Single-crystal X-ray diffraction data have been obtained at several pressures to 5.6 GPa for synthetic LiScSiO4 olivine. The bulk modulus is 118 +/- 1 GPa, assuming K' = 4. This value is smaller than that of forsterite because compressibility of Li-O bonds in the M1 octahedral site is approximately twice that of the M1(2+)-O bonds in other isomorphs. Compressibilities of a, b, and c orthorhombic axes are 2.70, 2.80, and 2.61 (all x 10(-3) GPa(-1)), respectively. This nearly isotropic compression (axial compression ratios = 1.00: 1.04:0.97) contrasts with that of forsterite (1.00:1.99:1.55), fayalite (1.00:2.83:1.22), monticellite (1.00:1.85 :1.10), and chrysoberyl (1.00:1.30:1.17). These differences arise from the distinctive distribution of cations of different valences and consequent differences in M1-0 and M2-O Oond compressibilities. The Li M1 octahedron displays a significant decrease in polyhedral distortions with pressure, a behavior not observed in other olivines.

The crystal structure of (Mg1.54Li0.23Sc0.23)Si2O6 protopyroxene has been studied with single-crystal X-ray diffraction at pressures to 9.98 GPa and Raman spectroscopy to 10.4 GPa. A first-order displacive phase transformation from the Pbcn space group to P2(1)cn was observed between 2.03 and 2.50 GPa, which is characterized by a discontinuous decrease in a, c, and V by 1.1, 2.4, and 2.6%, respectively, and an increase in b by 0.9%, along with appearance of intensities of some 0kl reflections with k not equal 2n. This is the first substantiated example of protopyroxene having the symmetry predicted by Thompson (1970). Evidence for the phase transition from Raman spectroscopy is also presented. The prominent structural changes associated with the Pbcn-to-P2(1)cn transformation involve the abrupt splitting of one type of O-rotated silicate chain in low-pressure protopyroxene into S-rotated A and O-rotated B chains in high-pressure protopyroxene, coupled with a marked decrease in the O3-O3-O3 angles and a re-configuration of O atoms around the M2 site. The kinking angle of the silicate chain in the low-pressure phase at 2.03 Cpa is 165.9 degrees, whereas the angles are 147.9 degrees and 153.9 degrees for the A and B chains, respectively, in high-pressure phase at 2.50 GPa. Strikingly, the two types of silicate chains in the P2(1)cn structure alternate along the b axis in a tetrahedral layer parallel to (100). Such a mixed arrangement of two differently rotated silicate chains in a tetrahedral layer has not been observed in any other pyroxene structure. Compression anisotropy of the protopyroxene structure is affected by the phase transition. The relative linear compressibilities (beta(a):beta(b):beta(c)) are 1.00:1.72:0.99 for low-pressure protopyroxene, but are 1.00:1.28:1.65 for high-pressure protopyroxene. The bulk moduli of low- and high-pressure phases are 130(3) and 111(1) GPa, respectively. This study concludes that the Pbcn-to-P2(1)cn phase transition results from the differential compression between SiO4 tetrahedra and MO6 octahedra.

The crystal structure of Fe3+-wadsleyite, (Fe1.672+Fe0.333+)(Fe0.333+Si0.67)O-4, was determined by single-crystal X-ray techniques at six pressures to 8.95 GPa. The isothermal bulk modulus is K-m = 173(3) GPa [K-tau 0(1)= partial derivative K-T \partial derivative P = 5.2(9)], which is identical within error to bulk moduli observed for normal wadsleyites [beta-(Mg,Fe)(2)SiO4]. Compression of Fe3+-wadsleyite is significantly more isotropic than for beta-(Mg,Fe)(2)SiO4 because Fe3+ substitutes into both Si4+ tetrahedral sites and (Mg,Fe2+) octahedral sites. Ferric iron thus reduces the contrast between tetrahedral and octahedral compressibilities, which in turn reduces the compressional anisotropy. Bond distance analysis and octahedral compressibilities of the three symmetrically distinct octahedral sites reveal th;lt Fe3+ orders preferentially into M1 and M3, while M2 occupancy is close to pure Fe2+.

Single crystals of five wadsleyite compositions, beta-(Mg,Fe)2SiO4 with Fe/(Fe+Mg)=0.00, 0.08, 0.16, 0.25 and 0.40, have been synthesized at high temperature and pressure in a uniaxial, split-sphere apparatus. Crystal structures of these samples, determined by x-ray diffraction techniques, reveal that iron is significantly ordered: Fe is depleted in the M2 octahedron, while it is enriched in M1 and M3. The most iron-rich synthetic sample, which falls well outside previous estimates of wadsleyite stability, raises questions regarding published Mg2SiO4-Fe2SiO4 phase diagrams at transition zone conditions.

The unique cation-disordered crystal structures of two samples of phase E, a non-stoichiometric, hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions above 13 GPa and 1000-degrees-C, have been solved and refined in space group R3mBAR. The compositions and unit cells for the two materials, assuming six oxygens per cell, are Mg2.08Si1.16H3.20O6, a = 2.9701(1) angstrom, c = 13.882(1) angstrom, V = 106.05(4) angstrom3 for sample 1, and Mg2.17Si1.01H3.62O6, a = 2.9853(6) angstrom, c = 13.9482(7) angstrom, V = 107.65(4) angstrom3 for sample 2. The structure contains layers with many features of brucite-type units, with the layers stacked in a rhombohedral arrangement. The layers are cross linked by silicon in tetrahedral coordination and magnesium in octahedral coordination, as well as hydrogen bonds. Interlayer octahedra share edges with intralayer octahedra. Interlayer tetrahedra would share faces with intralayer octahedra. To avoid this situation, there are vacancies within the layers. There is, however, no long-range order in the occupation of these sites, as indicated by the lack of a superstructure. Selected-area electron diffraction patterns show walls of diffuse intensity similar in geometry and magnitude to those observed in short-range-ordered alloys and Hagg phases. Phase E thus appears to represent a new class of disordered silicates, which may be thermodynamically metastable.

The unit-cell dimensions and crystal structure of sillimanite at various pressures up to 5.29 GPa have been refined from single-crystal X-ray diffraction data. As pressure increases, a and b decrease linearly, whereas c decreases nonlinearly with a slightly positive curvature. The axial compression ratios at room pressure are beta(a):beta(b):beta(c) = 1.22:1.63:1.00. Sillimanite exhibits the least compressibility along c, but the least thermal expansivity along a (Skinner et al. 1961; Winter and Ghose 1979). The bulk modulus of sillimanite is 171(1) GPa with K' = 4 (3), larger than that of andalusite (151 GPa), but smaller than that of kyanite (193 GPa). The bulk moduli of the [Al1O(6)], [Al2O4], and [SiO4] polyhedra are 162(8), 269(33), and 367(89) GPa, respectively. Comparison of high-pressure data for Al2SiO5 polymorphs reveals that the [SiO4] tetrahedra are the most rigid units in all these polymorphic structures, whereas the [AlO6] octahedra are most compressible. Furthermore, [AlO6] octahedral compressibilities decrease from kyanite to sillimanite, to andalusite, the same order as their bulk moduli, suggesting that [AlO6] octahedra control the compression of the Al2SiO5 polymorphs. The compression of the [Al1O(6)] octahedron in sillimanite is anisotropic with the longest AII-OD bond shortening by similar to 1.9% between room pressure and 5.29 GPa and the shortest AII-OB bond by only 0.3%. The compression anisotropy of sillimanite is primarily a consequence of its topological anisotropy coupled with the compression anisotropy of the Al-O bonds within the [Al1O(6)] octahedron.

Crystal structures of HgBa2CaCu2O6+delta (Hg-1212) and HgBa2Ca2Cu3O8+delta (Hg-1223) have been determined on single crystals at room temperature conditions by X-ray diffraction techniques. Both tetragonal specimens are isomorphous with compounds in the Tl-Ba-Ca-Cu-O system. We observe excess oxygen in both samples on two sites near the z = 0 plane: at (1/2 00) and close to (1/2, 0.4, 0) - but no oxygen at (1/2 1/2 0) as reported in earlier studies. Refined Hg occupancies for both samples indicate a substitution of Cu for Hg of more than ten percent. If the substitution of Cu for Hg and excess 0 at (1/2 00) are coupled, then the effect is to insert a perovskite-like six-coordinated copper site at (000). No evidence was found for a split atom at the barium sites in these compounds.