Geoscience Reference
In-Depth Information
Where a specific standard is lacking, it is common
practice for laboratories offering petrographic services to
work in accordance with an in-house 'test procedure',
which adapts accepted standards from the nearest
material in terms of similarity.
the hand specimen, such as scratch testing to assess
relative hardness (Moh's scale), or acid drop testing (with
dilute hydrochloric acid) to determine if carbonate
minerals are present. Colour is usually assessed visually
in the hand specimen by comparison with standard
colour charts such as the Munsell rock colour chart
(Geological Society of America, 1991) or the Munsell soil
colour chart (Munsell, 1994). The hand specimen is then
examined using a low-power stereo-zoom microscope (
2
)
at magnifications of typically up to ×50. In certain
circumstances a slice of the sample may be finely ground
on one surface to aid the low-power microscopical
examination. In addition to being an observational tool,
the visual and low-power microscopical examinations
are used to determine the most appropriate location for
thin section specimens and/or highly polished specimens
to be taken for further, more detailed high-power
microscopical examination. Specimen preparation is
discussed in Sampling and sample preparation, p. 15.
High-power microscopical examination is conducted
using a polarizing microscope capable of magnifications
up to ×600 (
3
). Thin section specimens are examined in
plane-polarized or cross-polarized transmitted light.
V
ISUAL AND LIGHT MICROSCOPICAL EXAMINATION
Light microscopy (also called optical microscopy) is at
the heart of petrography and, combined with visual
examination, it forms the basis of commercial
petrographic examinations. The main suppliers of high-
quality light microscopes and their accessories are listed
in Appendix A. If required, light microscopy can be
supplemented with a range of complementary techniques
to achieve the objectives of the petrographic
examination. These are discussed in Complementary
techniques, p. 13.
Following arrival in the laboratory, samples of
geomaterials are first examined in the as-received
condition using the unaided eye. At this stage a few
simple physical and chemical tests may be performed on
2
3
2
A stereo-
zoom low-
power
microscope for
examining
samples in
hand specimen.
(Courtesy of
Leica
Microsystems
UK Ltd.)
3
A polarizing microscope set up for combined
transmitted light, reflected light, and fluorescence
microscopy of thin sections and highly polished
specimens. With attached digital image capture
system. (Courtesy of Leica Microsystems UK Ltd.)
