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  • 1985-1989  (11)
  • 1975-1979  (21)
  • 1
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Olivine inclusions in diamonds from kimberlites originating from the deep Upper Mantle contain significant amounts of chromium. It has been suggested that divalent chromium occurs in these olivines. This hypothesis is supported by recent Mössbauer and electronic spectral measurements at high pressures, which demonstrate that pressure-induced reduction takes place in compounds and minerals initially containing Fe3+, Mn3+, and Cu2+ ions. The process is facilitated at high temperatures. Low oxidation states of other metals such as Cr(II) are expected to be stabilized under the very high pressures and elevated temperature conditions in the Mantle. Since Cr2+ ions are susceptible to the Jahn-Teller effect, they are predicted by crystal field theory to be stabilized in certain distorted coordination sites, such as the olivine Ml site, all three sites of the β-spinel phase, and the 7-coordinated site of the strontium plumbate structure-type. The Cr2+ ions in olivines are stabilized in kimberlites intruded into the Crust by the high confining pressures in the diamond inclusions.
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  • 2
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 320 (1986), S. 55-56 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The red colour of the surface of Mars has long been attributed to Fe3+-bearing minerals in the regolith, implying that oxidizing conditions have prevailed. However, the apparent enrichment of iron in martian regolith, its mineralogy and the nature of the unweathered parent rocks remain poorly ...
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  • 3
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 255 (1975), S. 130-131 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Observations of the interiors of manganese nodules under the microscope5, together with electron microprobe analyses6"8, reveal the intimate associations and oscillatory intergrowths of the manganese- and iron-bearing minerals. Most nodules have formed about a nucleus consisting of pumice, altered ...
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  • 4
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 253 (1975), S. 614-616 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] One type of manganese mineralisation is related to calc-alkaline magmatism. Reports on porphyry copper deposits and numerous manganese ore deposits have revealed that the two mineralisations seem to be contemporaneous within calc-alkaline terrains11. Porphyry copper deposits consistently occur over ...
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  • 5
    Publication Date: 2013-08-31
    Description: Jarosites, which are present in terrestrial gossans capping oxidized sulfides associated with mafic igneous rocks, may also be present in Martian regolith. Spectral characteristics of jarosites are displayed in remote sensed reflectance spectra of bright regions of Mars surface. The occurrence of jarosite in the regolith would imply that acidic permafrost and sulfide ores exist beneath the surface of Mars.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington, Reports of Planetary Geology and Geophysics Program, 1986; p 176-177
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  • 6
    Publication Date: 2013-08-31
    Description: Large areas of Mars' surface are covered by oxidative weathering products containing ferric and sulfate ions having analogies to terrestrial gossans derived from sulfide mineralization associated with iron-rich basalts. Chemical weathering of such massive and disseminated pyrrhotite-pentlandite assemblages and host basaltic rocks in the Martian environment could have produced metastable gossaniferous phases (limonite containing poorly crystalline hydrated ferric sulfates and oxyhydroxides, clay silicates and opal). Underlying groundwater, now permafrost on Mars, may still be acidic due to incomplete buffering reactions by wall-rock alteration of unfractured host rock. Such acidic solutions stabilize temperature-sensitive complex ions and sols which flocculate to colloidal precipitates at elevated temperatures. Sampling procedures of Martian regolith will need to be designed bearing in mind that the frozen permafrost may be corrosive and be stabilizing unique complex ions and sols of Fe, Al, Mg, Ni and other minor elements.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Workshop on Mars Sample Return Science; p 46-47
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  • 7
    Publication Date: 2013-08-31
    Description: Paragenetic evidence that indicates that hisingerite may have formed during the evolution of Martian regolith is summarized. Hisingerite was once regarded as poorly crystalline iron-rich smectite or nontronite. However, recent electron microscopy and X-ray studies have revealed hisingerite to have an amorphous or gel structure containing a disordered array of (FeO6) octahedra and (SiO4) tetrahedra. It is just this coordination environment and degree of crystallinity that matches materials simulating the spectral properties of the bright regions of Mars. Therefore, hisingerite and basic ferric sulfate minerals appear to be major contributers to remote sensed reflectance spectral profiles of Mars.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington, Reports of Planetary Geology and Geophysics Program, 1986; p 175
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  • 8
    Publication Date: 2013-08-31
    Description: Pyrrhotite-pentlandite assemblages in mafic and ultramafic igneous rocks may have contributed significantly to the chemical weathering reactions that produced degradation products in the Martian regolith. By analogy with terrestrial processes, a model is proposed whereby supergene alteration of these primary Fe-Ni sulfides on Mars has generated secondary sulfides (e.g., pyrite) below the water table and produced acidic groundwater containing high concentrations of dissolved Fe, Ni and sulfate ions. The low pH solutions also initiated weathering reactions of igneous feldspars and ferromagnesian silicates to form clay silicate and ferric oxyhydroxide phases. Near-surface oxidation and hydrolysis of ferric sulfato- and hydroxo-complex ions and sols formed gossans above the water table consisting of poorly crystalline hydrated ferric sulfates (e.g., jarosite), oxides (ferrihydrite, goethite) and silica (opal). Underlying groundwater, now permafrost, contains hydroxo sulfato complexes of Fe, Al, Mg, Ni, etc., which may be stabilized in frozen acidic solutions beneath the surface of Mars. Sublimation of permafrost may replenish colloidal ferric oxides, sulfates and phyllosilicates during dust storms on Mars.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Workshop on Mars Sample Return Science; p 48-50
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  • 9
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract The blue colors of several minerals and gems, including aquamarine (beryl, Be3Al2Si6O18) and cordierite (Al3(Mg, Fe)2Si5AlO18), have been attributed to charge transfer (CT) between adjacent Fe2+ and Fe3+ cations, while Fe2+→Ti4+ CT has been proposed for blue kyanites (Al2SiO5). Such assignments were based on chemical analyses and on polarization-dependent absorption bands measured in visible-region spectra. We have attempted to characterize the Fe cations in each of these minerals by Mössbauer spectroscopy (MS). In blue kyanites, significant amounts of both Fe2+ and Fe3+ were detected with MS, indicating that Fe2+→Fe3+ CT, Fe2+→Ti4+ CT, and Fe2+ and Fe3+ crystal field transitions each could contribute to the electronic spectra. In aquamarines, coexisting Fe2+ and Fe3+ ions were resolved by MS, supporting our assignment of the broad, relatively weak band at 16,100 cm−1 in E∥c spectra to Fe2+→Fe3+ CT between Fe cations replacing Al3+ ions 4.6Å apart along c. A band at 17,500 cm−1 in E⊥c spectra of cordierite is generally assigned to Fe2+ (oct)→Fe3+ (tet) CT between cations only 2.74 Å apart. However, no Fe3+ ions were detected in the MS at 293K of several blue cordierites showing the 17,500 cm−1 band and reported to contain Fe3+. A quadrupole doublet with parameters consistent with tetrahedral Fe3+ appears in 77K MS, but the Fe3+/Fe2+ ratios from MS are much smaller than values from chemical analysis. These results sound a cautionary note when correlating Mössbauer and chemically determined Fe3+/Fe2+ ratios for minerals exhibiting Fe2+→Fe3+ CT.
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  • 10
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract Crystal field stabilization (CFS) plays a significant role in determining equilibrium phase boundaries in olivine→spinel transformations involving transition-metal cations, including Fe2+ which is a major constituent of the upper mantle. Previous calculations for Fe2SiO4 ignored pressure and temperature dependencies of crystal field stabilization enthalpies (CFSE) and the electronic configurational entropy (S CFS). We have calculated free energy changes (ΔG CFS) due to differences of crystal field splittings between Fe2SiO4 spinel and fayalite from: ΔG CFS=−ΔCFSE−TΔS CFS, as functions of P and T, for different energy splittings of t 2g orbital levels of Fe2+ in spinel. The results indicate that ΔG CFS is always negative, suggesting that CFS always promotes the olivine→spinel transition in Fe2SiO4, and expands the stability field of spinel at the expense of olivine. Because of crystal field effects, transition pressures for olivine→spinel transformations in compositions (Mg1−x Fe x )2SiO4 are lowered by approximately 50x kbar, which is equivalent to having raised the olivine→spinel boundary in the upper mantle by about 15 km.
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