Behavior of 10-A phase at low temperatures

The thermal evolution of 10-Å phase Mg3Si4 O10(OH)2·H2O, a phyllosilicate which may have an important role in the storage/release of water in subducting slabs, was studied by X-ray single-crystal diffraction in the temperature range 116-293 K. The lattice parameters were measured at several intervals both on cooling and heating. The structural model was refined with intensity data collected at 116 K and compared to the model refined at room temperature. As expected for a layer silicate on cooling in this temperature range, the a and b lattice parameters undergo a small linear decrease, αa = 1.7(4) 10-6 K-1 and αb = 1.9(4) 10-6 K-1, where α is the linear thermal expansion coefficient. The greater variation is along the c axis and can be modeled with the second order polynomial cT = c293(1 + 6.7(4)10-5 K-1ΔT + 9.5(2.5)10-8 K-2(ΔT)2) where ΔT = T - 293 K; the monoclinic angle β slightly increased. The cell volume thermal expansion can be modeled with the polynomial VT = V293 (1 + 8.0 10-5 K-1 ΔT + 1.4 10-7 K-2 (ΔT)2) where ΔT = T - 293 is in K and V in Å3. These variations were similar to those expected for a pressure increase, indicating that T and P effects are approximately inverse. The least-squares refinement with intensity data measured at 116 K shows that the volume of the SiO4 tetrahedra does not change significantly, whereas the volume of the Mg octahedra slightly decreases. To adjust for the increased misfit between the tetrahedral and octahedral sheets, the tetrahedral rotation angle α changes from 0.58° to 1.38°, increasing the ditrigonalization of the silicate sheet. This deformation has implications on the H-bonds between the water molecule and the basal oxygen atoms. Furthermore, the highly anisotropic thermal ellipsoid of the H2O oxygen indicates positional disorder, similar to the disorder observed at room temperature. The low-temperature results support the hypothesis that the disorder is static. It can be modeled with a splitting of the interlayer oxygen site with a statistical distribution of the H2O molecules into two positions, 0.6 Å apart. The resulting shortest Obas-OW distances are 2.97 Å, with a significant shortening with respect to the value at room temperature. The low-temperature behavior of the H-bond system is consistent with that hypothesized at high pressure on the basis of the Raman spectra evolution with P.