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Raman Point Counting Procedure

Mineral proportions and rock textures

We have determined that the best way to obtain the proportions of minerals in a rock on a planetary surface is by point counting .  In this procedure, one hundred or more Raman spectra are taken along a linear traverse on the surface of rock or soil. A small laser beam size is used, so each spectrum covers only one or two mineral grains.  The volume proportion of each mineral phase is extracted from the frequency with which its signal appears among the spectra. Rare phases such as secondary minerals can be detected. From the linear image of the rock’s mineralogy, information about rock texture and grain size can be obtained. From this information, we can determine the environments in which the rock originated and was altered.

pointcount.gif (181012 bytes) Schematic view of grid points where measurements were made on a thin section of lunar rock 14161,7062:

Volume proportions of minerals were extracted from the frequencies of appearance  of their signals among a large number of spectra.

These measurements required  about 1-3 hours.

To demonstrate the value of this technique and to evaluate the ability of Raman analysis to determine mineralogy on a rough, unprepared surface, we have done a 100-point analysis by Raman spectrometry on a lunar rock fragment (sample 15273,7039) taken from a lunar soil, with each laser spot centered on a strict geometric grid.  Among  the 100 Raman points, 98 yielded good spectra. Sixty-two points are single phase, 30 are multiphase, and 6 gave unknown spectra. Results of  the modal analysis obtained by Raman spectroscopy are compared in the table  below to the results obtained by petrographic examination of a polished section made after the Raman analysis.

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Mineral proportions obtained from the 100- point Raman measurement Identification of the rock type, basalt, is evident from the mineralogy as determined by the Raman experiment. The characteristics of the pyroxenes and the high proportion of phosphate and cristobalite indicate an evolved lithology, namely, a KREEP basalt. Petrographic observation of a thin section made from the rock chip after the Raman measurements supports these conclusions.

Reference: Haskin L. A., Wang A., Rockow K. M., Jolliff B. L., Korotov R. L., Viskupic K. M., “Raman spectroscopy for mineral identification and quantification for in-situ planetary surface analysis: a point count method”, Journal of Geophysical Research (1997), Vol. 102, p19293-19306.

 

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