guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Light refraction: chagrin
Chagrin (surface roughness)
The upper and lower surfaces of minerals in thin section (in polished sections only the lower
surface) show relief due to minute roughness, unevenness and cracks. If the minerals have
higher or lower refractive indices than the embedding medium (epoxy resin, n = 1.54),
refraction and reflection of light rays will occur at this micro-relief. As a result, the grain
surfaces display a finely pitted structure in the microscope, which is referred to as chagrin
(French for grained leather). The larger the difference of the refractive indices between
mineral and epoxy resin, the more pronounced is the chagrin as the proportion of totally
reflected and scattered light increases (Fig. 4-19).
Figure 4-19 Chagrin in relation to the refractive index of a mineral.
Raith, Raase & Reinhardt – February 2012
Practical hints: The chagrin contrast may be modified by changing the illumination
conditions: (1) putting the auxiliary condenser lens into the light path and opening the aperture
diaphragm decrease the chagrin effect; (2) closing the aperture diaphragm and swinging
out the condenser lens increases the effect. For an estimation of the light refraction of an
unknown mineral using the chagrin contrast, the illumination conditions and magnification
should be kept constant (i.e., small aperture by closing the aperture diaphragm; 20x
objective).
Furthermore, it should be noted that the chagrin contrast is generally low in polished
microprobe sections, as light scattering is only occurring at the rougher lower surface of the
minerals.
Note also that a clouding due to finely disseminated mineral and fluid inclusions can give an
impression of higher chagrin than the host material actually has.
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