Single crystals of an Fe-bearing high-pressure magnesium silicate "anhydrous phase B," (Mg0.88Fe0.12)14Si5O24, have been synthesized at 15 GPa and 1800-degrees-C. A remarkable feature of this Fe-bearing variant is the significant partitioning of Fe into one of the six symmetrically distinct octahedral sites-a degree of Fe-Mg ordering not observed in any other rock-forming silicate quenched from very high temperature. The presence of Fe, as well as other octahedral cations such as Ca, Mn, or Al, may, therefore, expand the stability of anhydrous phase B into a pressure-temperature regime characteristic of the Earth's transition zone.

Single crystals of Ca-bearing majorite, a garnet with composition (Ca0.49Mg2.51)(MgSi)Si3O12, have been synthesized at 18.2 GPa and 2050-degrees-C. This sample is the first silicate garnet to display ordering on both octahedral and dodecahedral sites-behavior that may increase the compositional flexibility of garnet, affect clement partitioning at high pressure, and stabilize the garnet structure in the transition zone and upper portion of the lower mantle. The garnet is tetragonal [space group I4(1)/a, Z = 8, a = 11.5816(9), c = 11.5288(13) angstrom, V = 1546.39(29) angstrom3] and, like MgSiO3 majorite, displays twinning by twofold rotation about [110].

The crystal structures of two high-pressure magnesium silicates with mixed four- and six-coordinated Si have been solved and refined. The crystals were synthesized in the splitsphere, cubic anvil apparatus (USSA-2000) located at the Stony Brook High-Pressure Laboratory. Phase B, Mg12Si4O19(OH)2, crystallizes in space group P2(1)/c with 40 atoms in the asymmetric unit. Phase AnhB, Mg14Si5O24, occurs in space group Pmcb with 18 atoms per asymmetric unit. Final values of the standard R-factors were 0.056 and 0.029 for the observed reflections of phases B and AnhB, respectively. Both structures contain two types of layers, one with edge-shared Mg and Si octahedra, and the other with Mg octahedra and Si tetrahedra and the stoichiometry of humite for phase B, and forsterite for phase AnhB. Each octahedral layer is flanked by two of the tetrahedral layers, with a total of six layers per unit cell. Each Si octahedron shares all 12 edges with Mg octahedra, forming a cluster with 13 octahedral cations. This dense packing contributes to the relatively high zero-pressure densities of 3.368 and 3.435 g cm-3 for phases B and AnhB, respectively. This study also demonstrates that high-pressure materials do not always have simple structures.

Single crystals of lead aluminosilicate hollandite, with composition Pb0.8Al1.6Si2.4O8, have been synthesized at 16.5 GPa and 1450 degrees C. Crystals are tetragonal [space group I4, Z = 2, a = 9.414(3), c = 2.750(3) Angstrom, V = 243.7(3) Angstrom(3)]. Si and Al are disordered on the octahedral site. Pb is best modeled by two atom sites, one on the fourfold axis and the other split, lying off the fourfold axis. This feature was predicted by Post and Burnham (1986) but not previously observed.

A high-pressure single-crystal XRD study of KAlSi3O8 with the tetragonal hollandite structure has been completed to 4.47 GPa. The a axis is approximately twice as compressible as c, so c/a increases with pressure. This anisotropy is similar to that of the structurally related mineral stishovite, though KAlSi3O8 hollandite is approximately 53% more compressible along both axes. The relative incompressibility of the c axis can be explained by the strong cation to cation repulsive forces across the shared octahedral edge in the double chain; Si(Al)-O bonds perpendicular to c are more compressible than those in other directions. P-V data give an isothermal bulk modulus of 180(3) GPa using a Birch-Murnaghan equation of state with K(T)' = 4.0 and constraint of V0. The polyhedral bulk modulus of the Si(Al)O6 octahedron is 153(9) GPa, the smallest among rutile-related oxides. The KO8 tetragonal prism has a polyhedral bulk modulus of 181(43) GPa, which is unusually large for an alkali cation site. The volume of the K coordination polyhedron is constrained by the rigid tetragonal octahedral framework, so the bulk modulus is expected to be independent of the size and charge of the central cation.

High-pressure crystal structures are reported for two synthetic wadsleyite: crystals, beta-Mg2SiO4 (Fe00) and beta-(Mg0.75Fe0.25)(2)SiO4 (Fe25), at six pressures to 10.12 GPa. In both compositions, bulk compressibilities are equal to the average compressibility of divalent cation octahedra. Individual silicate tetrahedra, by contrast, are relatively rigid, though the Si-O-Si angle between tetrahedra in Si2O7 dimers decreases systematically with pressure. Wadsleyites display anisotropic compression. with the c axis approximately 40% more compressible than a or b. This behavior results from differential compression of (Mg,Fe)-O bonds; in each of the structure's three symmetrically independent octahedra, the longest and most compressible bonds are roughly parallel to the b axis. Although the linear compressibilities of Fe00 and Fe25 are similar, details of structural changes with pressure differ. Iron-enriched M1 and M3 octahedral sites in Fe25 are significantly less compressible than analogous Mg sites in Fe00.

The structural changes associated with the C2/m-P2(1)/m phase transition in cumming-tonite with (Fe + Mn)/(Fe + Mn + Mg) approximate to 0.50 have been studied with single-crystal Xray diffraction at various pressures up to 7.90 GPa and infrared spectroscopy up to 8.63 Cpa. With increasing pressure, the crystal transforms from C2/m to P2(1)/m symmetry at similar to 1.21 GPa, as determined by the appearance of reflections violating the C2/m space group. Infrared spectra provide additional evidence for the phase transition: A distinct splitting of OH stretching bands results from an increase from one to two nonequivalent OH positions. The C2/m-P2(1)/m transition is of weakly displacive first-order or tricritical character with apparent slope changes in the plots of the axial ratios alb and nle as a function of pressure. The unit-cell compression is considerably anisotropic with the a dimension in both C2/m and P2(1)/m phases being the most compressible. Major structural changes for the C2/m-P2(1)/m transition include: (1) One crystallographically distinct silicate chain becomes two discontinuously, coupled by the splitting of the M4-O5 bond, as well as M4-O6, into two nonequivalent bonds, and (2) the M4-cation coordination increases from sixfold to sevenfold. More importantly, we observed a change in the sense of rotation for the A chain while the crystal structure maintains P2(1)/m symmetry: It is O rotated, as the B chain, at 1.32 Gpa, but S-rotated at 2.97 GPa and higher pressures. As pressure increases from 1.32 to 7.90 Gpa, there is a switching of the nearest bridging O atoms coordinated with the M4 cation: The M4-O5B distance contracts from 2.944 to 2.551 Angstrom, whereas the M4-O6B distance increases from 2.754 to 2.903 Angstrom. Compression mechanisms for the low- and high-pressure polymorphs appear to be slightly different. In the C2/m phase, the behavior of the A and M4 sites controls the compression of the structure, whereas the response of the M1, M2, and M3 octahedra to pressure also plays a role in determining the compression of the P2(1)/m structure. The phase transition is regarded as primarily driven by the differential compression between the M4 and T sites, and the symmetry breaking provides a necessary tighter coordination for the M4 site, Based on our data, the obvious changes in the hyperfine parameters of Fe-57 in grunerite between 1.0 and 3.4 GPa, observed by Zhang and Hafner (1992), are likely to result from the C2/m-P2(1)/m structural transformation.