mineralogical features observed with polarizing light
Post on 15-Feb-2022
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Mineralogical Features Observed with Polarizing Light Microscopy
Uniaxial figure
Biaxial figure
Albite twins
Perthite
Tartan (gridiron) twinning
Determining the
optic sign for a
uniaxial mineral.
A uniaxial
negative sign
Centered uniaxial optic axis figure.
Note the large number of rings
indicating high birefringence.
Off center uniaxial optic axis.
Determining the sign of a
biaxial mineral.
Biaxial +
Biaxial -
Biaxial interference figures and the
determination of 2V.
Albite Twinning
Tartan Twinning
Carlsbad-Albite Twinning
Carlsbad Twinning
Perthites – plagioclase exsolved in K-feldspar
Cleavage in Pyroxenes – two cleavage directions at 90o
Cleavage in Amphiboles – two cleavage directions at 60o and 120o
Micas in plane light (left) and crossed polars (right)
Ms
Ms
Bio
Bio
Bio
Using Albite twins to determine
the composition of plagioclase
Using combined Carlsbad-Albite
twins to determine plagioclase
composition
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