New methods for Fourier transform infrared spectroscopy investigation and quantification of OH absorption in anisotropic nominally anhydrous minerals: the example of pyroxene

Fourier transform infrared spectroscopy (FTIR) is a powerful method for investigating and quantifying structurally bound OH, commonly reported as an H 2 O-equivalent (wt. % or µg g −1 ) oxide component, in nominally anhydrous minerals (NAMs). However, anisotropy in the absorbance depending on crystal orientation relative to the direction of the incident infrared beam causes large uncertainty. For orthorhombic orthopyroxene, the crystallographic and optical indicatrix axes coincide and can be found in randomly oriented grains using characteristic Si–O overtone bands and morphological or optical features. For monoclinic clinopyroxene, the infrared indicatrices of different OH-related defects are not always aligned with the axes of the optical indicatrix, complicating the determination of the total integrated absorbance and, thus, H 2 O quantification. An automatic FTIR polariser has been developed that enables the acquisition of large numbers of spectra between 0 to 180° with equal-angle intervals. With a new software solution, efficient processing, evaluation, and visualisation of the angle-resolved polarised measurements of large data sets are possible. This set-up allows recognition of spectral endmembers in the Si–O overtone region (wavenumber range 1250–2300 cm −1 ) that can be used to find groups of mutually perpendicular spectra and to obtain minimum and maximum OH absorption in the wavenumber range 3800–3000 cm −1 in different directions and sections. A new nomenclature for polarised FTIR measurements in low-symmetry crystals is proposed that includes the orientation of the analysed crystal section, as well as the direction of the electric field vector of the incident wave. From a large data set of Si–O overtone bands from diopside, augite, and omphacite, five distinct spectral endmembers aligned with axes of the optical indicatrix X , Y , and Z ; the crystallographic b and c axes; and the orthogonal direction ( a * ) were recognised. Detailed investigation using angular polarised measurements allowed the identification of combinations of endmember spectra suitable for a robust approximation of the total integrated absorbance and, thus, more accurate H 2 O content determination of a sample. For diopside and augite, cuts along the axes of the optical indicatrix are best suited to determine the total integrated absorbance, whereas, for omphacite, cuts along the crystallographic b and c axes and the orthogonal direction ( a * ) give the most reproducible results. In addition, Si–O overtones can help to evaluate sample homogeneity as spectra with the same Si–O overtone signal are measured in crystal sections with the same orientation and polarisation direction, and, thus, O–H absorption becomes directly comparable. The angular polarisation series combining Si–O overtones with OH absorbance bands analysis is a powerful tool for identifying families of bands with similar polarisation behaviour and for the detection of minor and overlapping absorption bands, leading to a better understanding of OH incorporation mechanisms in low-symmetry crystals.