Distinguishing the Feldspars

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay

The Principal Feldspars

Albite: NaAlSi₃O₈
Anorthite CaAl₂Si₂O₈
Other plagioclases form a continuous solid-solution series with any mixture of albite and anorthite possible. The composition of plagioclase is usually described in terms of the anorthite component. The plagioclase series consists of:

Anorthoclase: a solid solution of about equal proportions of K-feldspar and albite.
Perthite: an exsolution texture with K-feldspar enclosing plagioclase. If the texture is very fine it can be termed microperthite or cyptoperthite.
Antiperthite: an exsolution texture with plagioclase enclosing K-feldspar.
Peristerite: a microscopic- or smaller-scale exsolution texture in albite-rich plagioclase.

Celsian: BaAl₂Si₂O₈
Hyalophane: (K,Na,Ba)(Al,Si)₄O₈
The feldspar structure consists of zigzag chains of silica (and aluminum) tetrahedra enclosing rather large spaces where the alkali cations occur. The enclosing spaces are large, thus only large cations can be retained and there are no iron or magnesium feldspars. The closest mineral in composition to an iron-magnesium feldspar is probably the iron-manganese aluminosilicate cordierite, which has a completely different and unrelated structure.

Microcline: The principal potassium feldspar in granitic and metamorphic rocks. The low-temperature potassium feldspar.
Orthoclase: A high-temperature polymorph found in volcanic rocks and high-temperature metamorphic rocks.
Sanidine: The highest-temperature polymorph of potassium feldspar, found in volcanic rocks and very high-temperature metamorphic rocks.

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. Magmas are rarely sufficiently Na-rich to form albite.
Oligoclase (10-30% anorthite) is the principal plagioclase in granitic rocks.
Andesine (30-50% anorthite) is the principal plagioclase in intermediate rocks.
Labradorite (50-70% anorthite) is the principal plagioclase in gabbroic rocks and anorthosites.
Bytownite (70-90% anorthite) is the least common plagioclase. It is too calcic to form from most magmas and too sodic to form in most metamorphic rocks. It is the first feldspar to form in mafic magmas and may be present in the cores of zoned feldspars. It also forms from sodic metasomatism of metamorphic rocks.
Anorthite (90-100% anorthite) is too calcic to form magmatically and forms mostly from metamorphism of calc-silicate rocks.

Anorthoclase: a solid solution of about equal proportions of K-feldspar and albite. Generally a high-temperature mineral. The sodium-rich equivalent of orthoclase and sanidine.
Perthite is widely found in microcline in granitic rocks.

Celsian: BaAl₂Si₂O₈and Hyalophane: (K,Na,Ba)(Al,Si)₄O₈are rare and found, for some reason, mostly in manganese or deposits.

Created 20 December 1999, Last Update 20 December 1999