Feasibility of determining the quantitative OH content of garnets with Raman spectroscopy

Elizabeth H. Arredondo and George R. Rossman

Division of Geological and Planetary Sciences
California Institute of Technology, MS 170-25, Pasadena, CA 91125

American Mineralogist 87, 307-312

The Mineralogical Society of America
1015 18th St NW Ste 601, Washington, DC 20036-5274 USA




Two suites of garnets were examined by infrared absorption spectroscopy and depolarized Raman spectroscopy to determine if the Raman signal could be used as a quantitative measurement of OH content. To avoid the problems of determining the absolute Raman signal intensity, the integrated intensity of the OH bands was ratioed to the integrated intensity of silicate Si-O stretching bands and used as a proxy for the OH intensity. These were compared to the OH contents, expressed as wt% H2O, independently determined from infrared absorption. A somewhat useful trend developed between OH contents determined by Raman and IR for some grossular garnets with H2O contents less than 0.5%. Many colorless to near-colorless grossulars do not provide a useful Raman signal due to fluorescence, while other grossulars with H2O contents between 0.5 and 1.3 wt% and are strongly colored, fall far below the trend of the lower H2O-content samples. Spessartine-almandine garnets from the Rutherford #2 pegmatite all respond to the Raman experiment, but produce a confusing trend when compared to the wt% H2O. Coincidentally, a relatively smooth but decreasing trend was observed when the spessartine Raman OH intensity ratio was compared to the iron content. These observations suggest that Raman measurements by this method are not suitable, in general, for the determination of OH in garnets.

Figure Title  
1 Typical Raman spectra  


OH/Si ratio vs. wt% H2O for low water content grossular garnets

OH/Si ratio vs. wt% H2O for high water content grossular garnets

3 OH/Si ratio vs. wt% H2O for the spessartine-almandine garnets  
4 Optical absorption spectra showing where the Raman emissions occur  
5 OH/Si ratio vs. wt% Fe(tot)O for the spessartine-almandine garnets  
6 OH/Si ratio vs. wt% Fe(tot)O for the grossular garnets  
7 OH/Si ratio vs. wt% H2O for colored grossular  
8 OH/Si ratio vs. wt% H2O for grossular low wavenumber OH