guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Measuring thickness
2.3 Determination of thin section thickness
The birefringence of anisotropic minerals can be determined in approximation by comparing
the interference colours to those of the Michel-Lévy or Raith-Sørensen interference colour
charts (Ch. 4.2.3), or it can be done precisely using the compensation method. For both
methods the thickness of the mineral in thin section must be known. Commonly, the thin
section thickness is standardised to 25 resp. 30 μm. Abundant minerals such as quartz and
feldspar would then show an interference colour of first-order grey to white. If these minerals
are not present in the thin section, thickness is difficult to estimate for the inexperienced. In
such a case, the thickness can be determined via the vertical travel of the microscope stage.
For focusing the image, microscopes are equipped with focusing knobs for coarse and fine
adjustment. The instruction manuals supplied by the manufacturers contain information on
the vertical travel per graduation mark. This value is 2 μm for many microscopes.
For measuring the thickness, an objective with high magnification and small depth of field
(40x or 63x) is chosen and the surface of the cover slip is put into focus. This surface can be
recognised by the presence of dust particles. The inexperienced user may put a fingerprint
onto the surface! Now the fine adjustment is turned in the appropriate direction to decrease
the distance between sample and objective. Eventually, the focus plane has passed the cover
slip, and the interface between the embedding medium and the minerals appears. This is
recognised by the surface roughness and the grinding marks on the thin section. Reducing the
distance between sample and microscope stage further, the lower surface of the thin section
with all its uneven features comes into focus. The path through the thin section can also be
followed along cleavage planes and inclusions. Inexperienced users should repeat this
procedure a couple of times and maybe make notes about, and cross-check, the positions of
the various boundary surfaces using the scale at the focusing knobs.
Raith, Raase & Reinhardt – February 2012
For a precise determination of thickness using the upper and lower boundary surfaces of the
thin section, it is necessary to turn the focal adjustment in one direction only, eliminating the
mechanical backlash. If the thickness from the lower to the upper surface is measured, the
starting position of the focal plane must be in the 1 mm thick glass slide. If moving in the
reverse sense, the starting position must be in the 0.17 mm thick cover glass above the
mineral surface.
The number of graduation marks by which the fine adjustment is turned in order to shift the
focus from the lower to the upper boundary surface (or in the reverse direction) multiplied by
the value given per graduation mark (e.g., 2 μm) results in a vertical travel distance h in μm.
However, this distance is commonly not the true thickness of the section.
28