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.

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.