guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Extinction
Figure 4-40. Extinction angle in clinoamphiboles
A: Relation between composition and extinction angle ε = X,Y,Z Λ c in crystal sections parallel to
(010) for some important members of the clinoamphibole group; B: Method of determining the
extinction angle.
Example plagioclase: In plagioclase solid solutions, the orientation of the indicatrix in
relation to the axes a, b and c of the triclinic crystal varies, dependent on anorthite content
and the degree of Al/Si ordering. The vibration directions of the pair of waves change
accordingly in crystal sections of defined orientation. At a constant degree of ordering, the
vibration directions depend on anorthite content alone (Fig. 4-41 A,B).
Raith, Raase & Reinhardt – February 2012
Figure 4-41 A. Relation between indicatrix orientation and anorthite content in plagioclase
with maximum Al-Si order (from slowly cooled plutonic and metamorphic rocks).
The anorthite content of plagioclase solid solutions in such rocks can be determined reasonably well
by measuring the extinction angle ε. ε is the angle between the vibration direction X' and the trace of
(010) in a section orthogonal to the direction [100] (Method of Rittmann, 1929).
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