Rapid Identification of Mineral Phases of Metamorphic Rock with Quantitative WDS

Rocks are not simple materials but very complex collections of different chemical compounds or minerals in varying concentrations, sizes, layers, and inclusions. This complexity happens over time, when rock far beneath the earth’s surface is subject to high temperatures, or succumbs to the pressures of the rock layers above it. Such conditions change the chemical composition of the minerals that comprise the rock and remove its original texture in a process known as contact metamorphism. Continental collisions or magma can also cause the formation of metamorphic rock. Metamorphic rocks such as slate, marble, and quartzite, make up a large part of the Earth’s crust. Geologists frequently use scanning electron microscopy (SEM) and X-ray microanalysis to study the elemental compositions of the minerals in metamorphic rocks to learn more about temperatures and pressures that occur at great depths within the Earth and the chemical reactions which must have occurred during each phase of the rock formation. The goal of an analysis of geological samples is to:

• Accurately identify minerals within a sample
• Understand the spatial distribution of elements within a sample
• Identify the textural evolution and mineral phases— the conditions under which a mineral is stable—that occurred during the geological process.

Wavelength dispersive spectroscopy (WDS) on the SEM uses X-ray crystals to isolate the characteristic X-rays generated by individual elements to provide quantitative analysis of material types ranging from petrochemical, geochemical, metals, glass and ceramics, mining and cement. WDS mapping is widely used by geologists because it identifies basic information about the composition of a selected section of the sample. Energy dispersive spectroscopy (EDS) rapidly acquires the characteristic x-rays of different elements into an energy spectrum, and accompanying software is used to analyze the spectrum to identify the quantities and distribution of specific elements. EDS can also be used to create compositional maps. In SEM-based X-ray microanalysis, samples are mapped to provide basic information about how metamorphic rocks have formed. There are, however, some potential problems with elemental mapping that must be considered to correctly understand the sample.  Our next article will address these issues. In the meantime, click here to learn more about how an  x-ray microanalysis system gets microscopists to the final answer in the least amount of time and with complete confidence in their results.