INTRODUCTION

Standard staining methods have been developed for carbonate rocks that are specific for calcite (Friedman, 1959), ferroan calcite (Dickson, 1965) and magnesium-calcite (Choquette and Trusell, 1977). These techniques are applicable to many types of carbonate rocks, but except for a combined calcite and ferroan calcite stain, are not normally used in any combination because they are incompatible alkaline and acidic stains. Except for preparing and alternately staining serial thin sections, there would be little way to examine the relationship of intergrown calcite, ferroan calcite, magnesium-calcite and dolomite.

During a study of the mineralogy of coal balls, it became evident that standard staining techniques were inadequate to study the relationship of the carbonate minerals present in the coal balls. A combined calcite and ferroan calcite stain overpowered the thin section and made observation of the intergrowths of mineral grains difficult. Separate staining using a modified potassium ferricyanide stain and a titan yellow stain for ferroan calcite and magnesium calcite, respectively, revealed many details about the coal ball mineralogy, but could not be easily combined to yield textural information.

Experimentation with staining techniques led to the realization that separate sides of a thin section could be stained with otherwise incompatible stains and, when viewed in transmitted light, the stains would superimpose in such a way that information about the relationship of the minerals could be recovered.

METHOD

Step 1:

The thin section black must be smoothed to at least a semi-polish. Sanding to 600-1200 mesh grit is acceptable and depends on the grain size. Fine grained microdolomites require 1200 mesh or a 3 micron pre-polish. After sanding, the blank should be lightly etched (15 to 30 seconds) and 1% acetic acid or 1% hydrochloric acid are recommended.

Step 2:

The thin section blank is stained in the first stain. For this research the initial stain was a Titan yellow stain specific for magnesium calcite:

1.0 g Titan yellow

8.0 g sodium hydroxide

4.0 g EDTA

1.0 l water

The blanks were stained at room temperature for approximately 30 minutes, rinsed with distilled water and fixed with a 30 second immersion in:

200 g sodium hydroxide

1.0 l water

After fixing the blank was allowed to air dry.

Step 3:

The stained blank was then attached to the glass slide with epoxy mounting medium. Tests showed that mounting of the blank using a vacuum degassing processing resulted in less parting of the blank and also infilled porous areas that might "bleed-through" when the second stain was applied to the thin section.

Step 4:

The mounted blank was then reduced to a thin section using standard techniques. The surface was additionally smoothed using 600-1200 grit and lightly etched.

Step 5:

The thin section was immersed in a potassium ferricyanide (PFS) stain specific for ferroan calcite (combined PFS and Alizarin red S can also be used but represented "overkill" on the coal ball samples):

2.0 g potassium ferricyanide

100 cc 1.0% hydrochloric (or acetic) acid

If hydrochloric acid is used the staining time is very short (30-45 seconds) and care must be exercised to prevent complete etching away of the thin section. Acetic acid is more easily monitored and staining times range up to 5 minutes, but acetic acid may be ineffectual in some cases (on coarse dolomite intergrown with the calcite, for example).

After staining the thin section is rinsed with distilled water, air dried and mounted using permount or any similar acid mounting medium.

USE OF THE DOUBLE SIDED STAIN TECHNIQUE ON COAL BALLS

Many coal balls have a relatively unique mineralogy and consist of intergrown calcite, ferroan calcite and a "microdolomite" that is actually a metastable product from magnesium-calcite (Barwood, 1995). Unique features such as "bushes" of "microdolomite", botryoids and spherules have been observed using separate staining techniques. The superimposition of some of these features was inferred, but could not be observed directly.

Figures 1-5 illustrate how this staining technique can be used to see the relationship of the initial magnesium-calcite infillings of cells and plant stems to later ferroan calcite that is a diagenetic overprint.

The areas of these thin sections that were stained as magnesium-calcite were in reality a metastable, fine grained breakdown product from magnesium-calcite. Microprobe analysis of this material shows that it is compositionally:

CaO 50.59%

MgO 8.30%

FeO 0.13%

MnO 0.08%

X-ray diffraction analysis showed that this mineral was "kutnahorite", but kutnahorite is a manganese-magnesium member of the dolomite group (figure 6). Apparently this mineral represents a metastable product intermediate between magnesium-calcite and dolomite and probably has a formula of Ca(Ca,Mg)(CO₃)₂. Work is proceeding on characterizing this mineral.

REFERENCES

Barwood, H.L. 1995. Mineralogy and origin of coal balls. Geol. Soc. Am. Abst. with Programs, North-Central and South-Central Sections 27(3):37

Choquette, P.W. and F.C. Trusell. 1978. A procedure for making the titan-yellow stain for Mg-calcite permanent. J. Sed. Petrology 48:639-641

Dickson, J.A.D. 1965. A modified staining technique for carbonates in thin section. Nature 205:587

Friedman, G.M. 1959 Identification of carbonate minerals by staining techniques. J. Sed. Petrology 29:87-97

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