High-pressure vibrational spectra of humite-group minerals: Fluorine effect on thermodynamic properties and hydrogen bonds

Abstract

The humite-group minerals could be potential water carriers as the dehydration products of serpentine in subduction slabs. Fluorine is an important element incorporated into the crystal structures through the substitution mechanism of OH− = F−, and has significant impacts on the thermodynamic properties. We conducted in situ high-pressure Raman and Fourier transform infrared (FTIR) measurements on natural F-bearing clinohumite, humite and norbergite samples, up to approximately 19GPa. The isothermal mode Grüneisen and intrinsic anharmonic parameters were calculated for both the lattice and OH-stretching modes, and the anharmonic contribution to the heat capacities was further evaluated for these hydrous phases. As compared with the OH-clinohumite sample, the anharmonic contribution is larger in magnitude for the F-bearing clinohumite, while such anharmonic contributions are even more significant in the humite and norbergite samples with higher fluorine concentrations. Both the Raman and FTIR spectra indicate that the OH-stretching modes above 3450 cm−1, which are controlled by the neighboring HH, systematically show positive pressure dependences. On the other hand, the OH bands below 3450 cm−1 shift to lower frequencies at elevated pressure, due to F− or Ti4+ substitutions. The F− substitution of OH− group could alleviate local HH repulsion effect, and create new H sites with shorter hydrogen bond lengths. These hydrogen bond would get shorter (negative pressure dependences), and are likely to participate in the hydrogen bond symmetrization at high pressure.