A Color Illustrated Guide to Carbonate Rock Constituents, Textures, Cements, and Porosities

By Peter A. Scholle


Memoir 27 is one of AAPG's all-time best-sellers. This is a full-color collection of thin-section photographs depicting characteristics of carbonates. It is not a collection of classic examples, but rather a useful guide targeted to the explorationist or beginning petrographer. Over 375 full-color photos plus many black and white and SEM photos.

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    Although petrography is an extremely valuable tool for the identification of minerals and their textural interrelationships, it is best used (in many cases) in conjunction with other techniques.

    Precise mineral determinations are often aided by staining of thin sections or rock slabs, by X-ray analysis, or by microprobe examination. Where noncarbonate constituents are present in carbonate rocks they often are better analyzed in acid-insoluble residues than in thin section. Where detailed understanding of the trace element chemistry of the sediments is essential. X-ray fluorescence, microprobe, atomic absorption, or cathodoluminescence techniques may be applicable.

    Commonly, also, sediments may be too fine-grained for adequate examination with the light microscope. The practical limit of resolution of the best light microscopes is in the one to two micrometer (μm) range. Many carbonate and non-carbonate grains fall within or below this size range. Furthermore, because most standard thin sections are about 30 µm thick, a researcher examines 10 or 20 of these small grains stacked on top of one another, with obvious loss of resolution. Smear mounts or strew mounts (slides with individual, disaggregated grains smeared or settled out onto the slide surface) are an aid in examining small grains where the material can be disaggregated into individual components. However, in most cases, scanning and transmission electron microscopy have proved to be the most effective techniques for the detailed examination of fine-grained sediments.

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    Staining techniques are among the fastest, simplest, and cheapest methods for getting reliable chemical data on carbonate phases. The following list of minerals and their diagnostic stains is derived from the work of Friedman (1959), Dickson (1966), Milliman (1974), and others. The original papers, listed in the bibliography, will provide details about the exact application and methods.

    Aragonite Distinguished from calcite by use of Feigel's Solution. Aragonite turns black whereas calcite remains colorless for some time. Mixing Feigel's Solution requires 7.1 g of MnSO4. H2O; 2 to 3 g of Ag2SO4; 100 cc distilled water and a 1% NaOH solution. Difficult to prepare and store.

    Calcite Can be distinguished from dolomite with a simple stain of Alizarin Red-S in a 0.2% HCl solution (cold). Calcite and aragonite turn red, whereas dolomite remains colorless.

    Dolomite Can be. distinguished from calcite by the above method or one can stain specifically for dolomite with a number of organic stains including Titan Yellow, Trypan Blue, and Safranine 0. All these stains require boiling the sample in a concentrated NaOH solution.

    (Mg) Calcite Can be distinguished from aragonite and low-Mg calcite by use of Clayton Yellow stain. This is made by adding 0.5 g of Titan Yellow, 0.8 g of NaOH, and 2 g of EDTA to 500 ml of distilled water. The grain or section is etched in dilute acetic acid for 30 seconds, and is then put in Clayton Yellow solution for 30 minutes. Mg-calcite turns red while low-Mg calcite remains colorless. Mg-calcite can also be stained with Alizarin Red-S in 30% NaOH; calcite remains colorless whereas Mg-calcite turns purple.

    (Fe) Calcite Can be distinguished (along with ferroan dolomite) from normal calcite by the use of a potassium ferricyanide stain in a weak HCl solution (details in Dickson, 1966). Ferroan minerals turn pale to deep turquoise, and non-ferroan ones remain colorless.

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    In addition to being a fast and reliable method of determing the bulk mineralogy of carbonate rocks (aragonite, calcite, dolomite, siderite, etc.). X-ray diffraction allows the moderately accurate determination of the amount of magnesium (Mg) substitution in the calcite or dolomite lattice. For this, one needs to do careful scans at low chart speeds. By including an internal standard (galena for ancient carbonates and NaCI or CaF2 for modern sediments) one can determine very accurately (in terms of the angle, 2 θ) the reflection peak positions representing the (112) plane of the calcite crystal lattice. By matching that data against the chart given below (modified from Goldsmith, Graf, and Heard, 1961) one can approximate the magnesium content of the lattice to about 0.5 mol%. However, other cations besides magnesium can cause lattice spacing shifts and this data should thus be checked occasionally with microprobe or atomic absorption.

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    The microprobe offers the advantage of getting chemical analyses of areas as small as a few microns or scans of much larger regions. These photographs illustrate one application. The rock was filled with rhombs which were all presumed, on petrographic grounds, to be dolomite.

    When placed under the microprobe, they were analyzed for calcium (top photos, A, in each of the four series), magnesium (middle photos, B, in each of the four series), and iron (bottom photos, C). Note that the crystals in the two upper series all contain calcium and magnesium (A and B) while having outer, iron-rich zones (C). The lower two series have calcium and iron-rich zones (A and C) but are completely lacking the magnesium (B). Thus the two upper crystals are dolomite and fer-roan dolomite whereas the bottom two crystals have been dedolomitized (but without losing zonation) and are now calcite, and ferroan calcite. Simple petrographic distinction was impossible, although staining or cathodoluminescence might have revealed the same features.

    The main disadvantages of the technique are cost, time of sample preparation, need for accurate standards, and inability to distinguish many elements in the trace amounts in which they are found in carbonate rocks.

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