Quantitative analysis of H-species in anisotropic minerals by unpolarized infrared spectroscopy: An experimental evaluation

Abstract

Attempts have been made to use unpolarized infrared analyses on unoriented anisotropic crystals of nominally anhydrous minerals to determine H contents, rather than using the more demanding polarized techniques that are more accurate (given that a reliable calibration is available). In this context, different approaches have been either empirically or theoretically proposed for the quantification; however, the involved accuracy has not been systematically documented by experimental work of both polarized and unpolarized analyses. In this study, we present a careful evaluation of experimentally grown, gem-quality OH-bearing olivine, clinopyroxene, and orthopyroxene single crystals. The samples were prepared for polarized and unpolarized infrared analyses, and the obtained spectra were used to estimate the H2O contents. We show that, regardless of the applied protocol, a single unpolarized determination is inadequate for quantitative analysis and the uncertainty could be up to 80%. The unpolarized method of Paterson (1982), by considering the linear absorbance intensity either through a single analysis or by averaging the data from multi-grain analyses, commonly underestimates the H2O content, by a factor of up to 6. The other unpolarized calibration method by using the averages of integrated absorbances of unoriented grains is in general of good accuracy, mostly within ±25% even for analyses on 2 grains (with perpendicular indicatrix sections), and the accuracy is even better if as many as 10 gains of random orientations are involved, e.g., within ±10%. Therefore, the latter method may be safely applied to quantify H in anisotropic minerals if a reasonable number of randomly oriented grains are chosen for the analyses. However, the uncertainty is non-systematic, and both underestimates and overestimates of H are possible depending upon orientation. These results provide a basis for quantifying H-species in anisotropic minerals and for documenting the quantitative effect of H on the physical properties of the host phases.