guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Conoscopy
4.2.5.5 Practical hints for conoscopic observation
Mineral grains that are to be investigated conoscopically should meet specific requirements:
a. When using a strongly magnifying objective (M O = 40x, 50x, 63x or oil immersion 100x)
with a large numerical aperture (>0.6), the mineral grain should cover the field of view
completely.
b. The mineral grain must not be twinned within the observed frame, nor must it show
alteration effects or exsolution phases.
c. The mineral grain should have a suitable orientation.
The procedure of obtaining the conoscopic interference figure is carried out as follows:
a. Move the mineral grain into the centre of the crosshairs, use a high-magnification objective
(40x minimum) and focus.
b. Open all diaphragms below the stage.
c. Move the condenser into the uppermost position and put the auxiliary condenser into the
light path.
d. Put the analyzer in. Take out the ocular; use a pinhole if included with the microscope,
observe the image in the ocular tube or use the Amici-Bertrand lens and observe the enlarged
image through the ocular. In some microscopes, the Amici-Bertrand lens can be centred and
focused separately, which is important for quantitative work.
e. If the mineral grain is too small to fully occupy the field of view, an upper diaphragm that
is included in some microscopes can be used to reduce or eliminate the periphery around the
grain. If there is no Amici-Bertrand lens with upper diaphragm, a sharp conoscopic interference
figure may be obtained using a simple pinhole in place of the ocular.
Raith, Raase & Reinhardt – February 2012
Minerals that are strongly coloured may cause problems for the determination of their optic
sign if the first-order red plate is used. Therefore, the quartz wedge is recommended for
compensation, which allows observing the direction of movement of the isochromes.
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