guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Extinction
Extinction angle
The angle between a vibration direction and the morphological reference element (crystal
edge, cleavage, twin plane) in a crystal section is referred to as the extinction angle.
Extinction angles are useful for the characterisation of monoclinic and triclinic minerals.
Example clinopyroxene: The monoclinic members of the pyroxene group show a correlation
between extinction angle ε, measured between vibration direction Z (n z ) and the trace of
(100) in a crystal section parallel to (010), and chemical composition, i.e. the type of
clinopyroxene (Fig. 4-39). The angle can be expressed as ε = Z Λ c.
Figure 4-39. Extinction angle in clinopyroxenes
A: Relation between composition and extinction angle ε = Z Λ c in crystal sections parallel to (010) for
some important members of the clinopyroxene group. The c-direction corresponds to the traces of
(100) crystal faces or the traces of the {110} cleavage. B: Method of determining the extinction angle.
Reading I: position of reference morphological element parallel to the N-S line of the crosshairs.
Reading II: extinction position.
Raith, Raase & Reinhardt – February 2012
Practical hints: The precise determination of extinction angles requires specific crystal
orientations. In the case of clinopyroxene, these are crystal sections with the highest
interference colour (Fig. 4-39). The extinction angle is determined in the same way as
measuring angles between morphological planes (e.g., cleavage planes), whereby the
polarizers must be exactly oriented in E-W resp. N-S direction. The measurement is done in
two steps (Figs. 4-39 B): (1) Rotation of the morphological reference direction (trace of a
crystal face or cleavage) into N-S direction (= N-S line of the crosshairs) and reading the
angle value I from the vernier on the microscope stage; (2) rotation of the vibration direction
into N-S direction (which is equivalent to the extinction position of the crystal section) and
reading the angle value II at the vernier. The difference between the values is the extinction
angle ε.
Example clinoamphibole: The monoclinic members of the amphibole group show a
correlation between extinction angle ε, measured between vibration directions Z, Y or X and
the trace of (100) in a crystal section parallel to (010), and chemical composition, i.e. the type
of clinoamphibole (Fig. 4-40 A). The measurement of the extinction angle is shown in Fig. 4-
40 B and is carried out in the same way as for clinopyroxene.
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