The longitudinal acoustic and optical phonon branches along the Gamma-X direction of MgO at 35 GPa have been determined by inelastic x-ray scattering using synchrotron radiation and a diamond-anvil cell. The experimentally observed phonon branches are in remarkable agreement with ab initio lattice dynamics results. The derived thermodynamic properties, such as the specific heat C(V) and the entropy S are in very good accord with values obtained from a thermodynamically assessed data set involving measured data on molar volume, heat capacity at constant pressure, bulk modulus and thermal expansion.

Phonon-dispersion curves along the threefold axis and the phonon density of states of the external modes in rhodochrosite, MnCO3, have been measured by inelastic neutron scattering employing the triple-axis spectrometer at the Dhruva reactor, Trombay. Lattice-dynamics computations based on a transferable potential model reveal the differences in the phonon properties of the isostructural minerals, calcite and rhodochrosite, and their manifestations in various thermodynamic quantities. The calculated dispersion curves and phonon density of states are in good agreement with experiments. The obtained results provide a strong experimental basis to validate the potential model.

One of the major goals of geophysics is to predict the thermodynamic properties and phase transitions of minerals at high pressure and temperature. A key requirement of predicting the thermodynamic properties of solids is an accurate description of its density of states. In this paper, we review our ongoing program aimed at providing a microscopic understanding of the vibrational adn thermodynamic properties of silicate minerals. Lattice dynamical model calculations backed by Raman scattering an inelastic neutron scattering studies have been undertaken to study the phonon dispersion relations, density of states and thermodynamic properties of several minerals. The calculations enabled a microscopic interpretation of the experimental data and have been successful in predicting the variations in the vibrational spectra in the various mineral phases of the Earth's mantle. The computed macroscopic thermodynamic properties and phase diagram of these minerals are in good agreement with experimental data. Molecular dynamics simulations of phase transitions of the MgSiO3 polymorphs have enabled an understanding of the relative stability and phase diagram of these minerals at pressures and temperatures characteristic of the Earth's interior.

The crystal structure of the high temperature phase of KAlSi2O6 at 700 dgC has been refined to R(%) = 5.4 by the least-squares method using 309 reflections. The thermal oscillation of K atom is great and the equivalent temperature factor Beq = 15.871(27) Å2. One of the principal axes of the thermal ellipsoids in K atom is close to the [111] direction and the magnitude of rms amplitude in this axis is larger than in other axes.

Electron- and ion-microprobe and Mossbauer analyses combined with X-ray structure refinement of a straw-yellow Mn-bearing alkali amphibole from Tirodi, Maharashtra, India, with no evidence of exsolution or inclusions, indicate a composition (Na0.30K0.03)(Na0.87Ca0.39Mn0.57Mg0.17)(Mg3.84Fe0.723+Mn0.38Li0.06)(Si7.88Al0.12)O22F0.40(OH)(1.60), nearly half-way between (Ca-Na) and (Fe-Mg-Mn) amphiboles. The unit-cell dimensions are: a = 9.704 (7), b = 17.990 (12), c = 5.297 (3) Angstrom, beta = 103.51 (5)degrees; space group C2/m and Z = 2. The single-crystal X-ray structure refinement (1363 reflections, R(all) = 0.039) was done to characterize the crystal-chemical features that might act as indicators of incipient exsolution phenomena due to immiscibility between (Ca-Na) and (Fe-Mg-Mn) amphibole compositions. An Fe-57 Mossbauer spectrum shows all iron to be in the trivalent state. All manganese is in the divalent state, and preferentially occurs at the split M4' site. The value of the beta angle is related to the cation content at M4, and is consistent with the intermediate composition. Mn-C(2+) is distributed almost equally over the three octahedral sites M1, M2, M3. A small amount of Li (detected by ion-microprobe) is ordered at the M3 site with concomitant substitution of Fe3+ at the M2 site for charge balance.

Inelastic neutron scattering measurements of phonon spectra from a powder sample of fayalite at various temperatures is reported. The measured spectra have been satisfactorily analyzed on the basis of a lattice dynamical model. The phonon density of states is used to derive the specific heat, which are found to be in good agreement with experimental data.

Raman spectra of oriented single crystals of calcite were measured at simultaneous high pressures and temperatures up to 40 kbar and 350{degrees}C. The fluorescence frequency shift of Sm:YAG was utilized for accurate pressure determination at elevated temperature, which exhibits negligible temperature shift in this temperature range. With increasing pressure the calcite-CaCO{sub 3}(II) and CaCO{sub 3}(II)-CaCO{sub 3}(III) transformations are observed at 14.5 and 18.5 kbar, respectively, and CaCO{sub 3}(III) remains the stable phase beyond 40 kbar. At elevated temperature and pressure, the width of the CaCO{sub 3}(II) stability field decreases and, at temperatures greater than 200{degrees}C, CaCO{sub 3}(III) transforms to aragonite. The CaCO{sub 3}(III)-aragonite phase boundary is insensitive to pressure over the 20 to 40 kbar interval. Calcite-CaCO{sub 3}(II) phase transition is first order and reversible, the CaCO{sub 3}(II)-CaCO{sub 3}(III) transition exhibits kinetic irreversibility, and the CaCo{sub 3}(III)-aragonite transition is sluggish and irreversible. Lattice dynamical calculations along T to F direction in calcite indicate an unstable phonon mode. The atomic displacements associated with this mode are consistent with those required for a continuous, displacive calcite-CaCO{sub 3}(II) phase transition. 29 refs., 3 figs.

High temperature experimental study of a leucite single crystal was carried out using heating-quenching and heating-cooling-heating processes. We observed that in the heating process, the transition temperatures, Tc1 and Tc2 were 655-degrees-C and 675-degrees-C, respectively. After quenching, in heating process (before cooling), transition temperatures were found at 640-degrees-C and 660-degrees-C, respectively. It is concluded that this hysteresis is not due to the first order transition because there is no existence of hysteresis in heating-cooling-heating process nor an abrupt volume change at the transition. Moreover, the reflections of 200, 020, and 002 were always observed in the temperature range 21-degrees-C - 900-degrees-C. Our results do not confirm the existence of I4(1)/a low temperature form, or Ia3d high temperature form including a transient phase I4(1)/acd.

Inelastic neutron scattering measurements and rigid-ion model calculations of phonon dispersion relation in the geophysically important mineral fayalite is reported. The measured Σ4TA branch is found to be softer in fayalite as compared to that in forsterite Mg2SiO4. The rigid-ion model calculations are in satisfactory agreement with the experimental data.

Cristobalite, a high temperature phase of silica, SiO2, undergoes a (metastable) first-order phase transition from a cubic, Fd3BARm, to a tetragonal, P4(3)2(1)2 (or P4(1)2(1)2), structure at around 220-degrees-C. The cubic C9-type structure for beta-cristobalite (Wyckoff 1925) is improbable because of two stereochemically unfavourable features: a 180-degrees Si-O-Si angle and an Si-O bond length of 1.54 angstrom, whereas the corresponding values in tetragonal alpha-cristobalite approximately 146-degrees and 1.609 angstrom respectively. The structure of the beta-phase is still controversial. To resolve this problem, a symmetry analysis of the Fd3BARm-->P4(3)2(1)2 (or P4(1)2(1)2) transition in cristobalite has been carried out based on the Landau formalism and projection operator methods. The starting point is the ideal cubic (Fd3BARm) C9-type structure with the unit cell dimension a (7.432 angstrom) slightly larger than the known a dimension (7.195 angstrom at 250-degrees C) of beta-cristobalite, such that the Si-O-Si angle is still 180-degrees, but the Si-O bond length is 1.609 angstrom. The six-component order parameter driving the phase transition transforms according to the X4 representation. The transition mechanism essentially involves a simultaneous translation and rotation of the silicate tetrahedra coupled along <110>. A Landau free-energy expression is given as well as a listing of the threes types of domains expected in alpha-cristobalite from the beta-alpha-transition. These domains are: (i) transformation twins from a loss of 3-fold axes, (ii) enantiomorphous twins from a loss of the inversion center, and (iii) antiphase domains from a loss of translation vectors 1/2 <110> (F-->P). These domains are macroscopic and static in alpha-cristobalite, and microscopic and dynamic in beta-cristobalite. The order parameter eta, couples with the strain components as epsilon-eta-2, which initiates the structural fluctuations, thereby causing the domain configurations to dynamically interchange in the beta-phase. Hence, the alpha-beta cristobalite transition is a fluctuation-induced first-order transition and the beta-phase is a dynamic average of alpha-type domains.

Ilvaite, Ca(Fe2+, Fe3+)Fe2+Si2O7O(OH) with double octahedral chains of Fe2+(A) and Fe3+(A) ions with Fe2+(B) octahedra occurring above and below shows electron delocalization at the A sites and a semiconductor-insulator transition (∼370K) followed by a crystallographic transition (346 K). In addition, it shows two magnetic transitions at 123 and 50 K. The magnetic structure derived from powder neutron diffraction consists of A site ribbons composed of two single ferromagnetic (Fe2+−Fe3+−Fe2+−Fe3+…) chains, whose spins are aligned antiparallel to each other below 123 K; the Fe2+(B) spins are weakly coupled to their A site neighbors and are mostly ordered below 50 K. Both A and B site spins are nearly parallel and antiparallel to b. Below 123 K, the Fe2+(A) and Fe3+(A) moments increase rapidly with decreasing temperature, showing a sharp increase at 50 K indicating the second transition, whereas Fe2+(B) moment increases very slowly below 123 with a substantial increase at 50 K indicating considerable spin fluctuation between 123 and 50 K. The moments extrapolated to 0 K are 3.6, 4.6, and 3.6 μB for Fe2+(A), Fe3+(A) and Fe2+(B) sites respectively. The difference in the thermal evolution of the magnetic moments at A and B sites is due to the frustration of Fe2+(B) spins, which occurs at the apex of a square pyramid, a corner common between two triangles with respect to two sets of Fe2+(A)-Fe3+(A) pairs with opposite spin directions.

The temperature dependence of the absorption spectra of ilvaite, Ca(Fe,Fe)FeSiO(OH), shows strongly one dimensional transport behaviour with no singularity at the Pnam-P2/a phase transition point near 335 K. Polarized single crystal transmission measurements were carried out between 300 K and 450 K in a frequency range between 600 and 23 000 cm. No Drude —absorption at low energies was found at any temperature. A macroscopic, thermodynamic model based on Landau-Ginzburg theory is given which accounts for the observed macroscopic properties of the structural phase transition and its coupling with the Fe-Fe ordering. This ordering scheme is discussed on an atomistic level and compared with the behaviour of magnetite and trans-(CH).

The elastic stiffness constants of sodalite Na4Al3Si3O12Cl (cubic, a=8.882 Å;Z=2), a framework-type aluminosilicate with cubo-octahedral cages, have been determined by an ultrasonic method. The measured values are C11,88.52(71); C12,38.70(50); and C44,36.46(33) GPa. Because of its structural flexibility, the bulk modulus KS(55.30 GPa) and shear modulus μ (31.30 GPa) values in sodalite are considerably smaller than those in densely packed cubic silicate structures such as spinel and garnet.