guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Colour and pleochroism
Pleochroic minerals of tetragonal, hexagonal and trigonal symmetry show two specific
absorption colours (dichroism), parallel to the vibration directions of the E- and O-waves.
Sections orthogonal to the crystallographic c-axis (= optic axis) generally show the absorption
colour of the O-wave as the microscope stage is rotated. Sections parallel to the c-axis show
an alternation between the absorption colour of the E-wave (E-W orientation of the c-axis)
and the O-wave (N-S orientation of the c-axis) for every 90° rotation of the stage (Fig. 4-11
and Fig. 4-12: example tourmaline).
Figure 4-11. Change of absorption colour of tourmaline during a 360° rotation of the
microscope stage. Shown are the four positions of a crystal section 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.
Pleochroic minerals of orthorhombic, monoclinic and triclinic symmetry possess three
characteristic absorption colours (trichroism) related to vibration directions parallel to the
principal indicatrix axes X, Y and Z. The determination of these colours must be performed
on selected crystal sections that are parallel to two of these axes (Fig. 4-14 A-C). The relation
between crystal orientation, vibration direction and absorption (pleochroism) for members of
the amphibole group is explained in Figs. 4-15,16,17. As the example of the amphiboles
demonstrates beautifully, there is a correlation between absorption colour and chemical
composition (Fig. 4-13).
Raith, Raase & Reinhardt – February 2012
Figure 4-13. Colours of amphiboles of different composition, in relation to the principal
vibration direction parallel b. The amphibole grains are oriented such that the b-axis is in
alignment with the lower polarizer. In these minerals, b is parallel to the indicatrix axis Y.
Hence, the colours displayed correlate with Y (cf. Fig. 4-15).
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