guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Conoscopy
Determining the optic sign of uniaxial minerals
For the determination of optic sign, crystal sections are required that are either isotropic or
show very low interference colours in orthoscopic view. The conoscopic interference figure
will then show a more or less centred, dark isogyre cross.
If the first-order red plate is inserted, the cross will appear in 1 st -order red, while the four
quadrants display two different sequences of interference colours, depending on the vibration
direction of the E- and O-waves. Two cases have to be distinguished:
Case A: In quadrants I and III the interference colours increase by one order (= addition),
while in quadrants II and IV the interference colours decrease by one order (= subtraction).
Thus, n e ' > n o , and hence the optic sign of the minerals is positive (Fig. 4-52; example
quartz).
Case B: In quadrants I and III the interference colours decrease by one order (= subtraction),
while in quadrants II and IV the interference colours increase by one order (= addition).
Thus, n e ' < n o , and hence the optic sign of the minerals is negative (Fig. 4-52; example
calcite).
The interference figure in high-birefringent minerals shows multiple isochromes. In order to
distinguish clearly between cases A and B, it is suggested to use the quartz wedge instead of
the first-order red plate. When inserting the quartz wedge, the isochromes in the four
quadrants shift in a characteristic way (Fig. 4-52,55).
Raith, Raase & Reinhardt – February 2012
Figure 4-52. Determination of the optic sign of a uniaxial crystal.
The distinction between positive and negative optic sign is made in sections perpendicular to the optic
axis by observing the addition and subtraction phenomena caused by insertion of the first-order red
plate.
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