SEM/X-Ray Imaging Method in Petrographic Modal Analysis

Long-Cheng Liang

Image analysis is a fast and accurate technique for conducting petrographic modal analysis. In a routine scanning electron microscope-based modal analysis, backscattered electrons (BSE) are used to generate atomic number contrast to determine the presence of various minerals. The minerals are then identified with the use of an energy-dispersive spectrometer. The number of pixels, displayed as a specific level of intensity for a particular mineral, can be integrated and then calculated into the modal percent of that mineral. This method works well in most cases except where samples contain minerals with similar average atomic numbers or backscattering coefficients, thus producing ambiguity in identifying minerals in the BSE image. This problem, however, can be overcome by sing a superimposed x-ray imaging method (SXI), which applies logic and function to combine multiple x-ray maps of various elements in a mineral. The area of each mineral can then be estimated by integrating the pixels which contain all characteristic elements for that mineral. If two or more minerals contain the same elements but different concentrations, the minerals can be distinguished by the pixel intensity in the x-ray maps.

An application of the SXI method on a sample consisting of quartz, albite, and orthoclase proves that this method can unambiguously identify minerals and provide modal analysis with an absolute percent error of less than 2%. In contrast, the use of the BSE method creates difficulty in distinguishing quartz from albite owing to the close backscattering coefficients (quartz 0.125 vs. albite 0.124). Therefore, I recommend that the SXI method be incorporated with the routine BSE method for an accurate modal analysis.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.