guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Conoscopy
Determining the optic sign of biaxial minerals
For the determination of the optic sign, crystal sections with lowest-possible interference
colours should be selected. For minerals with low 2V, acute bisectrix figures are also suitable
(Fig. 4-54). Appropriate grains will show low, but not zero birefringence in orthoscopic
mode. In general, it is sufficient to see one optic axis (melatope) within the field of view.
Ideally, the point of maximum curvature (melatope) of the isogyre should lie in the centre of
the field of view, such that the direction of curvature can be seen clearly. The convex side of
the hyperbola points to the acute bisectrix, the concave side to the obtuse bisectrix.
For the observation of the change of interference colour when a compensator is inserted, the
isogyres should be positioned such that the optic axial plane is in diagonal, NE-SW
orientation. In this orientation, the concave sides of the isogyres point NE and SW, or either
direction, if only a single isogyre is in the field of view. After inserting the first-order red
plate, the isogyres appear in 1 st -order red, while the interference colours in between the
isogyres change in accordance with the optic sign of the mineral (Fig. 4-54):
Case A: On the convex sides of the isogyres subtraction occurs (1 st -order orange-yellow
close to the melatope), while the concave domains show addition (2 nd -order blue close to the
melatope). The acute bisectrix is Z; the optic sign of the minerals is therefore positive (Fig.
4-54; example aragonite).
Case B: On the convex sides of the isogyres addition occurs (2 nd -order blue), while the
concave domains show subtraction (1 st -order orange-yellow). The acute bisectrix is X; the
optic sign of the minerals is therefore negative (Fig. 4-54; example muscovite).
Raith, Raase & Reinhardt – February 2012
Figure 4-54. Determination of the optic sign of a biaxial crystal.
The optic sign can be determined in sections perpendicular to the acute bisectrix or perpendicular to
one of the two optic axes. The distinction between positive and negative optic sign is made by
observing the addition and subtraction phenomena resulting from insertion of the first-order red plate.
The isogyres of the mica show distinct blue fringes of their concave side due to dispersion of the optic
axes.
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