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Sapphirine I (1979)

Higgins, John B. ; Ribbe, Paul H. ; Herd, Richard K.

Springer

Contributions to mineralogy and petrology 68 (1979), S. 349-356

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Microprobe analyses of 26 natural sapphirines from 17 localities indicate that the predominant chemical substitutions in this mineral occur along the solid solution joinVI(Mg,Fe)2++IVSi4+=VI(Al, Fe)3++IVAl3+. Chromium and manganese are minor substituents. Evidence for the substitution Si⇄Al+1/2Mg+1/2 vacancy is absent within the limits of analytical error. A partitioning scheme based on electrostatic charge balance considerations has been devised permitting calculation of Fe2+ and Fe3+ from total iron content. Results are in good agreement with previous Mössbauer studies which indicate Fe3+ is sometimes in octahedral and/or tetrahedral coordination. Distribution coefficients for Fe2+-Mg exchange equilibria between sapphirine-spinel and sapphirine-orthopyroxene are similar for most mineral pairs and suggest that most of the assemblages equilibrated at about the same temperature or that the exchange reactions are insensitive to temperature. Compositions of synthetic sapphirines as a function of temperature and pressure are qualitatively predictable from crystal chemical considerations. Changes in sapphirine composition along the MgSi= AlAl solid solution join toward more aluminous compositions stabilize the sapphirine structure at high temperatures and low pressures. The limited extent of MgSi=AlAl solid solution observed in natural sapphirines appears to be related to the requirements of geometrical fit among octahedra and tetrahedra in the almost idealized cubic closest-packed anion framework.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF01164519

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Sapphirine II (1979)

Higgins, John B. ; Ribbe, Paul H.

Springer

Contributions to mineralogy and petrology 68 (1979), S. 357-368

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract The crystal structure of aP21/a polymorph of sapphirine (a=11.286(3),b=14.438(2),c=9.957(2) Å, β=125.4(2) °) of composition [Mg3.7Fe 0.1 2+ Al4.1- Fe 0.1 3+ ]IV[Si1.8Al4.2]IVO20 was refined using structure factors determined by both neutron and x-ray diffraction methods to conventionalR factors of 0.067 and 0.031. respectively, forF obs〉2σ. The results of the two refinements agree reasonably well, but a half-normal probability plot (Abrahams, 1974) comparing the two data sets indicates that the pooled standard deviations of the atomic coordinates have been underestimated by a factor of two. The structure of sapphirine, solved initially by Moore (1969), consists of cubic closest packed oxygens with octahedral and predominantly tetrahedral layers alternately stacked along [100]. The layer in which 70% of the octahedral sites are occupied has an Mg-Al distribution characterized by Mg-rich octahedra sharing edges mainly with Al-rich octahedra. Mean octahedral bond lengths correlate well with Al occupancy determined by neutron site refinement if the relative number of shared octahedral edges is taken into account (see Table 1). The predominantly tetrahedral layer has 10% of the octahedral sites occupied by Al and 30% of the tetrahedral sites occupied by Al-Si in the ratio 2.33∶1. There are single chains of Al-Si tetrahedra parallel toz with corner-sharing wing tetrahedra (T5 andT6) on either side in the (100) plane. The meanT-O distance is highly correlated with Al occupancy, XAl, as determined from the neutron site refinement: $$\langle T - O\rangle = 1.656 + 0.105X_{Al} (r^2 = 0.995).$$ Details of the neutron refinement are summarized below.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF01164520

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Soft X-ray spectroscopy of ferrous silicates (1978)

Dodd, Charles G. ; Ribbe, Paul H.

Springer

Physics and chemistry of minerals 3 (1978), S. 145-162

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ISSN: 1432-2021

Source: Springer Online Journal Archives 1860-2000

Topics: Chemistry and Pharmacology , Geosciences , Physics

Notes: Abstract Energy gaps and electrical conductivities in the ferrous silicates, Fe2SiO4 and FeSiO3, depend primarily on Fe-O bonding and may be studied by ultraviolet and soft X-ray spectroscopy. We have measured FeLII–III X-ray band spectra under conditions of “minimal” (I4, at 4.0 keV) and “high” (I10, at 10.0 keV) self absorption to determine 3d orbital energy levels, to delineate d states in the valence band, and to construct band gap models. Absorption spectra, I4/I10, were computed to determine vacant orbital levels in the gap. A difference function (I4–I10) has been proposed to identify X-radiation at photon energies above the measured LIII absorption edge, including high-energy, double-vacancy satellites and radiative transitions involving the anti-parallel (spin-down) d 6 electron in the ground state. The proposed band gap model for Fe2SiO4 is consistent with that of Nitsan and Shankland (1976), including an intrinsic transition of 6.5 eV and an energy gap of 7.8 eV. The 3d orbital energy level electronic structures are in general agreement with levels computed by Tossell et al. (1974) for [FeO6]10− in FeO using an SCF Xα cluster MO method. A high-energy, double-vacancy satellite was found at ∼710.7 eV, and is presumed to originate from an LIIIMII,III initial state. The intensity of these satellites for the ferrous silicates and other iron compounds, and corresponding Fe LII/LIII intensity ratios are correlated with differences in band gap magnitudes and gap structure. Fe LII/LIII intensity ratios are not well correlated with iron oxidation state.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00308118

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