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We present a detailed [sup.29]Si NMR spectroscopic study of isotopically enriched samples of forsterite and of anhydrous and hydrous wadsleyite and ringwoodite ([alpha], [beta], and [gamma] phases of [Mg.sub.2]Si[O.sub.4]), which complement previous extensive studies of these minerals by XRD and vibrational spectroscopy, [sup.VI]Si is not detected in any of the phases at levels of about 0.1 to 0.5%. When coupled with recent theoretical calculations on ringwoodite, this suggests the possibility of re-ordering of high-temperature octahedral-tetrahedral disorder during cooling. Cross-polarization ([sup.29]Si{[sup.1]H } CPMAS) NMR supports the protonation of O1 oxygen atoms in hydrous wadsleyite without formation of significant amounts of Si-OH groups, in contrast, new NMR peaks appear in hydrous ringwoodite that cross-polarize very rapidly, indicating very short Si-H distances and the presence of Si-OH, as expected from models in which much of the [H.sup.+] substitutes into [Mg.sup.2+] vacancies. Static NMR spectra provide new constraints on chemical shift anisotropies in wadsleyite and are fully consistent with the cubic structure of ringwoodite. Spin-lattice relaxation in all phases is much better fitted by a stretched exponential function than with a more conventional '[T.sub.1]' exponential, as expected when relaxation is dominated by paramagnetic impurities. However, the effects of paramagnetic impurity on ion contents on relaxation, and on the formation of newly observed minor peaks that may result from 'pseudo-contact shifts,' appear to depend on mineral structure, and will require considerable future study to understand in detail. Keywords: NMR spectroscopy, forsterite, wadsleyite, ringwoodite, water in mantle

Lattice parameters for gold nanocrystals, quartz, and vesuvianite have been determined by electron diffraction in routine transmission electron microscopy (TEM) work, with precision and accuracy near to 0.1%, after correction for elliptical distortion. The distortion, measured in three different microscopes, is constant for each microscope and may be easily eliminated. Variable camera constants have been avoided by positioning the oriented specimen on the eucentric plane and using parallel illumination. The current flowing in the first intermediate lens was kept fixed, assuring constant conditions of the TEM projecting system, with no further diffraction focus applied. Application of this method to micas from metamorphic rocks produced deviations between measured and expected values up to 0.8%. Although easy species distinction is still possible, minor crystal chemical differences within the sample may be lost. Likely causes of these deviations are the possible heterogeneous samples, as well as beam damage leading to cation loss with subsequent variation in basal spacings. Keywords: Electron diffraction, lattice parameters, elliptical distortion, mica

We have investigated by first principles the compressional behavior of low-pressure (LP) and high-pressure (HP) MgSi[O.sub.3] clinoenstatite. We have carefully examined unit-cell shapes, chain angles, and polyhedral volume responses, such as angle variances and quasi-elongations, under pressure at room temperature. We have observed opposite behavior of the tetrahedra in the S-rotated and O-rotated chains with pressure in the LP phase, with a slight increase (decrease) in angle variance and quasi-elongation in the former (latter). Inspection of zone center modes of both phases under pressure reveals a transition path that converts the S-rotated chain in the LP phase into the O-rotated chain in the HP phase. This conversion is related to a slight softening of an [A.sub.g] 'metastable' Raman mode under pressure. Keywords: Phase transition mechanism, low-pressure clinoenstatite, high-pressure clinoenstatite, Raman, IR, phonon frequency

We report the lattice thermal expansion of zircon-(xenotime-)type LuP[O.sub.4] and LuV[O.sub.4] in the temperature range of 25-1000[degrees]C from high-temperature powder XRD studies. The details of the high-temperature crystal chemistry of both phases have been determined from Rietveld analysis of the powder XRD data. Both the compounds show appreciably higher thermal expansion than analogous zircon-type silicates. Despite isomorphism, the axial thermal expansion of LuV[O.sub.4] shows significant anisotropy compared to LuP[O.sub.4]. In the studied temperature range, the average axial thermal expansion coefficients of LuV[O.sub.4] are [[alpha].sub.a] = 6.0 x [10.sup.-6] and [[alpha].sub.c] = 7.2 x [10.sup.-6] ([degrees][C.sup.1] and those of LuV[O.sub.4] are [[alpha].sub.a] = 3.6 x [10.sup.-6] and [[alpha].sub.c] = 11.8 x [10.sup.-6] ([degrees][C.sup.-1]). However, the average volume thermal expansion coefficients are almost identical. The differences in the thermal expansion behavior of the two structures originate from differences in the expansion and distortion of the Lu[O.sub.8] polyhedra. The Lu[O.sub.8] polyhedron in LuV[O.sub.4] shows about 30% higher thermal expansion than that in LuP[O.sub.4]. The overall thermal expansion behaviors of these two structures are predominantly related to the distortion in the Lu[O.sub.8] unit, inter-cation distances and spatial arrangement of the Lu-O bonds in the structure. Keywords: Thermal expansion, crystal chemistry, zircon, xenotime, phosphates, vanadates

We have measured Raman spectra of the postperovskite (PPv) phase in CaIr[O.sub.3] up to 30 GPa to constrain the Gruneisen parameter ([gamma]). We identified a total of 4 strong modes between 200 and 650 [cm.sup.-1], which is in contrast with the Raman spectra of [Mn.sub.2][O.sub.3] and MgGe[O.sub.3]-PPv where at least nine different modes have been detected. We found no sign of a phase transition in the Raman spectra of PPv CaIr[O.sub.3], which supports the stability of the PPv phase up to 30 GPa and room temperature in CaIr[O.sub.3]. The spectroscopic Gruneisen parameter, [[gamma].sub.sp.0] = 1.66~1.72, constrained from our Raman data, is in excellent agreement with the thermodynamic Gruneisen parameter, [[gamma].sub.th,0] = 1.75 [+ or -] 0.05, calculated from recent XRD measurements (Martin et al. 2007) on CaIr[O.sub.3]-PPv synthesized at high pressure and temperature similar to our starting material. Our result suggests that [[gamma].sub.sp] constrained by Raman measurements provides a reasonable estimate on the [[gamma].sub.th] of the PPv phase in CaIr[O.sub.3]. Keywords: Postperovskite, CaIr[O.sub.3], Gruneisen parameter, Raman sepctroscopy, thermal equation of state

A recent force field model is used in this paper to analyze, with a nano-scale simulation, the structural and the vibrational properties of a rock system under the application of various high pressures. Our results show that this model reproduces accurately the equilibrium structural properties for calcite, gives comparable results for aragonite with the observed data, and predicts the structural, elastic and vibrational properties of the post-aragonite phase. In addition, we show that the transition from the aragonite to the post-aragonite phase appears at a transition pressure of 35.2 GPa. The longitudinal wave velocity increases rapidly by a factor of 1.71. In contrast the shear wave velocity increases very slowly up to the transition pressure, and thereafter increases rapidly, changing by 45% to 80 GPa. In addition, our calculations for the vibrational properties show that with the new potential model presented here, we observe a softening mode in the [GAMMA]-F direction. The phonon frequencies of the post-aragonite phase show more modes than for calcite and aragonite. Keywords: Molecular dynamics, calcite rock, lattice dynamic

The electronic structures and lattice dynamics of pressure-induced complex phase transitions [bcc [right arrow] hR1(110.5[degrees]) [right arrow] distorted-hR1(108.2[degrees]) [right arrow] bcc] in vanadium as a function of pressure up to 400 GPa have been investigated with an ab initio method using density functional perturbation theory (DFPT). At ambient pressure, the soft transverse acoustic phonon mode corresponding to Kohn anomaly appears at a wave vector q = 2kF along [[xi]00] [GAMMA] [right arrow] H high symmetry direction. The nondegenerate transverse acoustic branches [TA.sub.1] on and [TA.sub.2] on show an exceptionally large split at high symmetry point N (0.5 0.5 0.0). The lattice dynamical instability starts at a pressure of 62 GPa (V/[V.sub.0] = 0.78, where [V.sub.0] is experimental volume of bcc-V at ambient conditions), derived by phonon softening that results in phase transition of bcc [right arrow] hR1 (alpha = 110.5[degrees]). At compression around 130 GPa (V/[V.sub.0] = 0.67), the rhombohedral angle of hR1 phase changed to 108.2[degrees], and the electronic structure changed drastically. At even higher pressure, [approximately equal to] 250 GPa (V/[V.sub.0] = 0.57), lattice dynamic calculations show that the bcc structure becomes stable again. ab initio calculation | phonon | structure phase transition

A method for analyzing problems involving defects in lattices is presented. Special attention is paid to problems in which the lattice containing the defect is infinite, and the response in a finite zone adjacent to the defect is nonlinear. It is shown that lattice Green's functions allow one to reduce such problems to algebraic problems whose size is comparable to that of the nonlinear zone. The proposed method is similar to a hybrid finite-boundary element method in which the interior nonlinear region is treated with a finite element method and the exterior linear region is treated with a boundary element method. Method details are explained using an anti-plane deformation model problem involving a cylindrical vacancy. [DOI: 10.1115/1.2710795]

The hexagonal Al-rich high-pressure phases are members of a structural family with the same type of framework composed of double-ribbons of edge-sharing octahedra, but variably occupied trigonal and hexagonal channels. This family includes jaffeite, fluoborite, yeremeyevite, painite, and synthetic Ba indates. This kinship broadens the spectrum of elements potentially accommodated by these phases in the mantle. Keywords: Al-rich high-pressure phases, jaffeite, fluoborite, painite, framework of octahedra

A lattice-automaton model is used to investigate the short-term behavior of conservative transient racers. Results suggest that at least nine mixing events are required before the biodiffusion model becomes an appropriate description of bioturbation.