guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Double refraction
4.2.3.1 Observation without analyzer (plane-polarized light mode)
In plane-polarized light, anisotropic minerals can only be distinguished from isotropic
minerals if characteristic grain shapes are observed (e.g., elongate or platy habit), if the grains
display relief changes such as chagrin contrast as the stage is turned (only in minerals with
large differences in n z ' and n x '), or if the absorption colour changes with changing orientation
(pleochroism).
The birefringence (∆n) of minerals is commonly not large enough to create distinct chagrin
effects, with the exception of carbonates. Fig. 4-22 shows the striking change of chagrin in
calcite and dolomite due to their extreme birefringence (∆n = 0.172 Cal resp. 0.177 Dol ).
Figure 4-22. Change of chagrin in calcite and dolomite during a 360˚ rotation of the stage.
Crystal sections are oriented parallel to the c axis. Shown are the four positions where the vibration
directions of the two waves in the crystal coincide exactly with the polarizer directions. In these
positions, only the E-W vibrating wave is passing through the crystal. The large difference between the
refractive indices of the O- and E-waves causes the change in chagrin (∆n = 0.172 Cal resp. 0.177 Dol ).
The majority of minerals show no or very little pleochroism. Exceptions include tourmaline,
members of the amphibole group, Fe-Ti-rich biotites as well as less common minerals such as
piemontite, sapphirine, dumortierite, yoderite and lazulite (Fig. 4-10).
Raith, Raase & Reinhardt – February 2012
Pleochroic minerals of tetragonal, hexagonal and trigonal symmetry show two characteristic
absorption colours parallel to the vibration directions of the E- and O-waves (dichroism).
Crystal sections normal to the crystallographic c-axis (= optic axis) only show the absorption
colour of the O-wave as the stage is rotated. Crystal sections parallel to the c-axis show an
alternation between the absorption colour of the E-wave (E-W orientation of c) and the O-
wave (N-S orientation of c) every 90˚ during stage rotation (Ch. 4.2.1, Figs. 4-11,12).
Pleochroic minerals of orthorhombic, monoclinic and triclinic symmetry show three
characteristic absorption colours parallel to the principal indicatrix axes X, Y and Z
(trichroism). Crystal sections normal to one of the two optic axes show the absorption colour
of the Y vibration direction as the stage is rotated. An identification of the absorption colours
in the X, Y and Z directions requires specific crystal sections (Ch. 4.2.1, Figs. 4-14−17).
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