Page 101 - Atlas of Sedimentary Rocks Under The Microscope

In the making of thin sections. it is generally assumed that the rock
will contain some quartz or feldspar. These show first-order grey and
white interference colours in a thin section of standard thickness and
neither should show a first-order yellow or red colour. Thus a thin
section in which quartz or fe ldspar shows colours in Newton's scale
higher than first-order white is too thick. In making thin sections of
limestones or evaporites where quartz and feldspar are absent, it is very
difficult to estimate thickness: only an experienced thin section maker
can do so accurately. With limestones. where the minerals show high­
order interference colours, the section should be ground until sparitc
crystals and the internal structures of shell fragments arc clear. Micrite
will remain difficult to resolve even at high power.
It is usual to cover the section. either by painting the surface with a
transparent cellulose lacquer, or better still with a glass cover slip as
lacquer tends to scratch easily. This is done traditionally using Canada
balsam diluted in xylene. but t he process of heating the mixture at the
correct temrcraturc for the correct time requires some experience. We
have found that it is quite satisfactory to fix the cover glass by either
using the same epoxy resin which was used to attach the rock to the
microscope slide. or by using a clear lacquer painted or sprayed onto
the surface of the rock. As in the process of fixing the rock to the
microscope slide. care must be taken to ensure that no air or gas
bubbles arc trapped between the cover glass and the rock. This is
particularly important if the material has been applied by a spray.
because some of the propellant may be dissolved in the clear lacquer.
Any bubbles which arc visible in the liquid after spraying should be
allowed to burst before applying the cover slip. Only sufficient lacquer
or Canada balsam to cover the slide with a thin layer of liquid should
be applied.
The cover slip should touch the liquid on the slide at one end and be
allowed to fa ll slowly onto the liquid. If any air bubbles are visible they
can be extruded by gentle pressure on the cover glass. The excess
lacquer or epoxy resm must be extruded to render it as thin as possible.
othcn\ isc the minerals cannot be brought into focus with a high-power
lens because of the short working distance of lenses of magnification
more than x 40.
Finally when the mounting material has set hard, the excess can be
scraped from round the edges of the cover glass with a razor blade or
sharp knife.

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