Feldspars I: Plagioclase group

Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals that make up as much as 60% of the Earth’s crust.

In petrology the group is split into two major groups: Alkali Feldspar and Plagioclase

feldspar-classification

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.

The Plagioclase series (or AlbiteAnorthite Series) is a group of feldspar minerals that essentially have the same formula but vary in their percentage of sodium and calcium. Albite and Anorthite are the end members of the series, with the intermediary minerals Oligoclase, Andesine, Labradorite, and Bytownite.

The acclaimed Dana’s System of Mineralogy lists the intermediary members as individual minerals, whereas the IMA does not recognize them as individual mineral species. Thus, some consider Oligoclase and Andesine to be a variety of Albite, and Labradorite and Bytownite to be a variety of Anorthite.

List of plagioclase group members as used in petrology.

Name Composition Formula
Anorthite An100Ab to An90Ab10 CaAl2Si2O8
Bytownite An90Ab10 to An70Ab30 (Ca,Na)Al(Al,Si)Si2O8
Labradorite An70Ab30 to An50Ab50 (Ca,Na)Al(Al,Si)Si2O8
Andesine An50Ab50 to An30Ab70 (Na,Ca)Al(Si,Al)Si2O8
Oligoclase An30Ab70 to An10Ab90 (Na,Ca)Al(Si,Al)Si2O8
Albite An10Ab90 to AnAb100 NaAlSi3O8

The Plagioclase Feldspars are  important rock-forming minerals and occur in numerous mineral environments. In many instances, it is very hard to tell apart one from another. The percentages of sodium and calcium for each member listed above are an approximate value; the actual percentage can vary as much as 20 percent.

See also: Feldspars II: Alkali feldspars
For more photos of plagioclases see also: PLAGIOKLAZY – czyli nieco „koloru” na fragment grupy skaleni
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3 thoughts on “Feldspars I: Plagioclase group

  1. Pingback: Feldspars II: Alkali feldspars – mineralcollectionblog

  2. Pingback: PLAGIOKLAZY – czyli nieco „koloru” na fragment grupy skaleni | REALGAR

  3. Pingback: PLAGIOKLAZY – czyli nieco „koloru” na fragment grupy skaleni | REALGAR

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