A COMPARISON OF THE RAMAN SPECTRA OF THE HUMITE MINERAL GROUP AT HIGH PRESSURES

Humite group minerals n(Mg,Fe)2SiO4 - (Mg,Fe)(OH)2 have been suggested as possible candidates for water storage and transport in the mantle, and clinohumite in particular has been proposed as the source of ilmenite lamellae in Alpine ultrahigh pressure metamorphic terranes via its decomposition at high pressure and temperature. In this study, a comparison is made between the Raman spectra and energy dispersive diffraction pattern of norbergite (n = 1), chondrodite (n=2), humite (n=3), and clinohumite (n = 4) up to 20 GPa to correlate the structural and vibrational properties of the humite mineral group.

The observed vibrational modes in the Raman spectra of the humite minerals increase in frequency with pressure, although the change in frequencies with pressure is much steeper in norbergite than for chondrodite, humite, and clinohumite. In norbergite, antisymmetric stretching modes of the SiO4 tetrahedron merge. However, the SiO4 tetrahedron do not merge in higher end members. In addition, the intensity of the antisymmetric stretching mode for clinohumite decreases in relative intensity with increasing pressure compared to the intensity of the symmetric stretching mode. In the spectra of norbergite, these two modes retain their relative intensities with increasing pressure. The most striking difference between the spectra of norbergite and chondrodite/humite/clinohumite is in the deformation modes of the brucite layer, which within the chondrodite/humite/clinohumite structure retain their intensities with increasing pressure, while these modes are not observed in the spectra of norbergite.

The nature of the Raman spectra and their evolution with pressure are correlated with the structural properties of the minerals in terms of the interactions between olivine and brucite layers and the crystal chemistry of the humite group minerals. (Currently we are collecting data at the Advanced Photon Source, using energy dispersive diffraction, to further constrain the change in the structure of the humite mineral group with increasing pressure).