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The mineral woodruffite, Zn2+ (Mn4+1− x Mn3+ x )O2 · y H2O, x ~ 0.4 and y ~ 0.7, is the first known example of a new type of Mn oxide characterized by large tunnels that measure 3 and 4 octahedra (6.9 × 9.2 A) on a side. These tunnels are rectangular in cross-section and are the largest of any yet reported in natural or synthetic Mn oxides. The thermal stability of woodruffite is comparable to that of todorokite and other large-tunnel Mn oxide phases, breaking down at ~300 °C and eventually transforming to a spinel-type structure. The woodruffite structure may serve as a model for a new class of octahedral molecular sieves with enhanced capabilities as catalysts and selective cation-exchange agents.

X-ray intensities for the “tetragonal almandine-pyrope phase” (TAPP), *(FeMg) VIII (AlMgFe) 4 [IV (SiAl)O 4 ] 3 where * refers to a capped tetrahedral site, reported by Harris et al. (1997) were recollected using the identical crystal, and the crystal structure was refined from these new data. The basic structure of the previous report is confirmed; however, some of the details regarding cation occupancies are revised. The chemical composition of this phase corresponds to a high-pressure origin; however, significant questions remain regarding the crystallization pressure of this phase.

Structural and volume compressibility data for reedmergnerite, NaBSi 3 O 8 , 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. The compression for reedmergnerite is similar to that of low-albite, wherein bending of the (Al.B)-Oco-Si angle compresses the Na-bearing zigzag channels. In contrast, micro-cline compresses by shearing the four-membered rings, which in turn compresses the K-bearing channels. At about 4 GPa, a new bond between K and Obm appears that alters the compression mechanism and explains the discontinuity in the pressure variation of crystallographic parameters observed by Allan and Angel (1997). Thus, the compression mechanism of the alkali feldspars is dominated by the compression of alkali containing channels. However, because of low symmetry, this can be accomplished in several ways. The observed variety of compression pathways resulted from T-O-T angle bending energetics that were coupled with the effects of alkali cation bonding.

The crystal structure of (Mg (sub 1.54) Li (sub 0.23) Sc (sub 0.23) )Si 2 O 6 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 Okl reflections with knot =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 GPa 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 SiO 4 tetrahedra and MO 6 octahedra.

Phase egg, first described by Eggleton et al. (1978), was synthesized and its composition determined to be AlSiO3OH. The crystal structure of AlSiO3OH, including the position of the hydrogen, has been solved and refined from high-resolution X-ray powder diffraction. The resulting lattice constants are a 5 7.14409(2) A ˚, b 5 4.33462(1) A ˚, c 5 6.95253(2) A ˚, and b5 98.396(1)8. The space group is P21/n; Z 5 4, V0 5 212.99(1) A ˚ 3 , and the zero

The structural changes associated with the C2/m-P2 1 /m phase transition in cummingtonite with (Fe+Mn)/(Fe+Mn+Mg) nearly equal 0.50 have been studied with single-crystal X-ray diffraction at various pressures up to 7.90 GPa and infrared spectroscopy up to 8.63 GPa. With increasing pressure, the crystal transforms from C2/m to P2 1 /m symmetry at approximately 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 a/b and a/c 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 M4-05 bond, as well as M4-06, 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 Aa, whereas the M4-O6B distance increases from 2.754 to 2.903 Aa. 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 57 Fe 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.

The unit-cell dimensions and crystal structure of kyanite at various pressures up to 4.56 GPa were refined from single-crystal X-ray diffraction data. The bulk modulus is 193(1) GPa, assuming K9 = 4.0. Calculated unit-strain tensors show that kyanite exhibits more isotropic compressibility than andalusite or sillimanite. The most and least compressible directions in the kyanite structure correspond approximately to the most and the least thermally expandable directions. The analysis of the distortion of the closest packing in kyanite indicates that the most compressible direction of the structure (along [012]) corresponds to the direction along which the closest-packed O monolayers are stacked. The bulk moduli for the A11, A12, A13, and A14 octahedra are 274(43), 207(14), 224(26), and 281(24) GPa, respectively, and those for the Si1 and Si2 tetrahedra are 322(80) and 400(95) GPa, respectively. Four A1O 6 octahedra that all become less distorted at higher pressures do not display clearly dominant compression directions. The average unshared O-O distance for each octahedron is considerably more compressible than the shared O-O distance. The high-pressure behaviors of the A11 and A14 octahedra are very similar but different from those of the A12 and A13 octahedra. Bulk moduli for the three Al 2 SiO 5 polymorphs (kyanite, sillimanite, and andalusite), as well as those for the AlO 6 octahedra in their structures, appear to decrease linearly as their volumes increase. The significantly larger bulk modulus and more isotropic compressibility for kyanite than for andalusite or sillimanite are a consequence of the nearly cubic close-packed arrangement of O atoms and the complex edge-sharing among four distinct AlO 6 octahedra in the kyanite structure.

Single-crystal X-ray diffraction data have been obtained at several pressures to 5.6 GPa for synthetic LiScSi04 olivine. The bulk modulus is 118 :t I 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 MP+-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 offorsterite (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 M 1-0 and M2-0 bond compressibilities. The Li M 1 octahedron displays a significant decrease in polyhedral distortions with pressure, a behavior not observed in other olivines.

ABSTRACf Pressure-volume X-ray diffraction data were collected at eight pressures to 17.2 GPa for synthetic single-crystalforsterite, Mg2Si04.Birch-Murnaghanequation-of-state parameters were determined to be Ko = 125(2) GPa and K~ = 4.0(4). To ensure hydrostatic conditions, we monitored the diffraction-peak shapes while the sample was in different pressure media: 4:1 methanol-ethanol, He, and Ar. The diffraction peaks for forsterite in 4:I methanol-ethanol at 12.2 GPa were broadened to such an extent that we believe that any data collected at similar pressures and in this medium may be suspect and should be examined critically. The diffraction-peak profiles for the sample in He were sharp and well-formed, suggestingHe provides an excellent medium at pressures ~ 17.2 GPa. However, upon decompression, the diffraction peaks of the sample in He also showed significant broadening. This broadening suggeststhat with increasing pressure He diffused into the crystal, but with the release of pressure the He was unable to diffuse out quickly. The diffraction peaks regained their sharpness after several months.

Single crystals of lead aluminosilicate hollandite, with composition Pbo.sA11.6Si2.40s, have been synthesized at 16.5 GPa and 1450 0c. Crystals are tetragonal [space group 14, Z = 2, a = 9.414(3), c = 2.750(3) A, V = 243.7(3) A3]. 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.