guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Microscope
The oculars of infinity-corrected microscopes are adjusted as follows (whereby the ocular
with the crosshairs should be placed in the right tube of the binocular):
(1) Look through the oculars and close the left eye. The crosshairs in the right ocular is
observed with the right eye and focused with the diopter adjustment ring on this ocular. Then
the object image is focused by carefully adjusting the stage height with the fine adjustment
knob.
(2) Now close the right eye and observe the object image with the left eye through the left
ocular, without adjusting the stage vertically. If the image is not properly focused, the
adjustment must be made using the diopter adjustment ring on the left ocular.
When focusing the object image, it is important that both eyes are relaxed and focused to
infinity.
1.6 Polarizer and analyzer
The polarizer (polarizing filter or nicol prism) resides below the condenser. It can be swung in
and out of the light path, and in many microscopes it is possible to rotate the polarizer about a
vertical axis. The light source emits waves that vibrate randomly in all possible planes. The
polarizing filter used in modern microscopes consists of a stretched polyvinyl film which
reduces the randomly vibrating light waves to waves of a single vibration direction
(polarization plane). For simplicity, the term “pol-waves” is introduced here for the planepolarized
waves leaving the polarizer.
When passing through the thin section, the pol-waves may undergo diverse modifications
(refraction, absorption, birefringence etc., Ch. 4.2). In order to detect and quantify such
modifications, the polarization plane should correspond to an easily identifiable reference
direction in the field of view. In modern microscopes, the reference direction for the lower
polarizer is E-W, parallel to the “horizontal” crosshair. There are microscopes, however,
where the orientation of the polarizers is different (lower polarizer N-S, analyzer E-W).
Specific optical-microscopic phenomena relate directly to the polarizer orientation (such as
pleochroism and relief change, where the vibration direction of the lower polarizer provides
the reference direction in the field of view).
Raith, Raase & Reinhardt – February 2012
Therefore, apart from ensuring that the polarizers are aligned, the general orientation of the
polarizers must be known to the operator before starting to work on thin sections. This routine
check may be performed with a colored tourmaline crystal, whether as a loose grain or as a
prismatic section in thin section. The maximum absorption will be observed if the c-axis is
oriented perpendicular to the polarizer direction (Fig. 1-10). The same check can be done with
biotite in sections roughly orthogonal to (001). Biotite is a common mineral in many rocks,
and thus thin sections containing biotite should be easily available. Biotite in sections roughly
orthogonal to (001) displays its prominent cleavage and shows its maximum absorption (i.e.
deepest color) if its basal plane (001) or cleavage is sub-parallel to the lower polarizer
direction (Fig. 1-10). Biotite should not be used for alignment, though, as it is monoclinic.
The angles between its (001) plane and any of the crosshairs may be close to zero, but it can
also deviate by as much as 10°, depending on composition.
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