guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Light refraction: relief
4.2.2 Light refraction (relief, chagrin, Becke line)
Refractive indices are important characteristics for identifying minerals. Their values cannot
be determined directly in thin section, but must be estimated from a comparison with the
embedding medium (epoxy resin; n about 1.54) or with minerals of known refractive indices,
using light refraction effects. Criteria for doing this are relief, chagrin and the Becke line.
Relief
Mineral grains that have higher or lower refractive indices than their immediate surroundings
show distinct contours and appear to lie higher or lower than the adjacent materials. The
relief is a result of refraction and internal total reflection of light rays at the interface between
the mineral grain and its surroundings. The higher the difference between the respective
refractive indices, the more pronounced are grain outline and relief. Both disappear if the
refractive indices of adjacent materials are identical (Fig. 4-18).
Raith, Raase & Reinhardt – February 2012
Figure 4-18. Generation of relief by refraction of light rays at grain boundaries. The lower
sequence shows, from left to right, grains of fluorite (Fl), K-feldspar (Kfs), albite (Ab),
muscovite (Ms), clinozoisite (Cz), garnet (Grt) and zircon (Zrn) in quartz (n qz = 1.544-1.553).
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