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

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 %Fe³⁺ and H₂O from single crystal measurements, in addition to other chemical, mineralogical, and structural properties. Raman spectral measurements of amphibole minerals are used here to estimate %Fe³⁺ (relative to total Fe) and H₂O (wt%) contents using partial least squares (PLS) multivariate modeling. The accuracy of our model for prediction of %Fe³⁺ 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 R² value of 0.85. The model for prediction of H₂O has an internal RMSE of ± 0.09 wt% over the range from 0.1 to 1.9 wt% with an R² value of 0.95. Additional compositional model variables for predicting FeO, Fe₂O₃, MgO, CaO, Cr₂O₃, Al₂O₃, and TiO₂ have high R² values above 0.82; the R² value for SiO₂ is 0.63. Reliable models could not be achieved for MnO, Na₂O, and K₂O. 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.