Physics and Chemistry of Minerals

 Mg-Fe partitioning experiments between (Mg,Fe)2SiO4 spinel and (Mg,Fe)O magnesiowüstite were carried out at pressures of 17–21.3 GPa at temperatures of 1400 and 1600 °C, using a multi-anvil apparatus, in order to determine interaction parameters of spinel and magnesiowüstite solid solutions and also to constrain the equilibrium boundaries of the postspinel transition in the Fe-rich side in the system Mg2SiO4-Fe2SiO4. The obtained values of the interaction parameters were 3.4 ± 1.5 and 13.9 ± 1.4 kJ mol−1, respectively, for spinel and magnesiowüstite solid solutions at 19 GPa and 1600 °C. The partitioning data in the system Mg2SiO4-Fe2SiO4 at 1400 and 1600 °C showed that the transition boundary between spinel and the mixture of magnesiowüstite and stishovite has a negative dP/dT slope. Using the above interaction parameters and available thermodynamic data of the Mg2SiO4 and Fe2SiO4 end members, the transition boundaries of spinel to the mixture of magnesiowüstite and stishovite were calculated. Within the uncertainties of the data used, the calculated boundaries are in good agreement with the boundaries at 1400 and 1600 °C experimentally determined in this study. The dissociation boundary of Fe2SiO4 spinel to wüstite and stishovite, calculated from the thermodynamic data, has a negative slope of −1.5 ± 0.6 MPa K−1.

The 29-Si NMR spectra of natural and synthetic leucites (KAlSi2O6) are found to contain a number of resonances which are interpreted in terms of the known structure of low-temperature (tetragonal) leucite. Computer simulation of the spectra suggests that the most distorted tetrahedral lattice site T 1 contains a higher proportion of Al than the other two tetrahedral sites. The occurrence of some ordering of the tetrahedral Si and Al in leucite is confirmed by Mossbauer studies of synthetic iron-containing leucites, including the fully ferric end-member KFeSi2O6, in which the three tetrahedral sites can be distinguished. On replacement of about half this Fe by Al, the most distorted of these sites is lost from the spectrum, reflecting the preference of Al for this site. A linear relationship is found between the unit cell dimensions of all these leucites and their iron content.

The elastic properties of six silicate garnet end members, among the most important rock-forming minerals, are investigated here for the first time via accurate ab initio theoretical simulations. The Crystal program is used, which works within periodic boundary conditions and allows for all-electron basis sets to be adopted. From the computed elastic tensor, Christoffel’s equation is solved along a set of crystallographic directions in order to fully characterize the seismic wave velocity anisotropy in such materials. Polycrystalline isotropic aggregate elastic properties are derived from the computed single-crystal data via the Voigt-Reuss-Hill averaging procedure. Transferability of the elastic properties from end members to their solid solutions with different chemical compositions is also addressed.

A series of Ti-substituted pyrope crystals was synthesized in the system MgO-(Na2O)-Al2O3-TiO2-SiO2-H2O at PH20 = Ptot between 25 and 30 kbars and 975 and 1000° C, using graphite heated piston-cylinder devices. The crystals, ranging up to 500 μm in diameter, were studied by X-ray, electron-microprobe and FTIR-microscope spectrometric techniques. The pyrope crystals were colourless when hem/mt or mt/wu buffers were used during the synthesis, and pale blue with the wu/iron buffer and in unbuffered runs. Sodium was not found in the synthetic crystals, titanium was always near 0.06 Ti atoms pfu, independent on the Ti-excess used in the starting material. A substitution Al2+[6]+Si4+[4]+4O2-= Ti4+[6]+□[4]+[(OH)3O^5-, providing charge balance for octahedral Ti4+-substitution is found to be compatible with all properties (number, widths, position, integrated intensity) of the stretching vibrations of defect hydroxyls, which have energies 3684, 3568, 3525 cm-1.

The effects of heating and cation exchange on the solid acidity of montmorillonite were investigated using n-butylamine titration in non-aqueous system and diffuse reflectance Fourier transform infrared spectroscopy. The number of total, Brønsted, and Lewis acid sites showed the same modulation tendency with increasing heating temperature, reaching a maximum at 120 °C and subsequently decreasing until it reaches a minimum at 600 °C. The Lewis acid sites result from unsaturated Al3+ cations, and their number increased with the heating temperature due to the dehydration and dehydroxylation of montmorillonite. The generation and evolution of Brønsted acidity were mainly related to interlayer-polarized water molecules. Water adsorbed on the unsaturated Al3+ ions also acted as a Brønsted acid. The acid strength of the Brønsted acid sites was dependent on the polarization ability of the exchangeable cation, the amount of interlayer water, and the degree of dissociation of the interlayer water coordinated to exchangeable cations. All cation-exchanged montmorillonites exhibited different numbers of acid sites and various distributions of acid strength. Brønsted acidity was predominant in Al3+-exchanged montmorillonite, whereas the Na+- and K+-exchanged montmorillonites showed predominantly Lewis acidity. Moreover, Mg2+- and Li+-exchanged montmorillonites exhibited approximately equal numbers of Brønsted and Lewis acid sites. The Brønsted acidity of cation-exchanged montmorillonite was positively correlated with the charge-to-radius ratios of the cations, whereas the Lewis acidity was highly dependent on the electronegativity of the cations. The acid strengths of Al3+- and Mg2+-exchanged montmorillonites were remarkably higher than those of monovalent cation-exchanged montmorillonites, showing the highest acid strength (H 0 ≤ −3.0). Li+- and Na+-exchanged montmorillonites exhibited an acid strength distribution of −3.0 < H 0 ≤ 4.8, with the acid strength ranging primarily from 1.5 to 3.3 in Li+-exchanged montmorillonite, whereas only weaker-strength acid sites (1.5 < H 0 ≤ 4.8) were present in K+-exchanged montmorillonite. The results of the catalysis experiments indicated that montmorillonite promoted the thermal decomposition of the model organic. The catalytic activity showed a positive correlation with the solid acidity of montmorillonite and was affected by cation exchange, which occurs naturally in geological processes.

To investigate the equation of state of α-MnS at high pressure and the possibility of a phase transition, the compression curve was measured at 298 K from 0 to 21 GPa using powder x-ray diffraction with a diamond anvil cell. The compression data are fit to a thirdorder Birch-Murnaghan equation of state, with parameters K 0 = 72(2) GPa and K′ 0 = 4.2(13). To compare present results with previous work, the data sets from three previous investigations (Clendenen and Drickamer 1966; Wakabayashi et al. 1968; Kraft and Greuling 1988) are refit to a Birch-Murnaghan equation of state. In the low pressure region (P < 10 GPa), the results of Clendenen and Drickamer (1966) agree with the present data; however the results of Wakbayashi et al. (1968) differ by more than 10%. A greater discrepancy between the present and previous results occurs above 10 GPa. Kraft and Greuling (1988) reported a structure transition at 7 GPa, and Clendenen and Drickamer (1966) observed a structure distortion at approximately 10 GPa; the present data show no evidence of either transition, and are well fit by a single equation of state from 0 to 21 GPa. Nonhydrostatic stress is discussed as one possibility for the discrepancy.

The X-band EPR and magnetic susceptibility in the temperature range 4.2–300 K study of the shungite-I, natural nanostructured material from the deposit of Shunga are reported. Obtained results allow us to assign the EPR signal to conduction electrons, estimate their number, N P, and evaluate the Pauli paramagnetism contribution to shungite susceptibility. A small occupation (~5%) of the localized nonbonding π states in the zigzag edges of the open-ended graphene-like layers and/or on σ (sp 2+x ) orbitals in the curved parts of the shungite globules has been also revealed. The observed temperature dependence of the EPR linewidth can be explained by the earlier considered interaction of conduction π electrons with local phonon modes associated with the vibration of peripheral carbon atoms of the open zigzag-type edges and with peripheral carbon atoms cross-linking different nanostructures. The relaxation time T 2 and diffusion time T D are found to have comparable values (2.84 × 10−8 and 1.73 × 10−8 s at 5.2 K, respectively), and similar dependence on temperature. The magnetic measurements have revealed the suppression of orbital diamagnetism due to small amount of large enough fragments of the graphene layers.