Energy Dispersive Analysis
Scanning electron microscopy is useful not only for examining sediment textures, but, when equipped with an energy dispersive analyser, it can be used effectively for mineral identification and semi-quantitative chemical analysis. The analyses are rapid (seconds) and require relatively little sample preparation. In most cases, small chips of the sample can be mounted on a small plug with no polishing or cutting required. The sample is then coated with a gold-palladium alloy (or other conductive metal) and is inserted into the SEM.
Here, a potassium feldspar from the Cretaceous Frontier Formation of Wyoming is shown with an accompanying analytical spectrum. Although the grain might be identifiable as a feldspar on the basis of its crystal shape, cleavage, and other features alone, the energy dispersive analysis provides additional chemical data which allows positive identification.
The analytical trace (lower photo) shows major peaks for Si and K (the main K peak has the long, pale blue line over it) with only very minor peaks for other elements. This indicates a rather pure K-feldspar composition.
Although energy dispersive analysis on the SEM provides an excellent tool for mineral identification, it is not ideally suited for quantitative analytical work. Detailed determination of mineral composition or analysis of trace element contents of small crystals is best done using polished samples on an electron microprobe.
Figures & Tables
A Color Illustrated Guide To Constituents, Textures, Cements, and Porosities of Sandstones and Associated Rocks
This book is designed as a companion volume to AAPG Memoir 27. As with its predecessor volume, the purpose of this book is to provide identified illustrations of important grains, textures, cements, and porosity types for geologists who may not be specialists in the petrography of sandstones and associated sedimentary rocks.
Sandstone petrography is of particular interest to the explorationist for several reasons. First, it can provide valuable information on the detailed composition of sedimentary rocks. From this, one can often draw conclusions about the lithology, climate, and tectonic history of the source area, as well as predicting the response of such units to a variety of subsurface diagenetic environments. Second, one can acquire significant data on the grain size, sorting, and rounding of sedimentary grains. For Iithified sediments this may be the only way to obtain such data, which may be useful in determinations of the transport mechanisms and depositional environment of the sediment. Third, information may be obtained on the postdepositional alteration history of sedimentary rocks. This may include data on compaction, cementation, leaching, fracturing, porosity types, and other factors. These are essential for a proper understanding of reservoir rocks and, commonly, petrography provides the only technique forgathering accurate data on such diagenetic factors.
This book is intended as an introduction for exploration geologists or students and is by no meansa complete textbook or treatise. However, it does include a wide variety of color photographs of terrigenous clastic grains, cements, and textures of sandstones and common accessory rock types. Although most of the illustrations are of features seen with the petrographic microscope, some scanning electron micrographs are included. The illustrations were made from samples having as wide a range of lithologies, geologic ages, and localities as possible to insure a fairly representative presentation. In addition, the photographs were generally selected to show the most common grain and textural types encountered by the geologist and to present typical, rather than spectacular, examples of most features. Thus, the book shouId have applicability to any sandstone petrographic study.
This volume focuses on the descriptive aspects of petrography and includes no text other than figure captions. Bibliographies are provided in each section of the book. For more detailed descriptive and interpretive information, the references listed in both the general and specific bibliographies should be consulted.
The major emphasis of th is book is on the fou... major fabric elements of sandstones: framework grains; detrital fine-grained matrix; cements; and pore space.