9.3 Polishing
For X-ray analysis, mapping and BSE imaging it is extremely desirable to avoid
topographic effects: specimens therefore should be flat and well polished.
Polishing procedures for ore microscopy can be adapted to rocks consisting
predominantly of silicates. Starting with a flat ground surface, polishing is
carried out with progressively finer grades of abrasive (typically carborundum
or emery for the coarser grades and diamond or alumina in the later stages).
Paper or woven nylon laps are preferable to cloth with a ‘nap’, since they have
less tendency to produce surface relief between minerals of different hardnesses.
Either rotating or vibrating motion of the lap is used, the former being prefer-
able. The specimens should be thoroughly cleaned after each stage, to avoid
transfer of abrasive material in pores and cracks. For soft phases a final hand
polish using very fine alumina may be necessary. A single-stage polishing
technique using only alumina has been described by Allen (1984). Special
procedures are required for electron backscatter diffraction studies (Section
4.8.3), the damaged surface layer left by conventional polishing being removed
by a final polish with alkaline colloidal silica slurry (Lloyd et al., 1981).
Polished thin sections are usually made by polishing a section that has
previously been ground to a thickness somewhat greater than 30 mm(Fig.9.5).
However, the final thickness is then poorly constrained, which is undesirable for
polarised-light microscopy. This can be avoided by polishing one face of the
rock slice first, temporarily mounting it face down on a glass slide, grinding off
surplus material to give a thickness of 30 mm, mounting permanently with epoxy
resin on another glass slide (ground face down), and finally removing the first
slide(seeFig.9.5).Afterpolishing,specimensshouldbecleanedbywashingina
solvent that does not attack the mounting medium (e.g. ethanol or petroleum
ether), preferably using an ultrasonic bath to dislodge remnants of polishing
materials. When surface contamination is especially important (e.g. for light-
element analysis) plasma cleaning is desirable (Isabell et al., 1999).
9.4 Etching
Chemical etching enables chemical and crystallographic differences to be con-
verted into topography that can be observed in secondary-electron images. (It is
inappropriate for quantitative EMPA, for which flat, smooth, surfaces are
required, and etching may alter surface composition.) Carbonates can be etched
with dilute hydrochloric acid (1–5%), acetic acid (20%), or EDTA, the most
delicate effect being obtained with the last two. In some cases heavy etching to
remove carbonate cement, leaving exposed grains of quartz etc. for SEM study,
158 Sample preparation