Although light-microscope 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 greatly 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, inductively coupled plasma mass spectrometry, ion microprobe, electron microprobe, atomic absorption or cathodoluminescence techniques may be applicable; and where it is desirable to know the temperatures, water sources, and/or pore fluid compositions involved in cementation, fluid inclusion geothermometry, stable isotope geochemistry, strontium isotope geochemistry and a number of other analytical techniques may provide useful information.
In addition, many 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 1-2 μm range. Many carbonate and noncarbonate matrix constituents fall within or below this size range. Furthermore, because most standard thin sections are about 30 μm thick, a researcher typically sees 10 or 20 such small grains stacked on top of one another in a micritic limestone, 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. In most cases, however, scanning and transmission electron microscopy have proved to be the most effective techniques for the detailed examination of fine-grained sediments.
The bibliography for this chapter (and those in many previous chapters as well) provides references to techniques useful in supplementing standard petrographic analysis. Although many of the techniques require sophisticated and expensive equipment, others, such as thin-section staining, production of acetate peels, or concentration of insoluble residues, can be done in any laboratory and at very little cost.
Because of the potential desirability of supplemental techniques, it is often useful to prepare epoxy-cemented thin sections without coverslips. These sections can be examined under a light microscope, either by placing a drop of water and a coverslip on the sample during viewing, or by using mineral oil or index of refraction oils with or without coverslips. Such examination involves some loss of resolution, but does allow the cleaning and drying of the surface of the section and subsequent staining, cathodoluminescence, or microprobe examination. One can even partially or completely immerse the thin section in acetic or hydrochloric acid and decalcify the section, thereby sometimes enhancing organic structures or insoluble-mineral fabrics. Finally, uncovered thin sections can be ground thinner in cases where examination of very fine-grained sediments is needed.
Clearly, one can spend years analyzing a single sample using all possible techniques. Efficient study requires a thorough understanding of all available tools and proper application of the most useful and productive of these.
Staining techniques are among the fastest, simplest, and cheapest methods for getting reliable mineralogical, and some qualitative elemental, data on carbonate phases. The following list of minerals and their diagnostic stains is derived from the work of Friedman (1959),, Dickson (1965, and 1966), Milliman (1974), and others. The original papers, listed in the bibliography, will provide details about the exact application and methods.
Aragonite - can be distinguished from calcite by the use of Feigelʼs Solution. Aragonite turns black whereas calcite remains colorless for some time. Mixing Feigelʼs Solution requires 7.1 g of MnSO4•H2O; 2 to 3 g of Ag2SO4; 100 cc of distilled water and a 1% NaOH solution. Difficult to prepare and store.
Figures & Tables
A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis
This volume expands and improves the AAPG 1978 classic, A Color Illustrated Guide to Carbonate Rock Constituents, Textures, Cements, and Porosities(AAPG Memoir 27). Carbonate petrography can be quite complicated. Changing assemblages of organisms through time, coupled with the randomness of thin-section cuts through complex shell forms, add to the difficulty of identifying skeletal grains. Furthermore, because many primary carbonate grains are composed of unstable minerals (especially aragonite and high-Mg calcite), diagenetic alteration commonly is quite extensive in carbonate rocks. The variability of inorganic and biogenic carbonate mineralogy through time, however, complicates prediction of patterns of diagenetic alteration. This book is designed to help deal with such challenges. It includes a wide variety of examples of commonly encountered skeletal and nonskeletal grains, cements, fabrics, and porosity types. It includes extensive new tables of age distributions, mineralogy, morphologic characteristics, environmental implications and keys to grain identification. It also encompasses a number of noncarbonate grains, that occur as accessory minerals in carbonate rocks or that may provide important biostratigraphic or paleoenvironmental information in carbonate strata. With this guide, students and other workers with little formal petrographic training should be able to examine thin sections or acetate peels under the microscope and interpret the main rock constituents and their depositional and diagenetic history.
- carbonate rocks
- color imagery
- problematic fossils
- sedimentary rocks