Plagioclase

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PlagioclaseFeldsparUSGOV.jpg
Plagioclase feldspar

Plagioclase feldspars are a very important group of igneous rock forming tectosilicate minerals. Plagioclase comprises a solid solution series: NaAlSi3O8 (Albite)- CaAl2Si2O8 (Anorthite)

Plagioclase feldspars range between two endmembers: the sodic feldspar endmember albite and the calcic feldspar endmember anorthite. The composition of plagioclase feldspar is typically determined by the percentage of anorthite (%An) in system. The members of this series are known as the sodic-lime feldspars, and as a group are called the plagioclase feldspars, which comes from Greek meaning "oblique fracture" in reference to the two not-quite-right-angle cleavages. There are four intermediate plagioclase feldspars that are recognized between albite and anorthite based on the %An. The following table expresses the compositions of the different plagioclase feldspars in terms of %An, as well as the percentage of albite in the system.

Plagioclases
Name% NaAlSi3O8(%Albite)% CaAl2Si2O8(%Anorthite)
Albite100-900-10
Oligoclase90-7010-30
Andesine70-5030-50
Labradorite50-3050-70
Bytownite30-1070-90
Anorthite10-090-100
Labradorite displaying typical iridescent effect termed labradorescence
Enlarge
Labradorite displaying typical iridescent effect termed labradorescence

Albite is named from the Latin albus, in reference to its unusually pure white color. It is a relatively common and important rock-making mineral associated with the more acid rock types and in pegmatite dikes, often with rarer minerals like tourmaline and beryl.

Anorthite was named by Rose in 1823 from the Greek meaning oblique, referring to its triclinic crystallization. Anorthite is characeristic of the mafic igneous rocks such as gabbro and basalt.

The intermediate members of the plagioclase group are very similar to each other and normally cannot be distinguished except by optical means.

Oligoclase is common in granite, syenite, diorite and gneiss. It is a frequent associate of orthoclase. The name oligoclase is derived from the Greek for little and fracture, in reference to the fact that its cleavage angle differs significantly from 90°. Sunstone is mainly oligoclase (sometimes albite) with flakes of hematite.

Andesine is a characteristic mineral of rocks such as diorite which contain a moderate amount of silica and related volcanics such as andesite.

Labradorite is the characteristic feldspar of the more basic rock types such as diorite, gabbro, andesite or basalt and is usually associated with one of the pyroxenes or amphiboles. Labradorite frequently shows an iridescent display of colors due to minute inclusions of another mineral. It is named after Labrador, where it is a constituent of the intrusive igneous rock anorthosite which is composed almost entirely of plagioclase. A variety of labradorite known as spectrolite is found in Finland.

Bytownite, named after the former name for Ottawa, Canada (Bytown), is a rare mineral occasionally found in more basic rocks.

See also

References

  • Alkali feldspar U. Texas (http://www.tmm.utexas.edu/npl/mineralogy/Science_of_Minerals/AlkalFfeldsparSeries.htm)
  • Webmineral (http://webmineral.com/data/Plagioclase.shtml)
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