TESTING THE GEOCHEMICAL ACCURACY OF ANALYTICAL OPTICAL MINERALOGY

Geologic and geochemical analysis of rocks is complicated by sample heterogeneity. When commercial tools or funding is not available to researchers, can an accurate geochemical analysis of a heterogeneous sample be obtained using optical mineralogy?

In this study, a heterogeneous igneous mafic rock commercially sold as Star Galaxy countertop was analyzed with both optical and commercial techniques. The in-depth optical analysis resulted in a volume percent of phases (minerals) present and an elemental volume percent was then obtained using stoichiometric conversions. Powdered mineral separates and whole rock samples will be used for both X-ray Diffraction (XRD) and X-ray Fluorescence Spectroscopy (XRF) analysis. XRD will yield phase percentages and individual phase chemistry in some cases, and XRF will yield whole rock geochemistry as well as detailed information on phenocryst chemistry.

Though XRD, XRF, and other commercial analytical techniques provide accurate information, the information provided from each test is limited. For example, a mineral separate of pyroxene with exsolution lamellae will yield an orthopyroxene and clinopyroxene with XRD, but a single pyroxene composition with XRF. In-depth optical mineralogy and petrographic analysis provide both phase and chemical information (e.g. Michel Lévy method, and index oils for pyroxenes). Commercial analysis has been favored over optical mineralogy and petrographic analysis because it is fast and offers higher accuracy. Traditional analysis (e.g. optical, petrographic, etc.) can provide detailed, accurate compositional data when other resources are not available.

Analytical accuracy of the optical analysis will be determined when comparing results against the two commercial analyses. This comparison will also determine if an accurate analysis of a heterogeneous rock can be performed with limited resources such as a petrographic microscope, index oils, etc. Optical analytical techniques are an important component of undergraduate training, and the resources are readily available to most students. I hope to highlight the importance of petrographic analysis to researchers and students who may not have access to expensive or off-site analytical tools.