IDENTIFYING TYPES OF ZONING IN MINERALS USING RAMAN MICROSCOPY

As minerals grow in sub-volcanic magma chambers, zones may form, representing changes in magma composition and temperature. There are three main types of concentric zoning profiles including normal (slow continuous cooling), reverse (mixing of new magma in the magma chamber), and oscillatory (small local variations). Therefore, zoning profiles may be used as indicators of petrogenetic processes in the magma chamber. Optically continuous zones can be identified using polarized light microscopy, but compositional data cannot be obtained to distinguish types of zoning profiles. A simple and accurate means of identifying the composition of the zones in the minerals, specifically in a thin section, is needed. The Electron Microprobe, Scanning Electron Microscope (SEM-EDS), and Raman Microscopy are all used to obtain compositional analyses. The Raman Microscope is the least expensive of these instruments and identifies compositions by comparing the acquired spectra to those of known compositions (e.g. those entered in the RRUFF database). So, there is a need for Raman spectra covering a wide range of compositions, especially for the compositions in a broad solid solution series like the plagioclase feldspars. To develop this necessary range, spectra from feldspar samples of known compositions were acquired both from literature and the existing RRUFF database to build a spreadsheet of spectral data of known compositions. Gaps in the compositions were filled in by obtaining precise compositional data on select feldspar specimens through the use of the Electron Microprobe and single crystal XRD, and then obtaining the Raman spectra for these additional minerals. From the data collected, we were able to develop a sort of calibration curve for the feldspars relating the main Raman peaks to the mineral composition. This graphed relationship enables the identification of the types of zoning relationships in feldspars (normal, reverse, oscillatory, etc.) using Raman Microscopy. This technique has the potential to be used with other solid solution series (olivine, pyroxenes, annite-phlogopite, etc.) to determine zoning relationships, geothermobarometry, or other factors affecting the growth and composition of these minerals.