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Hydrogen incorporation in aluminous orthopyroxene may control the generation of melt and dominate the seismic properties at the base of the Earth's lithosphere. To clarify the substitution mechanism of H, we have synthesized, characterized, and refined the crystal structure of this potentially significant variant of orthopyroxene. The experimentally produced crystals ate small needles up to approximately 20 x 20 x 100 [micro]m in size. Electron microprobe chemical analysis indicates about 11.7 wt% [Al.sub.2][O.sub.3]. FTIR spectra indicate 7500 ppmw [H.sub.2]O with absorbance features qualitatively similar to natural mantle orthopyroxenes. TEM imagery indicates that the phase is pure orthopyroxene with low concentrations of defects and inclusions. Cell-parameter refinement from single-crystal X-ray diffraction gives a = 18.1876(7) [Angstrom]; b = 8.7352(7) [Angstrom]; c - 5.1789 (5) [Angstrom], V- 822.79(11) [[Angstrom].sup.3], which is 1.2% smaller than pure Mg anhydrous orthoenstatite. The crystal structure has been refined from single-crystal X-ray intensity data measured using a rotating anode X-ray generator, micro-focused X-ray beam, and CCD detector system. The refined structure indicates about 5% vacancy in M2 and significant Al occupancy in both M1 and T2, consistent with its composition, [([Mg.sub.095,0.05]).sup.M2], [([Mg.sub.0.79][Al.sub.0.21]).sup.M1], [([Al.sub.0.25][Si.sub.0.75]).sup.T2] [Si.stup.T1] [O.sub.6]. The existence of hydrous orthopyroxene in the mantle could absorb water released from olivine on decompression to delay the onset of melting in the spinel stability region in mantle peridotite compositions. Keywords: Orthopyroxene, crystal synthesis, high-pressure studies, X-ray data, crystal structure

Raman spectra in the range 80 to 4000 [cm.sup.-1] of wadsleyite II ([Fo.sub.90] with 2.0 wt% [H.sub.2]O and [Fo.sub.88] with 2.7 wt% [H.sub.2]O) have been measured in a diamond-anvil cell with solid rare-gas pressure-transmitting media to 51.4 GPa at room temperature. The ambient Raman spectrum of wadsleyite II is closely similar to wadsleyite modified with bands in frequency regions where the Si[O.sub.4] tetrahedral and OH stretching vibrations of hydrous ringwoodite occur. The most intense, characteristic wadsleyite II modes at 709 and 911 [cm.sup.-1] ([Si.sub.2][O.sub.7] and Si[O.sub.3] symmetric stretching vibrations, respectively) shift continuously to 51.4 GPa showing no evidence for a change in the crystal structure. A striking feature in the high-pressure Raman spectra of wadsleyite II is a significant growth in intensity in the mid-frequency range (300-650 [cm.sup.-1] at [10.sup.-4] GPa and 400-750 [cm.sup.-1] at 51.4 GPa) under compression accompanied by the appearance of new Raman modes near 40 GPa, perhaps a result of resonance electronic Raman scattering. In the OH stretching frequency range, the Raman spectrum of wadsleyite II exhibits at least six modes and their high-pressure behavior agrees with that of [Fo.sub.90] hydrous wadsleyite: OH stretching modes above 3530 [cm.sup.-1] remain approximately constant up to at least 21.8 GPa whereas OH modes at frequencies Keywords: Hydrous wadsleyite II, Raman spectroscopy, diamond-anvil cell, transition zone, high pressure

The single-crystal elastic properties of phase A have been investigated by Brillouin spectroscopy at ambient conditions. The Voigt-Reuss-Hill average for the adiabatic bulk and shear moduli are [K.sub.s] = 106(1) GPa and [mu] = 61(1) GPa, respectively. The present acoustic measurements resolve discrepancies between the bulk moduli obtained in earlier compression studies. The axial compressibility of the hexagonal (P[6.sub.3]) structure is highly anisotropic with the c-axis, which is perpendicular to the distorted close-packed layers, approximately 21% stiffer than the a-axis, in agreement with previous compression studies. The hydration of forsterite to phase A decreases the bulk and shear moduli by about 18 and 21%, respectively, while both compressional [V.sub.p] and shear [V.sub.s] wave velocities decrease by about 7%. These results suggest that water could be identified seismologically if phase A is present in abundance in cold subducted slabs. Keywords: Single-crystal elasticity, phase A, Brillouin spectroscopy, water in subduction zones

High pressure optical and near infrared spectra of a single crystal of ringwoodite with composition [([Mg.sub.0.90][Fe.sub.0.10]).sub.2]Si [O.sub.4] were measured to 21.5 GPa. The spectrum at ambient pressure shows a strong band at 12 265 [cm.sup.-1] with two shoulders at 8678 [cm.sup.-1] and 17 482 [cm.sup.-1]. The bands at 12 265 [cm.sup.-1] and at 8678 [cm.sup.-1] are due to spin-allowed crystal field transitions of octahedral [Fe.sup.2+], while the band at 17 482 [cm.sup.-1] is most likely due to [Fe.sup.2+] [right arrow] [Fe.sup.3+] charge transfer. The absorption edge due to ligand-to-metal charge transfer occurs close to 30 000 [cm.sup.-1]. With increasing pressure, both the crystal field and the charge transfer bands shift to higher frequencies. Whereas this is expected for the crystal field bands, this blue shift is surprising for an intervalence charge transfer band. Moreover, neither the crystal field nor the charge transfer bands broaden or intensify significantly with pressure. These results have major implications for radiative heat transfer in the Earth's mantle. It has commonly been assumed that radiative heat transfer is blocked in the mantle, because it was believed that the red shift and the increased intensity of charge transfer bands with pressure would effectively make mantle minerals opaque throughout the near infrared and visible range. Our results demonstrate that this effect does not occur for ringwoodite with a Mg/Fe ratio realistic for the Earth's mantle. Quite to the contrary, the mean free path of photons in ringwoodite actually increases with pressure, because the absorption bands move away from the maximum of the blackbody radiation.

The survival of coesite in ultrahigh-pressure (UHP) rocks is most commonly attributed to rapid exhumation, continuous cooling during uplift, and inclusion in strong phases that can sustain a high internal over-pressure during decompression. Exceptions to all of these criteria exist. Perhaps less attention has been paid to the role of fluid infiltration in the preservation of coesite. We used infrared spectroscopy to measure water contents of coesite and coesite pseudomorphs in a variety of UHP rocks. In all cases, OH concentrations in coesite are below the detection limit of ~100 ppm [H.sub.2]O. The silica phases surrounding coesite, however, show varying amounts of [H.sub.2]O. This is most spectacularly observed in pyrope quartzites from the Dora-Maira massif that contain at least three phases of silica replacing coesite, also distinguished by varying color of cathodoluminescence (CL): palisade-textured quartz (< 100 ppm [H.sub.2]O, red-violet CL); 'mosaic' quartz, which is actually chalcedony (up to 0.4 wt% [H.sub.2]O, yellow/brown CL); and a rare, highly hydrated silica phase interpreted to be opal (~7 wt% [H.sub.2]O, dark blue CL). Very similar signatures are observed in a grospydite xenolith from the Roberts Victor kimberlite. The quartz replacing coesite in other UHP samples studied contains on the order of 500 ppm [H.sub.2]O or less, and most measurements are under the detection limit of our technique. We infer that palisade quartz forms under dry or nearly dry conditions and at high temperatures during dilation of the host phase. The formation of hydrous silica phases such as chalcedony and opal, however, must take place at much lower temperatures, after cracking of the host phase, which allows external fluids to infiltrate. Delay of fluid infiltration to low temperatures, where kinetics are slow even in the presence of water, is the most critical factor in the preservation of coesite.

Seven separate samples of hydrous ringwoodite with compositions ranging from F[O.sub.100] to F[O.sub.89] and hydrogen contents from 0.2 to 1.1 wt% were synthesized in the 5000 ton multi-anvil press at the Bayerisches Geoinstitut. Synthesis conditions ranged from 18 to 22 GPa and 1400 to 1500 [degrees]C. The crystals were characterized by single-crystal X-ray diffraction, electron microprobe, IR and Mossbauer spectroscopy, and by analytical and high-resolution transmission electron microscopy. The crystals are optically isotropic, and the Fe-bearing samples are deep blue in color. Mossbauer spectroscopy and ELNE spectroscopy applied to the Fe-bearing samples indicates about 10% of the iron is in the ferric state. High-resolution TEM examination of one of the Fe-bearing samples indicates that the crystals are homogeneous and free of significant inclusions or exsolution features. Infrared spectra show a broad absorption band extending from about 2500 to 3600 [cm.sup.-1] with maxima ranging from 3105 for the pure magnesian samples to 3150 [cm.sup.-1] for the F[O.sub.89] samples. The crystal structures of the seven ringwoodite samples were refined by X-ray single-crystal diffraction. Refinement of cation site occupancies indicates full occupancy of the tetrahedral site for all samples, whereas the occupancy of the octahedral site appears to decrease systematically with H content. The principal hydration mechanism involves octahedral cation vacancies. The IR spectra are consistent with protonation of the short O-O approach on the tetrahedral edge, which would imply partial Mg-Si disorder.

Wadsleyite [[Beta]-[(Mg,Fe).sub.2]Si[O.sub.4]] is known to accept variable amounts of H and may be an important reservoir of H in the transition zone of the mantle. The crystal structure of [Fo.sub.94.6] hydrous wadsleyite ([Mg.sub.1.730][Fe.sub.0.098] [Al.sub.0.008][Si.sub.0.0991][H.sub.0.355][O.sub.4]) synthesized at 1400 [degrees] C and 17 GPa was refined in space group I2/m from 1784 measured intensities of which 830 were unique, and, of these, 650 were of intensity greater than 4[Sigma]. Unit cell parameters are a = 5.6715(7), b = 11.582(2), c = 8.258(1) [Angstrom], and [Beta] = 90.397(9) [degrees]. The final R(F) for all reflections was 0.026 ([R.sub.w] = 0.024); goodness of fit was 1.68. An H position was located on the nonsilicate O atom (O1). Partial occupancy of a normally vacant tetrahedral site adjacent to M3 was observed. This is postulated to be the result of Si moving from M3 to the adjacent tetrahedral void on decompression. Deviation from orthorhombic symmetry appears to result from ordering of H, (Mg, Fe), and possibly Si, within the two nonequivalent M3 sites.

We report a structure refinement ofa [P2.sub.1]/m ferromagnesian cummingtonite crystal with Fe/(Mg + Fe) = 0.37 at 140 K; this crystal was inverted from the C2/m structure, which is stable at room temperature. The phase-transition temperature is [approximately]240 K. The most significant structural changes associated with the C2/m-to-[P2.sub.1]/m phase transition are as follows: (1) one crystallographically independent O-rotated silicate chain becomes two, with the A chain S rotated and the B chain O rotated; (2) the M4 site collapses, with a marked decrease (0.020 [Angstrom]) in the average M4-O6 bond distance; and (3) the largest decrease in the isotropic displacement factor is for the M4 cation. From 295 to 140 K, total changes of 14 and 11 [degrees] occur in the O5-O6-O5 kinking angles of the A and B chains, respectively. The volume of the M1 octahedron decreases more than that of the other three M[O.sub.6] polyhedra, whereas the volume of the M3 octahedron is essentially unchanged. All four M[O.sub.6] polyhedra become more regular as a result of the C2/m-to-[P2.sub.1]/m transformation. These results provide new evidence that the effective size of the M4 cation controls the relative stability of the C2/m and [P2.sub.1]/m structures. The results also provide a new explanation for the unresolved broadening of the cummingtonite Raman spectra measured at low temperatures by Wang et al. (1988).

The intrinsic anharmonicity plays an important role in the thermodynamic properties of minerals at the high-temperature conditions of the mantle. To investigate the effect of iron on the thermodynamic properties of olivine, the most abundant mineral in the upper mantle, we collected in situ high-temperature and high-pressure Raman spectra of natural Fo89Fa11 and synthetic Fo58Fa42 samples. Fo58Fa42 dissociates to enstatite + quartz + Fe2O3(+Fe) at 893 K. All the Raman-active modes systematically shift to lower frequencies at elevated temperatures, whereas to higher frequencies with increasing pressure. The Ag mode at ~960 cm-1 is more sensitive to the variations of temperature and pressure than other internal modes. The crystal-field splitting of the vibrational energy states becomes slightly weakened at high temperatures but strengthened at elevated pressures. We calculated the isobaric (γiP) and isothermal (γiT) mode Grüneisen parameters for these olivine samples. The intrinsic anharmonic parameters (ai) are negative for both the lattice and internal vibrations, and our calculations indicate that the intrinsic anharmonicity makes positive contributions to the thermodynamic properties of olivine at high temperatures, such as the internal energy (U), heat capacities (CV and CP), and entropy (S). Iron incorporation further increases the magnitudes of these anharmonic contributions. In addition, the Fe effect on the intrinsic anharmonicity may also apply to other thermodynamic properties in olivine, such as equations of state and equilibrium isotopic fractionations, which are important in constraining physical and chemical properties of the upper mantle. [ABSTRACT FROM AUTHOR]

The structure of rutile-type compounds shows no substantial changes with the addition of H, as proved by the comparative analysis of the detailed X-ray structures of a synthetic anhydrous crystal and a hydrous rutile crystal. The structures of the two types of synthetic anhydrous and hydrous rutile crystals were based on the single-crystal x-ray data. The a/c ratios and the O position were also nearly the same for both the structures.

A hypothetical, ordered model of hydrous wadsleyite has the formula Mg7Si4O14(OH)2 and belongs to a space group Pmmb. Nonequivalent O1 sites have two H atoms. IR and Raman O-H stretching absorptions exhibit strong C-axis polarization. Hydration relieves the underbonding of O1 and overbonding of bridging O2. The model predicts greater distortion of silicate tetrahedra on partial hydration, while full hydration of O1 decreases the distortion. Trivalent octahedral cations Fe3+, Cr3+ and Al3+ exhibit higher solubility in wadsleyite than in olivine under hydrous conditions. The model prediction of maximum water content suggests that several oceans of H2O may be present in the transition zone of the mantle.

Three-dimensional, single-crystal X-ray diffraction data is used to define the crystal structures of a synthetic H-bearing aluminous stishovite containing nearly 1.6 weight% of Al2O3, and a pure-silica stishovite. Analysis of the results of the experiments verify that Al is interchangeable with Si in octahedral sites in conditions simulating the conditions of earth's mantle.

Measurements of hysteresis loops were performed on thin permalloy rectangular platelets with various dimensions prepared by nanolithography. The particle coercivity, Hc, exhibits a sensitive size dependence. The measured Hc are compared with micromagnetic predictions utilizing two-dimensional models without magnetization variation through the thickness. Excellent overall agreement is found, indicating that a simplified two-dimensional approach is sufficient for characterizing reversal mechanisms for particles with thicknesses smaller than or comparable to the exchange length.