Page 72 - Carbonate Facies in Geologic History

Scanning· Electron Microscope 59

Peels may also be made more quickly by flooding the etched surface with
acetone and rolling on a thin sheet of acetate. Commercial acetate film (0.005
inches) is recommended. Place the roll offilm in the middle of the slab and spread
it on evenly to prevent entrapment of air bubbles. Let it dry 15 min after the
application and then peel it off. Such sheets may also be mounted between glass
slides to prevent wrinkling or may be previously glued to glass slides with a
silicone rubber cement. The etched and acetone soaked rock is then pressed onto
the slide. Plexiglass pieces may also be used. The "dry peel" technique is quicker
than using peel solution and is better for more porous rocks because the wet peel
solution tends to run down and harden in minute openings. This causes the peel
to tear when removed from the rock. Pore space may be outlined on polished rock
prior to acid etching by buffing with chrome oxide. This fine powder is transferred
on the peel. There are thus several advantages to the manufacture of dry peels
although generally better results (and less air bubbles) are obtained by use of the
solution. Peels are excellent for high magnification work where crystals of a few
microns need to be distinguished; the 30 micron thickness of a "thin section"
obscures small crystals. Peels also make good photographs. The petrographer
misses the color contrast of a thin section and, of course, no mineral identification
is possible through use of polarized light. Peels can be used effectively with
transmitted light through any type of microscope if the light is well diffused and
slightly oblique.

Cathode Luminescense (Sipple and Glover, 1965)

It is known that luminescence is produced when a broad beam of low energy
electrons is focused on a thin section or plaquette of a carbonate rock in a vacuum
chamber. This color glow outlines in great detail structures within calcite and
dolomite crystals which are not seen in polarized or white light. The very charac-
teristic orange luminescence of calcite is due to the presence of divalent Mn. This
luminescence is quenched by the presence of iron, cobalt, and nickel. Dolomite
shows a similar characteristic but with shifts in the spectrum to reds and yellows.
The trace elements are concentrated on surfaces of slow crystal growth. Thus
some idea of the trace element content of the water which precipitated the carbon-
ate cement may be obtained. Advantages of this technique to the petrographer
include the better recognition of growth stages in void-filling calcite cement and
in similar dolomite cements or replacements, the delineation of vein calcite which
is seemingly in optical continuity with the host rock and the better discernment of
fossils in altered carbonates (Sipple and Glover, 1965). The instrument for attach-
ment to the microscope, complete with vacuum chamber and pump, is commer-
cially available for about $3000.00 USA.

Scanning Electron Microscope

The Scanning Electron Microscope (SEM) is one of the latest developments in
electrical optical instrumentation. The image is derived from scanning the gold-
plated surface of a specimen with an electron beam and detecting secondary and