Geoscience Reference
In-Depth Information
There are two types of thin section making machines
on the market. The first is the lapping machine (
15
),
which grinds the sample to the required thickness on a
lapping plate that is fed with abrasive grit (carborundum
or aluminium oxide). The second type of machine grinds
with fine diamonds embedded in a bronze wheel without
the need for abrasive slurry. Lapping machines produce
a finer finish (with 5 μm aluminium oxide power) than
the diamond grinding wheel (typically utilizing 60 μm
diamonds). Also, diamond grinding wheels will cause
shattering of crystal grains within samples when
operated incorrectly. On the other hand, diamond
grinding wheel systems produce thin sections that are
noticeably cleaner when viewed through the microscope.
The conventional thickness of thin section specimens
is 30 μm, although this is occasionally varied depending
on the application; for example, 20-25 μm thick
specimens are sometimes used for concrete specimens as
it makes it easier to resolve the details of the cement
matrix. The plan size of thin section specimens tends to
be either small (25 mm × 75 mm), medium (50 mm ×
75 mm) or large (75 mm × 100 mm) (
16
). The size of slide
required depends on the grain size of the material being
thin sectioned, with small slides only being useful for
fine-grained rocks and grouts. The majority of materials,
including all concrete specimens, require the use of
medium or large area slides.
It is essential that the petrographer has a thorough
understanding of the thin section making process and
the sample preparation materials. Ideally the petographer
should have practical experience of thin-section
preparation. Due care must be taken to ensure that
artefacts of the thin section making process or storage
conditions are not mistaken for natural features of the
sample. Some of the more common pitfalls include:
• Over-heating during sample preparation may cause
mineralogical changes, for example, carbonation of
cements, or gypsum dehydrating to anhydrite.
• The use of water for cutting and grinding of water-
sensitive samples may cause them to hydrate and/or
dissolve away.
• Shrinkage microcracks caused by oven-drying prior
to resin impregnation must not be confused with
genuine microcracks in cementitious materials.
Cement-rich concretes and mortars are especially
prone to this.
• Open cracks and microcracks may become filled by
glue (epoxy resin) used to stick down the cover slip.
The glue is isotropic and could potentially be
misidentified as isotropic alkali-silica gel in
concrete, leading to an incorrect diagnosis of
alkali-silica reaction (ASR). Glue filling microcracks
can sometimes exhibit first-order interference
colours and could potentially be misidentified as
gypsum. In concrete this could lead to a
misdiagnosis of sulfate attack (
17
).
• Cracks caused by shrinkage of the impregnating
resin at the edge of specimens can easily be
misidentified as surface delamination of the sample.
• Abrasives and grinding debris produced during the
cutting and grinding processes can often be found
filling cracks and voids in thin sections. Care must
be taken to ensure that these are not mistaken for
the products of deleterious reactions.
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Suppliers of thin section preparation equipment and
consumables are listed in Appendix A.
F
INELY GROUND SLICES AND HIGHLY POLISHED
SPECIMENS
Finely ground slices are sometimes prepared to enhance
the visual and low-power microscopical examination
stage of petrographic examination. They are most
commonly used in concrete petrography where a typical
slice would be 100 mm square (minimum) by 25 mm
thick. One surface of the slice would be finely ground on
15
A lapping machine for thin section making.
(Courtesy of Logitech Materials Technologists and
Engineers.)
