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- D'Ippolito, Veronica, et al.
(författare)
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Crystallographic and spectroscopic characterisation of a natural Zn-rich spinel approaching the endmember gahnite (ZnAl2O4) composition
- 2013
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Ingår i: Mineralogical magazine. - : Mineralogical Society. - 0026-461X .- 1471-8022. ; 77, s. 2941-2953
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Tidskriftsartikel (refereegranskat)abstract
- The crystal chemistry of a natural, gem-quality, blue-grey Zn-rich spinel crystal from Jemaa, Kaduna State, Nigeria, was studied using electron microprobe, single-crystal X-ray diffraction, optical absorption and Raman spectroscopies. The composition of the crystal approaches the gahnite endmember (ZnAl2O4), ∼94 mol.%, with the remainder being dominated by a hercynite component (FeAl2O4). The unit-cell dimension is 8.0850(3) Å and the tetrahedral and octahedral bond distances are determined asT–O 1.9485(6) Å andM–O 1.9137(3) Å. Crystal chemical analysis resulted in the empirical structural formulaT(Zn0.94Fe2+0.03Al0.03)M(Al1.96Fe2+0.03Fe3+0.01)O4, which shows Zn and Al almost fully ordered in the tetrahedrally and octahedrally coordinatedTandMsites, respectively. Raman spectra obtained using the excitation of the blue 473.1 nm line of a Nd:YAG laser display three of the five Raman-active modes predicted for the general oxide spinel group of minerals. These are the Egmode at 420.6 cm–1and the T2gmodes at 510 cm–1and 661 cm–1, due to vibrations in the AlO6octahedra. Optical absorption spectra recorded in the UV/VIS-NIR-MIR range 2000 29000 cm–1show a dominant absorption band at ∼5000 cm–1which is caused by spin-allowed electronicd–dtransitions in Fe2+located at theTsites. The blue-grey hue exhibited by the sample is mainly due to spin-forbidden electronic transitions inTFe2+and toMFe2+↔MFe3+intervalence charge transfer, and the poor saturation of the colour is due to the small concentration of Fe2+and Fe3+.
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- Hålenius, Ulf, et al.
(författare)
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Oxyplumboroméite, Pb2Sb2O7, a new mineral species of the pyrochlore supergroup from Harstigen Mine, Värmland, Sweden
- 2013
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Ingår i: Mineralogical magazine. - : Mineralogical Society. - 0026-461X .- 1471-8022. ; 77:7, s. 2931-2939
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Tidskriftsartikel (refereegranskat)abstract
- Oxyplumboroméite, Pb2Sb2O7, is a new mineral of the roméite group of the pyrochlore supergroup (IMA 2013-042). It is found together with calcite and leucophoenicite in fissure fillings in tephroite skarn at the Harstigen mine, Värmland, Sweden. The mineral occurs as yellow to brownish yellow rounded grains or imperfect octahedra. Oxyplumboroméite has a Mohs hardness of ∼5, a calculated density of 6.732 g/cm3 and is isotropic with a calculated refractive index of 2.061. Oxyplumboroméite is cubic, space group Fdm, with the unit-cell parameters a = 10.3783(6) Å, V = 1117.84(11) Å3 and Z = 8. The strongest five X-ray powder-diffraction lines [d in Å(I)(hkl)] are: 2.9915(100)(222), 2.5928(32)(400), 1.8332(48)(440), 1.5638(38)(622) and 1.1900(12)(662). The crystal structure of oxyplumboroméite was refined to an R1 index of 3.02% using 160 unique reflections collected with MoKα radiation. Electron microprobe analyses in combination with crystal-structure refinement, infrared, Mössbauer and electronic absorption spectroscopy resulted in the empirical formula A(Pb0.92Ca0.87Mn0.09Sr0.01Na0.05)Σ1.93B(Sb1.73Fe3+0.27)Σ2.00X+Y[O6.64(OH)0.03]Σ6.67. Oxyplumboroméite is the Pb analogue of oxycalcioroméite, ideally Ca2Sb2O7.
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