guide to thin section microscopy - Mineralogical Society of America

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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). 71
Guide to Thin Section Microscopy Colour and pleochroism Raith, Raase & Reinhardt – February 2012 Figure 4-12. Determination of the colours relating to the vibration directions of the E- and O- waves of an optically uniaxial mineral, using tourmaline as an example. 72
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guide to thin section microscopy - Mineralogical Society of America

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