Optical Determination of Plagioclase Composition:
Several methods are commonly used to determine plagioclase An content under
the microscope. One of the easiest techniques to apply is the Michel-Levy
method, which relies on the extinction angles of Albite twins, the most
common plagioclase twin type. The measuring procedure is relatively simple:
Select a plagioclase grain with undistorted, easily seen polysynthetic twins
that show uniform illumination when the twin composition planes are oriented
exactly N-S (or E-W), as shown in the lower left. Perfectly oriented twins
for this method will be perpendicular to the microscope stage.
Rotate the stage from this position and measure the angle at which one set
of twins becomes extinct, as in the center picture below.
Rotate the stage from the N-S position in the opposite direction (counterclockwise
in this case), and measure the angle at which the alternate twin set goes
extinct (lower left photograph below). The two extinction angles should
be equal but can vary 4 or 5 degrees and still be useful. The average of
the two angles is used to determine the An content using graphs available
in most optical mineralogy books. This is pretty much a statistical method,
and several grains should be measured to attain reasonable accuracy for
the plagioclase composition.
Petrography
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