Recovering full crystallographic orientation in specimen coordinates from polarised μ-FTIR spectra

Abstract Fourier-transform infrared (FTIR) spectroscopy is the standard technique for quantifying trace hydroxyl (OH) in nominally anhydrous minerals because it is accessible, fast, and can distinguish hydrous defects by the orientation and number of OH dipoles in the defect complex. Although most minerals absorb infrared light anisotropically, most studies approximate the total absorbance (the sum over the three principal directions) from unpolarised spectra of randomly oriented crystals, discarding the OH dipole orientation information held by birefringent crystals. Polarised, spatially resolved FTIR (μ-FTIR) recovers the OH bond orientation of multiple grains relative to the crystal axes, provided the crystal orientations are known. We present a method that recovers the full crystallographic orientation of an anisotropic crystal in specimen coordinates, expressed as Euler angles, from polarised μ-FTIR spectra collected over a range of polariser angles along with reference spectra measured along three orthogonal directions. The method requires only a single diagnostic wavelength, making it more flexible than spectrum-range approaches because the user can avoid wavenumbers degraded by detector non-linearity or baseline issues. We validate it on the orientation dependence of the silica overtone region in olivine, using Monte Carlo simulations and crystals of known orientation measured by Electron Backscatter Diffraction (EBSD). We also provide an open-source Python implementation (FTIRkit). By recovering crystal orientation and OH content from a single FTIR measurement, the method provides a powerful tool for constraining water budgets, metasomatism, and magma sources in the mantle and lower crust, testing DFT predictions of OH-group orientation, and correcting CPO bias in deformation studies. The approach applies to any birefringent crystalline phase and can be extended to other polarised techniques (Raman, UV–Vis, and laser-direct IR) where principal-direction spectra and a model relating spectrum to orientation are available.