guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Colour and pleochroism
Figure 4-14 A-C. Change of absorption colour in crystal sections of biotite, actinolite and
aegirine-augite as the stage is rotated 360°. Shown are the four positions in which the
vibration directions of the two waves coincide exactly with the directions of the polarizers. In
these orientations only the E-W vibrating wave passes the crystal; the N-S wave is not
activated. Therefore, these crystal sections change their colour every 90° of rotation. In the
actinolite and aegirine-augite sections, these are the colours relating to the n y and n x’ waves,
and in biotite, the colours relating to the n z~y and n x waves.
As far as clinoamphiboles are concerned, the absorption colours parallel to X, Y and Z are
determined in two specific sections (Figs. 4-15,16,17):
(1) In crystal sections parallel to (010), the vibration directions Z and X are in the viewing
plane, except for some rare alkaline amphiboles. Such sections are commonly prismatic in
shape and can be recognised by their high interference colours (∆n = n z -n x ) (see Ch. 4.2.3).
Raith, Raase & Reinhardt – February 2012
(2) In crystal sections perpendicular to c, the vibration directions Y (parallel to b) and X’ are
in the viewing plane. These crystal sections are recognised by the characteristic intersection
of the {110} cleavage planes.
Crystal sections perpendicular to one of the two optic axes appear in a single colour
corresponding to Y as the stage is turned (n y wave only).
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