Ferric Iron, Hydrogen, and Major Element Quantification of Amphibole Minerals Using Raman Spectroscopy and Multivariate Analysis

ABSTRACT Quantification of Fe redox state and hydrogen content of amphibole provides information regarding the relationship between oxygen and water concentrations in terrestrial and planetary materials. Raman spectroscopy is a powerful technique due to its ability to characterize both %Fe 3+ and H 2 O from single crystal measurements, in addition to other chemical, mineralogical, and structural properties. Raman spectral measurements of amphibole minerals are used here to estimate %Fe 3+ (relative to total Fe) and H 2 O (wt%) contents using partial least squares (PLS) multivariate modeling. The accuracy of our model for prediction of %Fe 3+ is ± 8.11% (absolute) expressed as root‐mean‐square error (RMSE) of the entire data set, covering the range from 0 to 100% with an R 2 value of 0.85. The model for prediction of H 2 O has an internal RMSE of ± 0.09 wt% over the range from 0.1 to 1.9 wt% with an R 2 value of 0.95. Additional compositional model variables for predicting FeO, Fe 2 O 3 , MgO, CaO, Cr 2 O 3 , Al 2 O 3 , and TiO 2 have high R 2 values above 0.82; the R 2 value for SiO 2 is 0.63. Reliable models could not be achieved for MnO, Na 2 O, and K 2 O. The successful creation of our compositional models along with detailed analysis of the PLS model coefficients indicates that Raman spectroscopy can be used as a quantitative compositional tool in characterizing the amphibole mineral group. Quantifying amphibole compositions is useful for evaluating repositories of hydrogen, constraining the water budget of the terrestrial crust and interior, developing geothermobarometers and geohygrometers, and quantifying magma ascent rates.