pla gio clase
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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)
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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
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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
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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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Analyser out Analyser in
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)
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Plagioclase porphyroblasts in matrix ofmuscovite-rich schist
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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
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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
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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)
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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