Raman spectra of graphite as indicators of degree of metamorphism

Abstract

This study demonstrates the usefulness of Raman microspectroscopy an evaluation of the crystallinity of graphite in natural samples. The sensitivity obtained is at least as good as that for X-ray diffractometry. Laser Raman microprobe analyses of graphite separates, mostly from suites of regionally metamorphosed carbon-bearing pelites (Grew 1974), show that l) the spectra oferaphite accurately reflect the degree of crystallinity in terms of the (001) inplane crystallite size, 2) the same spectral features and changes with crystallinity occur in the spectra of natural graphite as in those of synthetic samples, 3) the Raman spectrum is a sensitive monitor of changes in graphite crystallinity, reflecting conditions from greenschist- to granulitefacies metamorphism, 4) significant changes in the Raman spectral profile coincide with transitions in the metapelites that delineate the chlorite, biotite + garnet, staurolite -r. kyanite + andalusite, and sillimanite zones, and 5) the graphite stmcture apparently records only the peak metamorphic temperatures and does not recrystallize under retrograde conditions.