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- Bonazzi, Paola, et al.
(författare)
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Multi-analytical approach to solve the puzzle of an allanite-subgroup mineral from Kesebol, Vastra Gotaland, Sweden
- 2009
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Ingår i: American Mineralogist. - : Mineralogical Society of America. - 0003-004X .- 1945-3027. ; 94:1, s. 121-134
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Tidskriftsartikel (refereegranskat)abstract
- Dark-brownish. euhedral crystals of an "allanite-like" mineral occur in a hematite-impregnated Mn-silicate rock at Kesebol, Vastra Gotaland, Sweden, associated with gasparite-(Ce), chernovite-(Y), rhodonite, andradite, manganoan calcite, and quartz. A structural study was carried out on single crystals-untreated, heated in air, and heated under inert atmosphere-combined with Mossbauer spectroscopy and TEM investigation. In all the untreated crystals the mean < M3-O > distance indicates that Me2+(Me = Mn, Fe) prevails at this site (< M3-O > in the range 2.169-2.180 angstrom), in contrast with chemical data obtained by EPMA that yield a simplified formula Ca(REE2/33+square(1/3))Me-3(3+)(SiO4)(Si2O7) O(OH), when normalized to Si = 3.00 apfu. Moreover, when a crystal is heated in air, all geometrical and structural variations indicate the development of an oxidation-dehydrogenation reaction, thus confirming that M3 is occupied by divalent cations before heating. The corresponding dehydrogenation is confirmed by a dramatic lengthening of the donor-acceptor distance. A crystal was annealed under inert atmosphere to verify possible effects of radiation damage on the polyhedral volumes. After prolonged annealing at 700 C, a slight decrease of the unit-cell parameters is observed, suggesting restoring of crystallinity from a "partially metamict" state. Nonetheless, even in the annealed crystal, the < M3-O > distance is still consistent with a dominance of divalent cations at the M3 site. For all the examined crystals, structural data point to an octahedral cation population as follows: M1 = (Me3+, Al); M2 = (Al, Me3+). M3 = (Me2+, Me3+). This assumption is also in agreement with the Mossbauer spectrum, which was fitted to two Lorentzian quadrupole doublets for Fe3+ and one for Fe2+. Values of the isomer shifts (0.36 and 0.37 mm/s for Fe3+; 1.11 mm/s for Fe2+) and the quadrupole splitting (1.96 and 1.02 for Fe3+; 1.90 for Fe2+) show that Fe2+ (similar to 12% of the total iron) is located in M3, while Fe3+ occupies M I and, to lesser extent, M2. TEM-EDS investigations have revealed chemical heterogeneities related to different degree of radiation damage. In particular, areas showing poor crystallinity are relatively enriched in Si and O with respect to the highly crystalline areas, thus suggesting that EPMA chemical data are biased by the presence of metamict areas enriched in SiO2 and likely in H2O. EPMA data were therefore corrected for the excess of silica. The cation population after correction is in keeping with the structural and spectroscopic data. Disregarding minor substitutions, the ideal chemical formula for the epidote-group mineral from Kesebol is CaREEFe3+AlMn2+(Si2O7)(SiO4)O(OH), which is related to ferriallanite-(Ce) by the substitutional vector (M3)(Mn2+) -> M3 (Fe2+).
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| 8. |
- Locock, Andrew J., et al.
(författare)
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New mineral names
- 2006
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Ingår i: American Mineralogist. - 0003-004X .- 1945-3027. ; 91:11-12, s. 1945-1954
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Tidskriftsartikel (refereegranskat)
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| 9. |
- Mitev, Pavlin D., et al.
(författare)
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Anharmonic OH vibrations in brucite: Small pressure-induced redshift in the range 0–22 GPa
- 2009
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Ingår i: American Mineralogist. - : Mineralogical Society of America. - 0003-004X .- 1945-3027. ; 94:11-12, s. 1687-1697
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Tidskriftsartikel (refereegranskat)abstract
- The uncoupled anharmonic OH-stretching vibrational frequency for the layered mineral Mg(OH)2(brucite) has been calculated in the pressure range 0(DFT) calculations were performed, followed by quantum-mechanical vibrational energy calculations.The following findings emerged: (1) The calculated d with the experimental literature value [taken as the average between the Raman and IR-measured slopes for Mg(OH) much smaller than that of traditional H-bond correlation curves in the literature. (3) The main origin of the small d are pressed toward each other. (4) At high pressure, the OH with respect to the variation of the D quadrupole coupling constant is approximately –1 kHz/GPa. −22 GPa. Quantum-mechanical electronic structureν(OH)/dP slope is –4 cm–1/GPa, in agreement2]. (2) The calculated ν(OH) vs. R(O···O) correlation is linear and the slope isν/dP and dν/dR(O···O) slopes is the small electric field variation as the mineral layers− ions show some tendency to be tiltedc axis, and a larger tilt angle leads to a larger ν(OH) downshift. (5) The pressure variation of the D quadrupole coupling constant is approximately –1 kHz/GPa.
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| 11. |
- Skorodumova, N. V., et al.
(författare)
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Stability of the MgCO3 structures under lower mantle conditions
- 2005
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Ingår i: American Mineralogist. - : Mineralogical Society of America. - 0003-004X .- 1945-3027. ; 90:06-maj, s. 1008-1011
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Tidskriftsartikel (refereegranskat)abstract
- The presence of carbon in the Earth makes the search for high-pressure carbon-containing phases essential for our understanding of mineral compositions of the Earth's mantle. In a recent study Isshiki et al. (2004) demonstrated that magnesite transforms into a new phase at lower mantle pressures. However, the structure of the emerging phase remained unknown. Here we show, by means of first principles calculations, that MgCO3 magnesite can transform into a pyroxene structure at 113 GPa, which further transforms into a CaTiO3-type structure at about 200 GPa.
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| 13. |
- Rimsa, Andrius, et al.
(författare)
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Brittle fracturing and fracture healing of zircon: An integrated cathodoluminescence, EBSD, U-Th-Pb, and REE study
- 2007
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Ingår i: American Mineralogist. - : Mineralogical Society of America. - 0003-004X. ; 92:7, s. 1213-1224
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Tidskriftsartikel (refereegranskat)abstract
- The entire population of magmatic oscillatory zoned zircons in a migmatitic granite from the Tjarnesjo intrusion, southwest Sweden, reveal fine-scale brittle fracturing. The oscillatory zoned fragments are rotated but not dispersed. Fractures between individual fragments are sealed by newly formed CL-bright zircon. Hydraulic fracturing is the most probable mechanism. The internal structure of fractured zircons and the LREE-enriched, low Th character of CL-bright zircon both suggest that cracks between oscillatory zoned zircon fragments were rapidly sealed after fracturing by CL-bright zircon, precipitated from hydrothermal fluids. Zircon fracturing and crack-sealing has been dated by SIMS ion-probe and U-Th-Pb isotopes to 920 +/- 51 Ma (lower intercept age, 2 sigma, MSWD = 1.09) with a limit for the Youngest possible age of 960 +/- 16 Ma (Pb-207/(206) Pb, 2 sigma, MSWD = 0.23) dated by sector-zoned rims forming overgrowths on the fractured cores.
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| 14. |
- Skogby, Henrik, et al.
(författare)
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Titanium incorporation and Ti-VI(3+)-Ti-IV(4+) charge transfer in synthetic diopside
- 2006
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Ingår i: American Mineralogist. - : Mineralogical Society of America. - 0003-004X. ; 91:11-12, s. 1794-1801
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Tidskriftsartikel (refereegranskat)abstract
- A series of Ti-doped diopside samples synthesized by fluxgrowth methods under reducing conditions were investigated by microanalytical methods and optical absorption spectroscopy, to assess Ti incorporation mechanisms, valence states, and related electronic transitions. Chemical characterization show that Ti occurs both in the tri-and tetravalent states, with Ti4+ preferentially ordered to the tetrahedral position whereas Till is restricted to the M1 position. Charge-balance is maintained by incorporation of Na and minor B, stemming from the flux compound. Polarized optical absorption spectra reveal three major absorption features. Two relatively narrow bands centered at 18 500 and 15 700 cm(-1), mainly polarized in the crystallographic c-direction, display absorbance values that correlate with the calculated Ti3+ (M1) concentration, and are assigned to spin-allowed d-d transitions in Till. A broader band centered around 24 500 cm-1 displays absorbance values that are well correlated with the product of Ti-VI(3+) and Ti-IV(4+). Based on the spectral characteristics of this band, including a strong polarization in the crystallographic b-direction, we assign this band to an Ti-M1(3+)-Ti-T(4+) intervalence charge transfer process.
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