guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Double refraction
Determination of birefringence using the interference colour: Birefringence is an
important property of anisotropic minerals and is crucial for mineral determination. However,
the amount of birefringence of a specific mineral depends on the orientation of the section. In
optically uniaxial minerals it varies from zero in the section perpendicular to the optic axis
(direction of optical isotropy), to a maximum value (|n e -n o |) in sections parallel to the optic
axis. In optically biaxial minerals it varies from zero in sections perpendicular to one of the
two optic axes, to a maximum value (n z -n x ) in the section parallel to the optic plane. For this
reason, the grains of each anisotropic mineral show different interference colours in thin
section dependent on their crystallographic orientation (Figs. 4-31). For routine mineral
identification, only the maximum birefringence (∆n = n z -n x ) values are critical (Fig. 4-32),
and these values are commonly listed in mineral-optical tables or compilations.
The determination of interference colours may be difficult in highly birefringent minerals
(such as carbonates: so-called high-order white). Wedge-shaped grain boundaries provide a
means to observe the colour spectrum, in relation to decreasing crystal thickness, down to
first-order black (Fig. 4-33).
Raith, Raase & Reinhardt – February 2012
Figure 4-33. A. Calcite grains in a thin section of marble appear in high-order white (+Pol).
B. The wedge-shaped margin of a calcite grain shows a spectrum of decreasing interference
colours from the interior plateau to the outer edge as the crystal thickness decreases from 25
to 0 μm. Five colour orders can be recognised using the red bands as a reference. C. A calcite
grain cut exactly parallel to the c axis would show an 8 th order white at a standard thickness
of 25 μm.
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