Page 71 - Carbonate Facies in Geologic History

58 Outline of Carbonate Petrography

Staining

A very useful review of stains for carbonate mineral identification is presented by
Friedman (1959) and the reader may refer also to Wolf et al. (in Chilingar et al.,
1967) for thorough discussion of techniques. The following are the most useful
stains.
1. Titian yellow in 30% NaOH. Stains Mg calcite and dolomite orange-red in
weak (5%) NaOH.
2. Clayton yellow in NaOH and EDT A. Stains Mg calcite orange-red (Winland,
1971).
3. Alizarin Red S in 30% NaOH. Stains Mg calcite and dolomite purple.
4. Alizarin Red S in 2% HCI. Stains calcite and aragonite red. Stains ferroan
dolomite and calcite purple.
5. K3Fe (CN 6 ) in dilute HCl-stains all carbonates dark blue if they contain iron.
Because most dolomite is iron-rich, this is a useful stain to distinguish dolomite
from calcite.
6. K3F (CN 6 ) and Alizarin Red S in 2% HCI (Evamy, 1963). Stains calcite red
and ferroan calcite purple.
7. Feigel's solution (Ag 2S0 4 in dilute solution of MnS0 4 ). Stains aragonite
black, leaving other minerals untouched.
Staining is more usefully employed on thin sections than on plaquettes, on
light-colored rock than on dark, and on coarse rather than on finely crystalline
fabric. Indeed, in the latter texture, crystalline boundaries tend to soak up the
stain and obscure its effect. Microscopic porosity (e.g., chalk texture) essentially
prohibits staining. The distinction between dolomite and calcite by stains is really
one of iron content as well as solubility. The above variations in effectiveness
indicate that considerable caution should be exercised in the use of stains for
mineralogical identification.

Acetate Surface Replicas-Peels

The use of cellulose acetate to make delicate replicas of etched surfaces of carbon-
ate rock was begun by paleontologists 30 years ago. It is now widely used to help
in the quick interpretation of carbonate textures and to view at high magnifica-
tion cements, micrite, and internal structure of bioclasts (Beales, 1960). Peels are
made by pouring a solution on a carbonate surface which has been polished with
1000 grit carborundum and lightly etched in weak acid. Depth of etching controls
the quality of the replica but varies considerably with the type of rock. The
following procedure and formula for peel manufacture is useful.
Make a creamy solution of cellulose acetate dissolved in amyl acetate or acetone in-
corporating drying inhibitors such as a little xylene and chloroform. Such a solution, if
tightly bottled, may be kept for a long time. After acid etching the rock surface, carefully
wash it by immersion in still water, and dry it in air. Flooding with acetone also quickly
removes the water. Level the rock slab in a sand box and pour acetate solution in the center
of the slab. Do not let it run over the edges. Let it dry overnight and peel off with a
dissecting knife or razor blade. Dip in HCl to remove plucked carbonate grains, wash, dry,
label with india ink and mount peel between glass slides.