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Plagioclase

Mineral structure: Framework silicate

Common mineral in Most silicate rocks (igneous and metamorphic), marbles

Rock-forming minerals associated with plagioclase: Quartz, K-feldspar, Pyroxene, Amphibole

Plagioclase in Gabbro Plagioclase in Schist

Analyser in Analyser in

Key optical features of plagioclase in thin section:

Colourless

Low relief

Low orderinterference colours

Multiple twinning(often untwinned in low to medium grade metamorphic rocks)

Common minerals that might be confused with plagioclase and occur in similar rocks:

K-feldspar(either simple or polysynthetitc twinning,perthitic texture),

Quartz(unaltered clean appearance)

Common alteration product: Clay minerals(low T), Epidote (moderate T)

Analyser out Analyser in
http://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Alterationhttp://web2.ges.gla.ac.uk/~minerals/Quartz.htmhttp://web2.ges.gla.ac.uk/~minerals/K-Feldspar.htmhttp://web2.ges.gla.ac.uk/~minerals/Pyroxene.htmhttp://web2.ges.gla.ac.uk/~minerals/Amphibole.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Alterationhttp://web2.ges.gla.ac.uk/~minerals/Quartz.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Exsolutionhttp://web2.ges.gla.ac.uk/~minerals/K-Feldspar.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Twinninghttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interferencehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Reliefhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Colourhttp://web2.ges.gla.ac.uk/~minerals/Amphibole.htmhttp://web2.ges.gla.ac.uk/~minerals/Pyroxene.htmhttp://web2.ges.gla.ac.uk/~minerals/K-Feldspar.htmhttp://web2.ges.gla.ac.uk/~minerals/Quartz.htmhttp://web2.ges.gla.ac.uk/~minerals/Intro.htm


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Note: Contrast with clean quartz.

Note: Dusty alteration of feldspar in contrast to adjacent clean quartz. Alteration is typically patchy

within the grain.

Alteration of Ca-rich plagioclase to Na-rich plagioclase (albite) plus epidote


Plagioclase shape in basalt

Plagioclase forms well shaped rectangular laths in volcanic rocks these crystals may be zoned. Plagioclase forms

as a matrix phase in metamorphic rocks, rarely can form porphyroblasts

http://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Shapehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Alterationhttp://web2.ges.gla.ac.uk/~minerals/Quartz.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Alterationhttp://web2.ges.gla.ac.uk/~minerals/Quartz.htm


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Note: Ophitic texture withpyroxene (above) and isotropic basaltic glass (below ). Both examples with

olivine andpyroxene.

Plagioclase (dusty altered, twinned grains) in matrix ofbiotite schist (with clean quartz)


Plagioclase porphyroblasts in matrix ofmuscovite-rich schist

http://web2.ges.gla.ac.uk/~minerals/Muscovite.htmhttp://web2.ges.gla.ac.uk/~minerals/Quartz.htmhttp://web2.ges.gla.ac.uk/~minerals/Biotite.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Twinninghttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Alterationhttp://web2.ges.gla.ac.uk/~minerals/Pyroxene.htmhttp://web2.ges.gla.ac.uk/~minerals/Olivine.htmhttp://web2.ges.gla.ac.uk/~minerals/Pyroxene.htm


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Zoned twinned plagioclase within gabbro and granite

Analyser in Analyser in

Note: Different compositions of plagioclase are in extinction in slightly different positions, hence

interference colours pick out different compositional zones.

Plagioclase phenocrysts in volcanic rocks showing oscillatory zoning


Plagioclase twinning and composition

Note:Extinction angles on multiple twins in plagioclase may be used to estimate feldspar composition

(30o = Ca-rich.) Rotate stage place one pair of the twin in extinction position,

measure angle of rotation to extinction position of other twin pair,dividethe total angle by 2 to determine

extinction angle.

Na-rich plagioclase (Albite) in greenschist facies metabasic rock

Analyser in Stage rotated by 25 o (i.e. Extn angle ca. 12 o)
http://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Extinctionhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Twinninghttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interferencehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Extinctionhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Twinning


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Ca-rich plagioclase (Labradorite) in gabbro

Analyser in Stage rotated by 70o (i.e. Ext n angle ca. 35 o)


Note: twinning may be polysynthetic..looks a bit like microcline ( K-Feldspar)

Take thePlagioclase testReturn to top

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http://web2.ges.gla.ac.uk/~minerals/Intro.htmhttp://web2.ges.gla.ac.uk/~minerals/Plagioclase.htm#Tophttp://web2.ges.gla.ac.uk/~minerals/Plagioclase_assessment.htmhttp://web2.ges.gla.ac.uk/~minerals/K-Feldspar.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Twinning

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