A combined Fourier transform infrared and Cr K-edge X-ray absorption near-edge structure spectroscopy study of the substitution and diffusion of H in Cr-doped forsterite

Single crystals of synthetic Cr-doped forsterite (Cr:Mg2SiO4) containing both Cr3+ and Cr4+ were partially hydroxylated in piston-cylinder apparatuses at 750–1300 ∘C and pressures from 0.5 to 2.5 GPa, with p(H2O) ≈Ptotal. The oxygen fugacity (fO2) was buffered by graphite-water, Ni–NiO, Re–ReO2, Fe2O3–Fe3O4 or Ag–Ag2O, and the silica activity (aSiO2) was buffered by powdered forsterite plus either enstatite (Mg2Si2O6), periclase (MgO) or zircon–baddeleyite (ZrSiO4–ZrO2). Profiles of OH content versus distance from the crystal edge were determined using Fourier transform infrared (FTIR) spectroscopy, and profiles of the oxidation state and coordination geometry of Cr were obtained, at the same positions, using K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The techniques are complementary – FTIR spectroscopy images the concentration and nature of O–H bonds, where Cr K-edge XANES spectroscopy shows the effect of the added H on the speciation of Cr already present in the lattice. Profiles of defect-specific absorbance derived from FTIR spectra were fitted to solutions of Fick's second law to derive diffusion coefficients, which yield the Arrhenius relationship for H diffusion in forsterite: log⁡10D̃[001]=-2.5±0.6+-(224±12+4.0±2.0P)2.303RT, where D̃ is the measured diffusion coefficient in m2 s−1, valid for diffusion parallel to [001] and calibrated between 1000 and 750 ∘C, P and T are in GPa and K, and R is 0.008314 kJK−1 mol−1. Diffusivity parallel to [100] is around 1 order of magnitude lower. This is consistent with previous determinations of H diffusion associated with M-site vacancies. The FTIR spectra represent a variety of Cr-bearing hydrous defects, along with defects associated with the pure Mg–Si–O–H system. It is proposed that all of the defects can form by interaction between the dry lattice, including Cr3+ and Cr4+, and fully hydroxylated M-site vacancies. The initial diffusive wave of hydroxylation is associated with neither reduction nor oxidation of Cr but with Cr4+ changing from tetrahedral to octahedral coordination. Superimposed on the H diffusion and concomitant change in Cr4+ site occupancy, but at a slower rate, producing shorter profiles, is reduction of Cr4+ to Cr3+ and potentially of Cr4+ and Cr3+ to Cr2+. In addition, by comparing FTIR data to trace element contents measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), constraints can be placed on absorption coefficients used for converting absorbance to H2O contents – our data support either wavenumber- or defect-dependent values of absorption coefficients. We estimate absorption coefficients of between 60 200 and 68 200 L mol−1 cm−1 for OH− associated with octahedral Cr3+ and an M-site vacancy and 18 700 to 24 900 L mol−1 cm−1 for two OH− associated with octahedrally coordinated Cr4+ and a Si vacancy (i.e. a “clinohumite-type” point defect).