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Raman Point Counting
Mineral proportions and rock
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 rocks 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.
||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.
|Mineral proportions obtained from the 100- point Raman
||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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