摘要:

AbstractHigh-resolution B K-edge X-ray absorption near-edge structure (XANES) spectra of K2O-SiO2-B2O3-P2O5glasses are reported using synchrotron radiation. Two prominent features, peak a at about 194.0 eV and peak b at about 198.0 eV, are observed. On the basis of the qualitative MO diagrams of BO33-and BO43-clusters, peak a is assigned to the transition of B 1s electrons to the unoccupied B 2pz(π*) states for threefold-coordinated B ([3]B),and peak b is assigned to the transition of B 1s electrons to the unoccupied B σ* states for fourfold-coordinated B ([4]B).B K-edge XANES spectroscopy is established as a method for "fingerprinting"[3]B and[4]B in borate and borosilicate minerals, glasses, and melts. Also, the relative proportions of [4JBand [3]Bin the borosilicate glasses are determined from the integrated peak areas for[3]B and[4]B edge peaks and are shown to be generally in good agreement with recent11B MAS NMR measurements. However, the surface and near-surface structure of powder particles of per boric glasses containing P2O5appears to have a lower proportion of[3]B entities than the bulk.

ISSN:0003-004X    

DOI:10.2138/am-1995-9-1001

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractRecently, correlations of23Na chemical shifts with structural parameters such as bond length, coordination number, and bond strength in inorganic compounds have been proposed. When considering only silicate materials we have found that some of these models lead to conflicting predictions.To test the applicability of these correlations to silicates, we collected23Na MAS NMR spectra of a pure, ordered albite (NaAlSi3O8) from 25 to 352°C. Over this temperature range, the Na-O bond lengths increase, and the measured isotropic shifts relative to 1 M NaCI decrease linearly from -7.1 ppm at room temperature to -7.8 ppm at 352°C. This agrees with the observed trend in silicates of increasing bond length leading to more negative values of δ but is not consistent with the bond-valence model, which predicts a change of several parts per million in the opposite direction over this temperature range.Quadrupolar coupling constants were also seen to change, decreasing linearly with temperature from 2.69 MHz at 20°C to 2.30 MHz at 352°C. Static spectra measured from 22 to 1020 °C yielded values consistent with this in the low-temperature range and continued to decrease linearly to 1.50 MHz at the highest temperature. The23Na spin-lattice relaxation times (T1) were collected between 25 and 1140 °C as well. T1decreases gradually up to about 500°C and then drops rapidly, indicating that there is a change in the relaxation mechanism at this temperature.

ISSN:0003-004X    

DOI:10.2138/am-1995-9-1002

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractCation ordering and structural changes in synthetic Mg2TiO4and Zn2TiO4spinels at temperatures across the polymorphic transition from the high-temperature cubic (Fd3m) to the low-temperature tetragonal (P4122) structure are examined by17O magic-angle spinning (MAS) NMR (9.4 T) and Rietveld structure refinement of powder X-ray diffraction data. The17O NMR spectra of cubic Mg2TiO4and Zn2TiO4are similar, each showing one broad peak, positioned at 303 and 301 ppm, respectively. At the transition to the tetragonal phase, spectra of both Mg2TiO4and Zn2TiO4show significant narrowing because of the onset of long-range cation ordering in the tetragonal structure. The17O NMR spectrum of tetragonal Zn2TiO4shows two narrow peaks, at 301 and 273 ppm, corresponding to the two crystallographically distinct O sites in the tetragonally distorted spinel, showing that '70 chemical shift is sensitive to octahedral Zn- Ti substitution in Zn2TiO4. In contrast, the17O NMR spectrum of tetragonal Mg2TiO4shows only one peak, at 298 ppm. The structures of cubic and tetragonal Mg2TiO4and Zn2TiO4are compared. Tetragonal Zn2TiO4exhibits greater distortion than Mg2TiO4. at the M1, O1, and O2 sites. These subtle structural differences do not explain differences in the17O NMR spectra.The17O NMR spectra of the cubic Mg2TiO4and Zn2TiO4show no change with quench temperature above the transition to the cubic phase, suggesting that short-range ordering does not occur in cubic Mg2TiO4and Zn2TiO4. A two-phase region is observed for both Mg2TiO4and Zn2TiO4, below 664 and 561°C, respectively, where the cubic and tetragonal phases are shown to be at equilibrium.The17O peak position of MgTiO3is observed at 398 ppm. This chemical-shift displacement of 100 ppm to high frequency of Mg2TiO4is related to increased distortion in MgTiO3.

ISSN:0003-004X    

DOI:10.2138/am-1995-9-1003

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractTransmission electron microscopy (TEM) revealed a commensurate modulated structure in a Na-rich alkali feldspar (Ab66Or29An5) from a basalt from Anhui Province, China. High-resolution TEM images with wavelike (001) lattice fringes directly reveal one-dimensional modulation waves in the crystal. The average modulation period along the b axis is 14d010(≅180 Å), although both larger and smaller spacings have been observed. The modulated structure can be described as periodic stacking of (010) layer domains following the albite twin law. The relationship between the extended cell (supercell) of the modulated structure and the normal alkali feldspar unit cell (subcell) is as follows: asup≅ asub, bsup≅ 14bsub, csup≅ csub, βsup≅ βsub(subscripts "sup" and "sub" indicate the supercell and subcell, respectively). A possible space group for the modulated structure is Pm.In situ heating and cooling experiments suggest that the phase transition in this feldspar thin foil might be a weakly first-order and displacive transformation. The transition temperature is close to 500 °C for the crystal examined. The modulated structure formed during the phase transition of the feldspar from C2/m symmetry to C1̅ at a rapid cooling rate.A heuristic nonlinear dynamics model for the arrangement of triclinic domains in the feldspar at the phase transition from monoclinic to triclinic symmetry was developed on the basis of a Landau potential, a damping term accounting for the structural differences among neighboring units, and a periodic force in the crystal along the b axis. Numerical solutions indicate that periodic modulation arises during the monoclinic to triclinic transition only under special conditions relatively far from equilibrium, and a nonperiodic arrangement of triclinic domains along the b axis is the more general case. Therefore, according to this model, the modulated structure is a metastable and intermediate structure between monoclinic (C2/m) and triclinic (C1̅) phases. This type of modulated structure in Na-rich alkali feldspars may indicate high-temperature growth as a monoclinic crystal, followed by very rapid cooling into the triclinic stability field

ISSN:0003-004X    

DOI:10.2138/am-1995-9-1004

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractDomain textures related to the I1̅-12/c displacive phase transition were observed by transmission electron microscopy (TEM) in synthetic samples from the Sr-rich region of the CaAl2Si2O8-SrAl2Si2O8join. Albite and, more rarely, pericline transformation twins are formed in samples with preexisting polysynthetic Carlsbad twins; they also cut preexisting b-type antiphase domains. Tweed textures associated with albite twin lamellae were observed in triclinic samples and, more rarely, in monoclinic samples close to the transition point. Progressive annealing of a monoclinic disordered precursor was performed to follow the monoclinic-triclinic transition at room temperature. Triclinic and monoclinic phases coexisted close to the transition, whereas tweed textures, rarely present in monoclinic grains, were retained in the triclinic phase after longer annealing. In situ high-temperature TEM experiments performed on a grain with pericline twins showed that the pericline twins disappeared at Tc, but, during cooling, tweed textures appeared and then were replaced by albite twins at lower temperatures. The tweed textures are interpreted as originating from strain modulations induced by the displacive transition rather than modulation in Al-Si order, and they form close to the transition point as precursors of the albite and pericline twinning.

ISSN:0003-004X    

DOI:10.2138/am-1995-9-1005

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractWe report structure refinements of three ferromagnesian cummingtonite crystals with Mg/(Mg + Fe) ≈ 0.63. An unheated crystal with XM4Mg= 0.051 has C2/m symmetry,but crystals heat treated at 600 and 700°C, with XM4Mg= 0.167 and 0.237, respectively, have P21/m symmetry. In P21/m cummingtonite from this study and from Hirschmann et ai. (1994), the A silicate chain is S rotated and the B chain is 0 rotated. This differs from the structure of P2, /m manganoan cummingtonite, in which both A and Bchains are 0 rotated (Papike et aI., 1969). Documented in nature for the first time, the S-rotated silicate chains in amphibole suggest that, like the P21/c to C2/c phase transition in clinopyroxene, the change of the A-chain configuration from S to 0 rotation is intrinsic to the P21/m to C2/m transformation in ferromagnesian cummingtonite.Documentation of different space groups for magnesio-cummingtonite crystals that differ only in site occupancies confirms that the M4-site population controls the relative stabilities of the C2/m and P21/m phases and places limits on the tolerance of the C2/m structure for Mg on the M4 site. At room temperature, ferromagnesian cummingtonite with XM4Mg>0.15 ± 0.02 has P21/m symmetry. In the P21/m structure, the separation between M4-O6A and M4-O6B and between M4-O5A and M4-O5B bond distances increases with increasing Mg content, as does the kinking angle (O5-O6-O5) of the A and B chains. Structural distortions away from C2/m symmetry, particularly in the configuration of the A chain, are greater for Fe-bearing cummingtonite than for Mn-bearing varieties at similar Mg concentration, indicating that for a given Mg content, ferromagnesian amphibole is more stable in the P21/m structure than manganoan amphibole.

ISSN:0003-004X    

DOI:10.2138/am-1995-9-1006

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractTransmission electron microscopy (TEM) of microstructures in the orthopyroxene of the Serra de Magé cumulate eucrite shows augite lamellae thicker than 500 Å, irregularly spaced within the orthopyroxene matrix and parallel to (100), with a*Opx≡ a*Aug, [101]*Opx≡ c*Aug, and b*Opx≡ b*Aug. Streaking along a* owing to stacking disorder was observed in selected-area electron diffraction (SAD) patterns. Slabs of clinopyroxene were observed, but no Guinier-Preston zones were detected. Single-crystal X-ray diffraction (XRD) data were collected and structure refinements completed in space group Pbca, converging to Robs= 2.81 and 5.17% for the two samples. Because of the presence of some Okl forbidden reflections with k = 2n + 1 and l = 4n + 2, structure refinement was attempted in space group P21ca, as suggested by previous studies, and in space groups P121/c1 and P1121/a, as suggested by the Laue symmetry of the reflections, which is 21m rather than mmm. However, the high residual indices R and the anomalous values of many displacement parameters and bond distances did not support the lower-symmetry space groups. The violations of the extinction conditions of the Pbca space group and the monoclinic symmetry of the reflections are due to overlap of reflections from the C2/c augite lamellae. From the intensity of the reflections forbidden in Pbca space group, the calculated augite content ranges from approximately 4 to 12%.

ISSN:0003-004X    

DOI:10.2138/am-1995-9-1007

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractTransmission electron microscope images of the zeolite ferrierite, prepared by ultramicrotomy as cross sections of the main-channel axis, revealed several types of crystallographic defects. Both shear faults and sigma transformations occur in the vug-filling bundles of ferrierite near Kamloops Lake, British Columbia. Sigma contraction {110} planes occur as discrete domains several unit cells thick. Shear faults involving displacements of (a ± b)/2 on {110} planes occur in lesser amounts. In addition, mordenite occurs in the Kamloops Lake sample, underscoring the frequent association of these zeolites, their crystal chemical and structural affinities, and the importance of TEM-scale observations in petrologic studies of fine-scale intergrowths of zeolites.

ISSN:0003-004X    

DOI:10.2138/am-1995-9-1008

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractSingle crystals of lead aluminosilicate hollandite, with composition Pb0.8Al1.6Si2.4O8, have been synthesized at 16.5 GPa and 1450 °C. Crystals are tetragonal [space group I4, Z = 2, a = 9.414(3), c = 2.750(3) Å, V = 243.7(3) Å3]. 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.

ISSN:0003-004X    

DOI:10.2138/am-1995-9-1009

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractLattice parameters of calcite were measured at simultaneous high pressures and temperatures up to 10 kbar and 500°C. Samples of Solnhofen limestone and distilled, deionized water were loaded in a hydrothermal diamond-anvil cell. In situ energy-dispersive X-ray diffraction was used to determine lattice parameters along five H2O isochores with densities of 1.040, 0.964, 0.760, 0.670, and 0.595 g/cm3from 30 to 500°C; these densities correspond to the ice-melting temperature of -7.7 °C and the liquid-vapor homogenization temperatures 92.4,274.5, 318.9, and 344.6 °C, respectively. In addition, data along the P axis were collected by hydrostatic compression at room temperature and along the T axis by heating at atmospheric pressure. Our results show that anisotropic thermal expansion of calcite continues up to 10 kbar, therefore making it a good double internal X-ray standard. Both a and c lattice parameters are fitted to second-order polynomials of pressure, temperature, and a cross term.

ISSN:0003-004X    

DOI:10.2138/am-1995-9-1010

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter