guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Double refraction
arranged as a function of birefringence and crystal (or thin section) thickness (Fig. 4-30). The
resulting, more complex hyperbolic colour arrangement has been calculated by Bjørn Eske
Sørensen using MATLAB. The correlation between the birefringence value of an unknown
mineral in a section of known thickness and the mineral's interference colour can be
conceived intuitively (Fig. 4-32, right-hand side). Similarly, reading the thin section thickness
from the reconfigured colour chart using the maximum interference colour of a known
mineral is straightforward. The dependence of interference colour on thin section thickness is
immediately evident. However, for getting the numerical correlation between retardation and
∆n, or between retardation and d, the Michel-Lévy chart is the better choice.
When using interference colours for the determination of minerals it has to be kept in mind
that the retardation which accumulates as the waves pass through the crystal is not only
dependent on ∆n of that particular crystal section, but also on the thickness of the sample. In
order to correlate interference colours and birefringence values directly, the thin section
thickness has to be known. Therefore, thin sections are prepared with a defined standard
thickness (commonly 25 or 30 μm). However, there may be reasons to deviate from these
standard values, for example, if the contrast of interference colours between low-birefringent
phases is to be increased; or, to improve the contrast between differently oriented grains of
extremely high-birefringent minerals, the interference colours can be reduced to a lower order
(as in ultra-thin sections of carbonates).
Raith, Raase & Reinhardt – February 2012
Figure 4-31. Relation between interference colour and grain (crystal) orientation of anisotropic
minerals, using quartz, diopside and anhydrite as examples.
A: The euhedral quartz crystals from a vein show creamy-white to dark grey first-order interference
colours. Creamy-white crystals: sections parallel to the c-axis [max. birefringence (n e – n o ) = 0.009];
dark grey crystal: section nearly orthogonal to c [circular indicatrix section showing n o ; birefringence =
0].
B: The diopside grains in a calcsilicate rock show different interference colours, dependent on crystal
orientation, ranging from second-order blue-green to first-order dark grey. Blue-green grain: section
parallel to (010) resp. optic axial plane [max. birefringence (n z – n x ) = 0.031]; dark grey grain: section
nearly orthogonal to one of the two optic axes [circular indicatrix section showing n y ; birefringence =
0].
C: The anhydrite grains show different interference colours, dependent on crystal orientation, ranging
from third-order red to first-order (near-)black. Pale red grains: section parallel to (010) resp. optic
axial plane [max. birefringence (n z – n x ) = 0.044]; black grain: section orthogonal to one of the two
optic axes [circular indicatrix section showing n y ; birefringence = 0].
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