Ge 114 - Optical Mineralogy: Laboratory Exercise #3
CONOSCOPIC OBSERVATION OF UNIAXIAL MINERALS
Reading: Nesse, Ch. 6 or Phillips (1971) pp. 103-120.
All questions or procedures in bold type should be answered or recorded in the lab
report.
Goals of this lab:
Recognize uniaxial interference figures
Determine the optic sign of a uniaxial mineral
The interpretation of interference figures is a rapid and powerful method of ascertaining
many of the optical properties of a mineral. It is particularly valuable in thin section
work. To observe an interference figure, cross the polarizers and search for a clear,
uniform part of the grain free of other crystals in different orientations; then open the
substage diaphragm and insert the movable substage condenser and Bertrand lens. The
condenser assembly must be moved to the top of its travel. The interference figure can
also be seen by removing both the Bertrand lens and the ocular and looking down the
microscope tube.
1. Examine interference figures of three uniaxial mineral thin sections (apatite, rutile and
calcite) cut perpendicular to the c-axis. Note that the black cross, whose center marks
the point of emergence of the optic axis on the interference hemisphere. It is very
faint for apatite. It will move during rotation of the stage if the optic axis is not
exactly parallel to the microscope axis, but the arms of the cross do not separate.
Count the number of color rings (expressed as number of orders of the
interference spectrum) visible in the three figures, and discuss this in terms of
the birefringences of the minerals involved.
2. Using the c section of rutile (uniaxial positive), observe and record the effect of
each of the three accessory plates (1/4  plate, 530 nm plate, and quartz wedge)
on the optic axis figure (and especially the isochromes). Do the same for the
(0001) beryl section (uniaxial negative). The differences you observe are
characteristic of U+ and U- minerals (p.69 Nesse).
3. Using the section labeled "quartz || pyramid", examine the appearance of the uniaxial
interference figure for the c-axis inclined about 45-60° to the microscope axis.
Ascertain from the motion of the isogyres upon stage rotation the approximate
location of the optic axis on the invisible outer portion of the interference hemisphere.
Knowing this, determine the optic sign.
4. Examine the flash figure obtained from one of the three thin sections cut parallel to
the c axes of beryl. Note how the dark cross of the flash figure breaks up into a pair
of hyperbolae that leave the field rapidly as the stage is rotated. Determine the optic
sign of the mineral using the flash figure. See p.72 Nesse on how to find the caxis. The c-axis can then be correlated with the slow- or fast-ray vibration direction.
(For minerals with high birefringence, it is extremely difficult, if not impossible, to
use the flash figure directly to check optic sign).
Revised: 18-Nov-2007
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