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

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 Albite–Anorthite 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.

Plagioclase var. labradorite (Ca,Na)[Al(Al,Si)Si2O8] in gabbro/anorthosite; Wolibórz (Volpersdorf), Kłodzko District, Lower Silesia (Dolnośląskie), Poland; 46 x 36 x 34 mm

Sunstone – reddish plagioclase (oligoclase) (Na,Ca)[Al(Si,Al)Si2O8] with inclusions of hematite Fe2O3 in granite; Bjordam Sunstone Quarry, Bjordam, Bamble (Bamle), Telemark, Norway; FOV: 33 mm

Crystal of plagioclase var. labradorite (Ca,Na)[Al(Al,Si)Si2O8] showing blue labradorescence in anorthosite; Golovinskoye deposit (Holowyne), Gorchin village, Chernyahovskiy district, Zhytomyr Oblast’ (Zhitomir Oblast’), Ukraine; crystal 34 x 29 mm

Rough plagioclase var. labradorite (Ca,Na)[Al(Al,Si)Si2O8] showing the typical multi-colour labradorescence; Southwestern Region, Tuléar Province, Madagascar; 78 x 45 x 12 mm

Plagioclase var. labradorite (Ca,Na)[Al(Al,Si)Si2O8] with blue labradorescence in gabbro/anorthosite; Wolibórz (Volpersdorf), Kłodzko District, Lower Silesia (Dolnośląskie), Poland; crystal 6 x 5 mm

Sunstone – reddish plagioclase (oligoclase) (Na,Ca)[Al(Si,Al)Si2O8] with inclusions of hematite Fe2O3; Bjordam Sunstone Quarry, Bjordam, Bamble (Bamle), Telemark, Norway; crystal 8 x 11 mm

See also: Feldspars II: Alkali feldspars

For more photos of plagioclases see also: PLAGIOKLAZY – czyli nieco „koloru” na fragment grupy skaleni