guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Light refraction: Becke Line
Becke Line
At the boundary between two substances of different refractive index (crystal–epoxy resin,
crystal–glass, crystal–crystal) a bright fringe, the Becke Line, is observed at high magnification.
When closing the aperture diaphragm, this fringe may show colours, particularly if the
minerals show a small difference in refractive index but a large difference in dispersion. The
Becke Line is named after Friedrich Becke (1855-1931).
If this phase boundary is defocused by raising or lowering the stage slightly, the light fringe
moves from one phase into the adjacent one: When lowering the stage it moves into the
higher refractive substance, and when raising the stage into the lower-refractive substance. An
explanation of this phenomenon is given in Fig. 4-20, left-hand side.
The intensity of the Becke Line decreases as the difference in refractive indices between the
adjacent substances decreases. It disappears completely if the indices are identical.
Differences in the refractive indices in the order of 0.001 to 0.002 can still be recognised. On
the other hand, the Becke Line is obscured at high differences in refractive index by the
strong chagrin.
Raith, Raase & Reinhardt – February 2012
Figure 4-20. Generation of the Becke Line (left); movement of the Becke Line at the quartzglass
interface as a result of raising or lowering the stage.
Practical hints: For observation of the Becke Line, a steeply inclined, but not vertical, clean
boundary between two phases must be chosen. Suitable are also broken-off grain boundaries
bordering on epoxy resin (in break-outs or at the thin section edge). The phase boundary is
then viewed with sufficiently high magnification (20x objective), the contrast is optimised by
closing the aperture diaphragm, and the sense of movement of the Becke Line is studied by
raising and lowering the stage slightly (Fig 4-20, right-hand side).
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