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Clinohumite is a potentially abundant silicate mineral with high water concentration (2~3 wt% H 2 O) that is generated from dehydration of serpentine-group minerals in subduction zones. Previous studies show that fluorine substitution (OH -  = F - ) can stabilize clinohumite to significantly higher temperature in subduction zones, although temperatures within the slabs are thought to be well within the stability field of both F-bearing and OH-clinohumite. We collected in-situ high-temperature Raman and Fourier transform infrared (FTIR) spectra for both the synthetic [Mg 9 Si 4 O 16 (OH) 2 ] and natural [Mg 7.84 Fe 0.58 Mn 0.01 Ti 0.25 (SiO 4 ) 4 O 0.5 (OH) 1.30 F 0.20 ] clinohumite samples up to 1243 K. Three OH bands above 3450 cm –1  are detected for both the natural and synthetic samples with negative temperature dependence, due to neighboring H-H repulsion in the crystal structure. Additional OH peaks are detected for the natural sample below 3450 cm –1  with positive temperature dependence, which could be explained by non-polar F -  substitution in the OH site. Hence, F -  substitution significantly changes the high-temperature behavior of hydrogen bonds in the humite-group minerals. On the other hand, we evaluated the mode Grüneisen parameters (γ i  P , γ i  T ), as well as the intrinsic anharmonic parameters ( a  i ) for clinohumite, chondrodite, and phase A, the dense hydrous magnesium silicate (DHMS) phases along the brucite–forsterite join. The estimated averaged anharmonic parameters ( a  i_avg ) for these DHMS phases are systematically smaller than those of olivine. To model the thermodynamic properties of minerals (such as heat capacity) at the high-temperature conditions of the mantle, the DeBye model, which simply approximates the lattice vibrations as harmonic oscillators, is commonly used. In contrast to forsterite, such quasi-harmonic approximations are valid for clinohumite at subduction zone temperatures, as the anharmonic contribution is no more than 2% when extrapolated to 2000 K. Hence, the classic DeBye model can reasonably simulate the thermodynamic properties of these DHMS phases in subduction zones.

american mineralogist

In-situ high-temperature vibrational spectra for synthetic and natural clinohumite: Implications for dense hydrous magnesium silicates in subduction zones