Synchrotron FTIR study of talc and 10-Å phase to 10 GPa

INTRODUCTION Talc and 10-Å phase are potentially important H 2 O-bearing phases in the Earth's upper mantle. Talc, Mg 3 Si 4 O 10 (OH) 2 , is common in hydrothermally-altered and SiO 2-metasomatised ultramafic rocks. It is stable to ~800°C at 3 GPa, decreasing to 700°C at 5 GPa [1]. Above ~4 5 GPa, and in the presence of excess H 2 O, it reacts to 10-Å phase, Mg 3 Si 4 O 10 (OH) 2 .xH 2 O. 10-Å phase is stable up to ~9 GPa [2]. Talc has a 2:1 layer structure, with sheets of edge-sharing octahedrally-coordinated Mg sandwiched between sheets of corner-sharing SiO 4 tetrahedra. Two OH groups form part of the octahedral coordination unit around each Mg, with the O-H bonds pointing towards the 6-fold rings in opposing tetrahedral sheets [perpendicular to (001)]. The 2:1 layer has monoclinic symmetry, but stacking arrangements typically reduce the symmetry to triclinic. Thus the space group is either C1 or C2/c. The structure of 10-Å phase is based on that of talc, but with the addition of H 2 O in the interlayer. However, both the amount of interlayer H 2 O, and its precise structural position, are uncertain. The compressibility of talc and 10-Å phase have previously been measured u sing a powdered sample in a diamond-anvil cell with synchrotron radiation [3]. The talc data, measured up to 6 GPa, varied smoothly with pressure. The 10-Å phase data show a discontinuity at between 0.4 and 0.6 GPa, with the volume measured below this pressure being above the extrapolated curve through the higher-pressure points. However, the ambient-pressure 10-Å phase 00l diffraction peaks were broad, suggesting heterogeneity in the spacing between successive 2:1 layers (averaging ~10 Å). More recent high-pressure synchrotron XRD experiments on talc and 10-Å phase in a multi-anvil press at Daresbury Laboratory have, however, demonstrated that the peaks are as sharp as the talc peaks at high pressures and temperatures [4]. Thus it appears likely that the increased volume of 10-Å phase at ambient pressure is caused by the structural change responsible for the broadening of the diffraction peaks. There have been several previous IR studies of talc at ambient pressure, focussing on the O-H stretching region. In pure talc there is a single sharp O-H stretching frequency of ~3677 cm-1. The high frequency of this vibration is characteristic of a strong O-H bond with little or …