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Amphibole
Mineral structure: Chain silicate
Common mineral in Granites, Diorites, Metabasics,
Calc-silicates
Rock-forming minerals associated with Amphibole: Plagioclase
Metabasic rocks may contain large amounts of amphibole at
moderate grades = Amphibolites.
Amphibole in Metabasic rock
Analyser out Analyser in
Key optical features of amphibole in thin section:
Green (more rarely Brown, Colourless or Blue)
Two cleavages (intersect at 120 o)
Pleochroic
Moderate interference colours
Inclinedextinction (typically
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Complete or partial alteration at lower temperature associated
with influx of fluids
Amphibole shape
Amphibole forms typically well-shaped crystals in rocks with
abundant quartz orplagioclase. Ideal crystal forms
are elongate with diamond-shaped or stretched hexagon cross
section.
In rocks with abundant Fe-Mg silicates amphibole typically is
equant
Amphibole in granite Amphibole in Calc-silicate
Analyer out
Amphibole in Gneiss Amphibole in contact metabasic
Analyser out
Amphibole colours and composition (images with analyser out)
Glaucophane (blue-mauve-colurless) Actinolite (blue green
green)
http://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Colourhttp://web2.ges.gla.ac.uk/~minerals/Plagioclase.htmhttp://web2.ges.gla.ac.uk/~minerals/Quartz.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Shape
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Tremolite (colourless) Hornblende (green brown)
Hornblende (brown)
Note: Different amphibole compositions associated with different
rock types and conditions
Glaucophane - Na-rich found stable at low T and high P
(blueschists)
Tremolite - Mg-rich found in metamorphosed carbonates and
peridotitesActinolite Fe-rich stable at moderate T (moderate T
metamorphic rocks)
Hornblende Fe-Al-rich stable at high T (igneous rocks and high T
metamorphic rocks)
Typically paler amphiboles contain less Fe and more Mg
Amphibole pleochroism in amphibolite
Analyser out Stage rotated by 90 o
http://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Pleochroism
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Note: Regional metamorphism may cause elongate amphiboles to
align in similar orientation. However
amphibole-rich rocks rarely contain a strong cleavage in
comparison to mica-rich lithologies.
Cleavage in Amphibole
Amphibole has two cleavages running parallel to the length of
the crystal. Thus elongate sections through
amphibole crystals show a single cleavage, cross sections show
two cleavages intersecting at 120 o. Note:
grains showing two well-developed cleavages may be tricky to
find
All images with analyser out
Inclined extinction in Amphibole
Tremolite in a calcite marble
Analyser in Analyser in
http://web2.ges.gla.ac.uk/~minerals/Calcite.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Extinctionhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Cleavage
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Actinolite in a metabasic rock
Analyser out Analyser in
Note: extinction angles measured in elongate crystals
typically
