Page 70 - Carbonate Facies in Geologic History

Acid Etching 57

study, is a necessary basic procedure for satisfactory observation and description.
A normal rock saw and a progressive polish with 400-600-1000 carborundum
grit is required. A good procedure is to cut a plaquette (about 3-5 em square and
a few mm thick) across the bedding and polish one side. A weak acid (acetic or
formic) in 1: 5 water solution is used to etch half of the already polished surface.
This improves the polish in a pure limestone and etches out dolomite rhombs,
clay, and such insoluble grains as quartz, silt and sand. Such preparations may be
routinely made and studied under a stereo-binocular scope with reflected light. A
light mineral oil on the surface permits fine observation at high magnification.
The closer the refractive index of the oil to that of calcite the better the visibility.
Except for the rock saw, no cumbersome or powered equipment is necessary. If
the saw can be mounted with an independent power source, the above procedure
is essentially adaptable to the field.

Thin Sections

Thin sections have long been the standard means of carbonate study. They are
expensive if purchased commercially costing several dollars each, depending on
the size. They are slow to manufacture without proper equipment. In many
respects, however, no substitute for them exists. For most purposes in carbonate
study, they should be ground just slightly thicker than the usual 30 microns to get
deeper color contrast under plane light. They should never have a permanent
cover-glass. (They can always be thinned if necessary and a temporary cover glass
mounted with immersion oil if needed.) Uncovered sections may be oiled,
scratched, plucked, stained, and acid etched under the microscope, using polar-
ized light to aid in determining the character of the residue. Because carbonate
minerals are optically difficult to distinguish from each other, the above simple
techniques of identification are useful.

Acid Etching
Use of progressive acid etching on a thin section or slab enables identification and
exact placement of dolomite, anhydrite, quartz, chert, and clay in the calcite
fabric. One may first use formic or acetic acid (about 1 part concentrated acid to 5
parts H 2 0) to dissolve calcite and preserve dolomite and later 1/10 Hel to dis-
solve the dolomite. The organic acid tends to coat the dolomite rhombs with a
fine residue of partly dissolved carbonate and makes them stand out clearly.
Progressive acid etching of calcite, when observed under a microscope, also gives
useful information about crystal structure and orientation.
Acid solution of carbonates may also be carried out on small chips of approxi-
mately equal volume, placed in a transparent glass finger plate. Progressive acidi-
zation of residue, with projection of polarized light through it, will determine both
kind and quantity of terrigenous impurities and dolomite-anhydrite in the lime-
stone. Water flushing and blotting with absorbent cotton is necessary during
application of multiple acids.