Page 34 - A Practical Introduction to Optical Mineralogy

THE MICROSCOPIC STUDY OF MINERALS POLISHING HARDNESS
1.6.5 Vickers hardness number (VHN) Oil immersion nearly always results in a decrease in reflectance
(Table 1.1), the reason being evident from examination of the Fresnel
This is a quantitative value of hardness which is useful to know when equation (Section 5.1.1), which relates the reflectance of a mineral to its
comparing the polishing properties of minerals (see Section 1.9).
optical properties and the refractive index (N) of the immersion
medium. Because it is then-Nand then+ N values in the equation that
1. 6. 6 Distinguishing features are affected, the decrease in reflectance that results from the increase in
N is greater for minerals with a lower absorption coefficient (see
These are given for the mineral compared with other minerals of similar
Table 1.1).
appearance. The terms harder or softer refer to comparative polishing
hardness (see Section 1.8). The colour of a mineral may remain similar or change markedly from
air to oil immersion. The classic example of this is covellite, which
changes from blue in air to red in oil, whereas the very similar blau-
bleibender covellite remains blue in both air and oil. Other properties,
1. 7 Observations using oil immersion in reflected-light such as bireflectance and anisotropy, may be enhanced or diminished by
studies
use of oil immersion.
To use oil immersion, lower the microscope stage so that the immer-
Preliminary observations on polished sections are always made simply sion objective is well above the area of interest on the well levelled
with air (RI = 1.0) between the polished surface and the microscope polished section. Place a droplet of recommended oil on the section
objective, and for most purposes this suffices. However, an increase in surface and preferably also on the objective lens. Slowly raise the stage
useful magnification and resolution can be achieved by using immersion using the coarse focus control, viewing from the side, until the two
objectives which require oil (use microscope manufacturer's recom- droplets of oil just coalesce. Continue to raise the stage very slowly using
mended oil, e.g. Cargille oil type A) between the objective lens and the the fine focus, looking down the eyepiece until the image comes into
section surface. A marked decrease in glare is also obtained with the use focus. Small bubbles may drift across the field but they should not cause
of immersion objectives. A further reason for using oil immersion is that any inconvenience. Larger bubbles, which tend to be caused by moving
the ensuing change in appearance of a mineral may aid its identification. the sample too quickly, may only be satisfactorily removed by complete
Ramdohr (1969) states: 'It has to be emphasised over and over again cleaning.
that whoever shuns the use of oil immersion misses an important diag- To clean the objective, lower the stage and immediately wipe the end
nostic tool and will never see hundreds of details described in this book.' of the objective with a soft tissue. Alcohol may be used with a tissue, but
not a solvent such as acetone, which may result in loosening of the
Table 1.1 The relationship between the reflectances of minerals in air (Ra;,) and objective lens. The polished section can be carefully lifted from the stage
oil immersion (Ron) and their optical constants, refractive index (n) and absorp- and cleaned in the same way.
tion coefficient (k). Hematite is the only non-cubic mineral represented, and two Most aspects of qualitative ore microscopy can be undertaken without
sets of values corresponding to the ordinary (o) and extraordinary (e) rays are resource to oil immersion, and oil immersion examination of sections
given. N is the refractive index of the immersion medium .
which are subsequently to be carbon coated for electron beam micro-
analysis should be avoided. The technique is most profitably employed
n k Ra;c( %) Roll( %)
(N = 1.0) (N = 1.52) in the study of small grains of low reflectance materials such as graphite
or organic compounds, where the benefits are a marked increase in
Transparent minerals resolution and image quality at high magnification.
fluorite CaF, 1.434 0.0 3.2 0.08
sphalerite ZnS 2.38 0.0 16.7 4.9
1.8 Polishing hardness
Weakly absorbing minerals
(o) 3.15 0.42 27.6 12.9
hematite Fe,O,
(e) 2.87 0.32 23.9 9.9 During the polishing process, polished sections inevitably develop some
relief (or topography) owing to the differing hardness of the component
Absorbing (opaque) minerals
galena PbS 4.3 1.7 44.5 28.9 minerals. Soft minerals tend to be removed more easily than hard
silver Ag 0.18 3.65 95.1 93.2 minerals. Also the surfaces of hard grains tend to become convex,
whereas the surfaces of soft grains tend to become concave. One of the
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