Volcanic rocks, cooling rapidly from high temperatures, retain the sanidine structure. On the other hand, microcline can be found only in plutonic rocks that cooled down very slowly.