Advanced FTIR Method Improves Pyroxene Water Analysis

A team led by Nils B. Gies has published a study demonstrating how modern Fourier‑transform infrared spectroscopy (FTIR) can be refined for analysing hydroxyl (OH) absorbance in anisotropic nominally anhydrous minerals such as pyroxenes.¹

The authors—Nils B. Gies, Jörg Hermann, Georgia Cametti, and Sonja Aulbach—describe new hardware and software solutions that enable acquisition of large sets of angle‑resolved, polarised spectra covering angles from 0° to 180°. By recording many orientations simultaneously, the method captures the full angular dependence of the OH vibrational bands. This dataset is then processed and visualised with custom algorithms designed specifically for anisotropic crystals. The result is a richer spectral dataset than what conventional single‑angle FTIR measurements provide.

Key to the approach is identifying spectral endmembers within the silicon–oxygen overtone region. These endmembers correspond to groups of mutually perpendicular spectra that reduce uncertainty when quantifying water content. In practice, this means that the technique separates overlapping absorptions more cleanly, yielding a more reliable estimate of H₂O present in the mineral lattice.

Beyond pyroxenes, the paper argues that the methodology could be applied broadly across nominally anhydrous minerals where directional effects influence infrared absorption. Because many silicates are also anisotropic, the improved accuracy may benefit gemological analyses as well.

The study’s experimental design involved measuring freshly cleaved surfaces of synthetic pyroxene samples under controlled laboratory conditions. Spectra were recorded at multiple incident angles using a rotating stage coupled to a high‑resolution FTIR spectrometer. Data processing employed a new algorithm that automatically aligns polarisation vectors and extracts the most orthogonal spectral components. Visualisations include colour maps showing intensity variations over the 0°–180° range, allowing researchers to spot subtle features invisible in traditional plots.

In sum, this research offers a concrete advance in mineral spectroscopy: it turns angle‑resolved FTIR from a niche capability into a routine analytical step for anisotropic minerals. The paper appears on pages 583–627 of volume 38 of Eur. J. Mineral. ¹