1 mm can be readily observed. However, there is no good way to get images of
areas larger than can be viewed with the normal microscope lenses or
assembled from image mosaics. For instance, a document scanner does not
yield very good images because the light is not orthogonal to the surface.
However, it may be useful if there are very large differences in reflectivity.
Although surfaces may be viewed in both plane-polarised light and under
crossed polarisers, the former is used most commonly for imaging surfaces. As
for transmitted light, anisotropic minerals may have different reflectances
(colours) for different orientations, but the effect is not large for most miner-
als. Hence, minerals are usually imaged in plane-polarised or unpolarised
light. In this situation the most important parameter is the reflectance of the
mineral. Reflectances have been recently tabulated for all new minerals, at
20 nm intervals from 400 to 700 nm, but values at four standard wavelengths
are generally used (470, 546, 589 and 650 nm). Most transparent minerals, such
as quartz and feldspars have reflectances in the range 5–10%. Other minerals
are much higher: oxide minerals 12–30%; sulphides 12–60% and metals
50–100%. Surfaces may also be etched or stained to bring out the contrast
between different minerals, sub-grain boundaries and crystal defects (see
Section 2.5.3.3 and review by Wegner & Christie, 1985 ).
Minerals can be imaged electronically by standard charge-coupled device
(CCD) cameras. However, such cameras are designed to mimic the human eye
and hence record light in three wide spectral bands. A better approach is to use
narrow (10 nm) bandwidth filters (Pirard, 2004). Such a system needs to be
carefully calibrated, but can give much better resolution of mineral phases,
especially those with reflectances greater than 5%. This method can distin-
guish mineral pairs that are problematic in BSE (e.g. chalcopyrite/pentlandite)
or X-ray maps alone (e.g. hematite/magnetite/goethite).
2.5.3.3 Nomarski (DIC) microscopy
A very useful optical technique for imaging surfaces is differential interference
contrast (DIC) or Nomarski microscopy. It has long been used by metallur-
gists to produce detailed images of polished surfaces, and has also been used by
petrologists to examine mineral and rock textures (Anderson, 1983, Pearce
et al., 1987, Pearce & Clark, 1989). It can reveal both the exterior shape of
crystals and their internal structure, and can be useful for distinguishing
crystals from a glassy matrix. It can be applied to thin sections and hence is
complementary to normal reflected and transmitted light methods.
Nomarski microscopy is an optical technique for imaging the micro-relief of
surfaces (<0.5 mm). A light beam is split by a prism; one beam is reflected off
the surface and allowed to interfere with the reference beam. Hence the method
24 General analytical methods