Single-crystal X-ray diffraction data have been obtained for synthetic Fe-Mg silicate spinels, gamma-(MgxFe1-x)2SiO4 (x = 1.00, 0.40, and 0.20). The measurements on gamma-Mg2SiO4 synthesized at 20 GPa and 1400-degrees-C, compared with previous data on a specimen synthesized at 22 GPa and 1000-degrees-C, provide evidence that approximately 4% of Si(tot), enters octahedral coordination in the sample synthesized at higher temperature. Fe-bearing silicate spinels synthesized at lower pressures display no evidence for Fe-Si disorder.

Structural and volume compressibility data for reedmergnerite, NaBSi3O8, were obtained by single-crystal X-ray diffraction at pressures up to 4.7 GPa. The bulk modulus was determined to be 69.8(5) GPa with the pressure derivative constrained to 4. Unit-cell compression is anisotropic, as indicated by unit strain tensors. Tetrahedral bond lengths and angles remained relatively constant over the pressure interval, whereas Na-O bonds decreased systematically. T-O-T angles underwent a variety of behaviors, remaining constant or decreasing with pressure.

Crystals of BaSi4O9 synthesized at 4 GPa and 1000 degrees C were determined to be isostructural with barium tetragermanate [trigonal, space group P3, a =11.2469(11) and c = 4.4851(6) Angstrom, V = 491.3(1) Angstrom(3)]. The structure (R = 2.4%) features a corner-linked framework of three-member silicate tetrahedral rings, which are cross-linked by isolated silicate octahedra. Ba cat-ions occupy tenfold-coordinated sites in channels defined by the silicate framework. This structure is 4.2% denser than the topologically similar benitoite form of high-pressure BaSi4O9, which was produced by grinding the barium tetragermanate-type crystals described in this report.

Structural and volume compressibility data for low albite were obtained by single-crystal X-ray diffraction methods at pressures up to approximately 4 GPa. The bulk modulus was determined to be 54(1) GPa, with a pressure derivative of 6(1). Unit cell compression is anisotropic, as indicated by unit strain tensors. In the softest direction, approximately perpendicular to (100), the structure is three times more compressible than in the stiffest direction. Intensity data were collected, and structures were refined at 0.00, 0.44, 1.22, 2.68, and 3.78 GPa. With increasing pressure, (1) the volumes of the TO4 tetrahedra do not vary, (2) the volume of the NaO7 polyhedron varies linearly with the volume of the unit cell, and (3) Si-O-Si angles increase or remain constant, but only Al-O-Si angles decrease, which is consistent with the smaller force constant of the Al-O-Si vs. Si-O-Si angle. We conclude that compression is accomplished through the bending of Al-O-Si angles, which squeezes together the chains of four-membered rings that run parallel to [001] and that are separated by zigzag channels containing Na atoms. The feldspar three-dimensional tetrahedral framework can be considered to be made up of these chains, which are linked together by O(c)-type atoms. The average value of the T-O(c)-T angle correlates with bulk moduli of alkali feldspars. Al-Si disorder tends to stiffen the T-O(c)-T angle in high albite, which in turn decreases the compressibility and thus can serve as a mechanism for pressure-dependent ordering of high to low albite.

A single crystal of intermediate orthopyroxene, (Mg0.56Fe0.44)2Si2O6, has been recovered from a synthesis experiment at approximately 1.1 GPa and 1600-degrees-C. This rapidly quenched crystal displays a high degree of disorder for orthopyroxene M1 and M2 octahedral sites (K(d) = 3.9). Comparison with low-pressure orthopyroxene quenched from similar temperatures indicates that pressures corresponding to the Earth's mantle transition zone have little effect on Mg-Fe ordering between M1 and M2, unlike intracrystalline ordering in several other dense magnesium iron silicates.

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.