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  • 1
    Electronic Resource
    Electronic Resource
    Microanalysis of Fe3+/ΣFe in oxide and silicate minerals by investigation of electron energy-loss near-edge structures (ELNES) at the Fe M 2,3 edge (1999)
    Springer
    Physics and chemistry of minerals 26 (1999), S. 584-590 
    Details
    ISSN: 1432-2021
    Keywords: Key words Ferrous/ferric ratio ; Electron energy-loss spectroscopy ; Electron energy-loss near-edge structure ; Transmission electron microscopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract The Fe M 2,3-edge spectra of solid solutions of garnets (almandine-skiagite Fe3(Al1–xFex)2[SiO4]3 and andradite-skiagite (Fe1–xCax)3Fe2[SiO4]3), pyroxenes (acmite-hedenbergite (Ca1–xNax)(Fe2+ 1−xFe3+ x)Si2O6), and spinels (magnetite-hercynite Fe(Al1–xFex)2O4) have been measured using the technique of parallel electron energy-loss spectroscopy (EELS) conducted in a transmission electron microscope (TEM). The Fe M 2,3 electron energy-loss near-edge structures (ELNES) of the minerals exhibit a characteristic peak located at 4.2 eV and 2.2 eV for trivalent and divalent iron, respectively, prior to the main maximum at about 57 eV. The intensity and energy of the pre-edge feature varies depending on Fe3+/ΣFe. We demonstrate a new quantitative method to extract the ferrous/ferric ratio in minerals. A systematic relationship between Fe3+/ΣFe and the integral intensity ratio of the main maximum and the pre-edge peak of the Fe M 2,3 edge is observed. Since the partial cross sections of the Fe M 2,3 edges are some orders of magnitude higher than those of the Fe L 2,3 edges, the Fe M 2,3 edges are interesting for valence-specific imaging of Fe. The possibility of iron valence-specific imaging is illustrated by Fe M 2,3-ELNES investigations with high lateral resolution from a sample of ilmenite containing hematite exsolution lamellae that shows different edge shapes consistent with variations in the Fe3+/ΣFe ratio over distances on the order of 100 nm.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002690050222
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  • 2
    Electronic Resource
    Electronic Resource
    Structure, ordering and cation interactions in Ca-free P2 1 /c clinopyroxenes (1998)
    Springer
    Physics and chemistry of minerals 25 (1998), S. 249-258 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract  A suite of 14 synthetic Ca-free P2 1 /c low-clinopyroxenes of compositions across the enstatite-ferrosilite (En-Fs) join have been studied by X-ray powder diffraction and Mössbauer spectroscopy. The crystal structure of one sample with composition X Fs=0.39 was determined by single-crystal X-ray diffraction. The powder diffraction data show that there is no significant (〈0.25% at X Fs=0.5) excess volume of mixing on the enstatite-ferrosilite join. The molar volumes are described by V=31.261(17)+1.677(23) X Fs cm3.mol(MSiO3)–1. The linear behaviour of volume with composition arises from the compensating effects of non-linear changes in the unit-cell parameters with composition. Unweighted fits to the data yielded the relationships: a=9.6100(15)+0.105(2)X Fs, b=8.8156(28)+0.218(12)X Fs+0.0481(117)X 2 Fs , c =5.1702(4)+0.0879(21)X Fs–0.0214(21)X 2 Fs , β=108.345(8)+0.947(30)X Fs–0.805(26)X 2 Fs . The strong positive deviation of β from linearity is directly correlated to the difference in site occupancies between M1 and M2 as determined by Mössbauer spectroscopy and arises directly from the expansion of M2-O1 and M2-O3 bonds upon initial substitution of Fe for Mg in enstatite. The hyperfine parameters from Mössbauer spectra are consistent with variations in the average local environment as recorded by the X-ray data. Asymmetric line broadening of the Mössbauer spectra provides evidence for next-nearest neighbour effects, and is consistent with no significant clustering of Fe or Mg within the samples.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002690050111
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  • 3
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    Accurate determination of ferric iron in garnets by bulk Mossbauer spectroscopy and synchrotron micro-XANES (2012)
    Mineralogical Society of America
    Details
    Publication Date: 2012-10-01
    Description: Measurements of Fe 3+ /Fe in geological materials have been intractable because of lack of access to appropriate facilities, the time-consuming nature of most analyses, and the lack of precision and reproducibility in most techniques. Accurate use of bulk Mössbauer spectroscopy is limited by largely unconstrained recoilless fraction ( f ), which is used to convert spectral peak area ratios into valid estimates of species concentrations and is unique to different mineral groups and compositions. Use of petrographic-scale synchrotron micro-XANES has been handicapped by the lack of a consistent model to relate spectral features to Fe 3+ /Fe. This paper addresses these two deficiencies, focusing specifically on a set of garnet group minerals. Variable-temperature Mössbauer spectra of the Fe 2+ -bearing almandine and Fe 3+ -bearing andradite end-members are used to characterize f in garnets, allowing Fe 3+ /Fe to be measured accurately. Mössbauer spectra of 19 garnets with varying composition were acquired and fit, producing a set of garnet-specific standards for Fe 3+ analyses. High-resolution XANES data were then acquired from these and 15 additional previously studied samples to create a calibration suite representing a broad range of Fe 3+ and garnet composition. Several previously proposed techniques for using simple linear regression methods to predict Fe 3+ /Fe were evaluated, along with the multivariate analysis technique of partial least-squares regression (PLS). Results show that PLS analysis of the entire XANES spectral region yields the most accurate predictions of Fe 3+ in garnets with both robustness and generalizability. Together, these two techniques present reliable choices for bulk and microanalysis of garnet group minerals.
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
    Published by Mineralogical Society of America
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  • 4
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    Experimental constraints in the CMAS system on the Ca-Eskola content of eclogitic clinopyroxene (2013)
    Schweizerbart
    Details
    Publication Date: 2013-12-10
    Description: The seismic X-Discontinuity, occasionally observed at ~300 km depth, has been attributed partly to the exsolution of the calcium-Eskola (Ca 0.5 0.5 AlSi 2 O 6 , CaEs) component from eclogitic clinopyroxene upon reaching the coesite–stishovite transition. To test if this is a viable mechanism, we have undertaken high-pressure experiments between 4 and 11.5 GPa at 1000–1350 °C in order to further investigate the factors controlling CaEs incorporation in clinopyroxene at upper mantle conditions up to coesite–stishovite transition. Bulk compositions had pyroxene stoichiometry in the simple system CaO–MgO–Al 2 O 3 –SiO 2 ± Na 2 O (CMAS ± Na), which restricts possible end-member components to diopside (di), calcium-Tschermaks (CaTs), calcium-Eskola (CaEs), and clinoenstatite (en), ±jadeite. All run products had a typical eclogitic mineral assemblage with clinopyroxene always in equilibrium with garnet and coesite or stishovite, and in some runs kyanite. Clinopyroxene is non-stoichiometric due to the defect-bearing CaEs component. CaEs and CaTs contents vary systematically with P , T and bulk composition. CaEs reaches up to 0.22 mol % at 4 GPa and 1200 °C, and for a given bulk composition CaEs decreases continuously with pressure from 4 to 11.5 GPa. Our experiments, along with experiments performed with more complex compositions reveal different systematics when Na is present, since it is only accommodated in clinopyroxene. Incorporation of a jadeite component drives down the CaTs content and also influences the CaEs content. Generally, maximizing the CaEs content requires a coexisting free SiO 2 phase and an elevated Al 2 O 3 content, but it is not clear if kyanite must be part of the assemblage, or whether buffering of Al by garnet is adequate. At P 〉 9 GPa, the systematics of clinopyroxene composition change as garnet becomes majoritic and also begins to accommodate significant Na. In both simple and complex bulk compositions CaEs content in clinopyroxene decreases steadily from ~4 GPa so that no sharp change occurs at pressures corresponding to the coesite–stishovite phase transition. Calculations reveal that only small (1–2 wt %) amounts of free stishovite can be exsolved from CaEs-bearing clinopyroxene, which are insufficient to produce a large enough impedance contrast to explain a significant contribution to the X-Discontinuity. If the X-Discontinuity is related to the coesite–stishovite phase transition in eclogite, as proposed in the literature, then free SiO 2 must be already present in the mineral assemblage rather than being exsolved from CaEs-bearing clinopyroxene upon entering the stability field of stishovite.
    Print ISSN: 0935-1221
    Electronic ISSN: 1617-4011
    Topics: Geosciences
    Published by Schweizerbart
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  • 5
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    Oxidation State of the Lithospheric Mantle below the Massif Central, France (2014)
    Oxford University Press
    Details
    Publication Date: 2014-12-14
    Description: Oxidation state is a sensitive indicator of geochemical processes within the upper mantle. Here we report results of a regional study of the oxidation state of spinel peridotite xenoliths from 45 volcanic centers distributed over ~20 000 km 2 in the Massif Central, France. The log f O 2 values relative to the fayalite–magnetite–quartz oxygen buffer (FMQ) were determined from the equilibrium between the Fe-bearing components in olivine, orthopyroxene and spinel, with the Fe 3+ content of spinel measured either by Mössbauer spectroscopy or by electron microprobe using secondary spinel standards. For the entire suite of samples, log f O 2 values range between FMQ – 0·47 and FMQ + 1·66. Our data confirm the presence of two distinct lithospheric mantle domains, previously reported in the literature, lying north and south of 45°30'N, respectively. The northern domain, with its refractory bulk composition, tends to record more oxidized conditions, having log f O 2 values mostly at or above FMQ + 1. The log f O 2 in the southern domain is more variable, including values below FMQ. Assuming that increasing equilibration temperatures among xenoliths reflect increasing depths of origin, samples from the northern domain suggest that the shallower part of the subcontinental lithospheric mantle (SCLM) is somewhat more oxidized than at deeper levels. On the other hand, such a general observation cannot be made for the southern domain. The high log f O 2 values of harzburgites suggest that they are more sensitive to resetting of their oxidation state by metasomatism than lherzolites. In terms of modally metasomatized xenoliths, the ‘melt’ leading to the addition of clinopyroxene apparently had a higher oxidation state (log f O 2 〉 FMQ + 1) than the agent responsible for crystallization of amphibole (log f O 2 ~ FMQ + 0·6). Furthermore, amphibole-bearing and amphibole-free peridotites exhibit the same range in f O 2 . Cryptic metasomatism can also reset oxidation state, sometimes very effectively. Metasomatic processes are probably the reason why the xenolith suite from the Massif Central records relatively high log f O 2 values compared with ‘normal’ non-cratonic SCLM. This study demonstrates the utility of using oxidation state to help characterize and delineate domains in the lithospheric mantle.
    Print ISSN: 0022-3530
    Electronic ISSN: 1460-2415
    Topics: Geosciences
    Published by Oxford University Press
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  • 6
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    Quantitative mapping of the oxidative effects of mantle metasomatism (2013)
    Geological Society of America (GSA)
    In: Geology
    Details
    Publication Date: 2013-05-21
    Description: The oxidation state of Fe in garnets in a garnet peridotite xenolith from the Wesselton kimberlite (South Africa) was quantitatively mapped using X-ray absorption near edge structure (XANES) spectroscopy. Maps of Fe 3+ /Fe were produced by recording the fluorescence intensity at discrete energies rather than recording the full spectrum at each point. The intensity at each point in the map was quantitatively converted to Fe 3+ /Fe with reference to a linear calibration derived from garnet standards for which Fe 3+ /Fe had been determined previously by Mössbauer spectroscopy. The resolution of these maps approaches that of elemental maps obtained using an electron microprobe. The maps reveal zoning in Fe 3+ /Fe between the core (0.075) and rim (0.125) that correlates with zoning of other elements. The rims record an oxidizing metasomatic event in the lithospheric mantle. The oxygen fugacity ( f O 2 ) of this metasomatism is considerably higher than expected from studies of homogeneous garnets that exhibit metasomatic signatures; such garnets may represent a re-equilibrated average of the original (core) and metasomatic (rim) f O 2 values. Metasomatism of the lithospheric mantle may thus have a greater impact on diamond stability than previously thought.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 7
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    Structural systematics of spinel and spinelloid phases in the system MFe2O4-M2SiO4 with M = Fe2+ and Mg (2012)
    Schweizerbart
    Details
    Publication Date: 2012-08-01
    Description: Spinels and structurally related spinelloid polytypes have been synthesised in the MgO–FeO–Fe 2 O 3 –SiO 2 system at 3.0–10.5 GPa and 1100–1200 °C. Here we report on the structural systematics of these phases as a function of composition, with particular emphasis on the structural changes induced by Mg incorporation. Analysis of the molar volume–composition relations in spinels suggests ideal or near ideal mixing behaviour. Spinelloids II, III and V are stable polytypes and their respective stability fields are confined in composition space, each related to specific structural constraints. A primary limitation to Si-Fe 3+ substitution is the differing structural relations between the tetrahedral sites in the three polymorphs. Mg incorporation in the spinelloid polytypes is generally restricted. Maximum Mg 2 SiO 4 contents follow the sequence spinelloid V 〉 spinelloid III 〉 spinelloid II, except for the additional stability field of the Mg 2 SiO 4 endmember (wadsleyite), which has the spinelloid III structure. A general Mg-Fe 3+ avoidance appears to limit Mg incorporation in the spinelloid structures. This effect is amplified by the preference of Mg for the octahedral site that is bonded to the bridging oxygen of the tetrahedral group, as determined by single-crystal refinements.
    Print ISSN: 0935-1221
    Electronic ISSN: 1617-4011
    Topics: Geosciences
    Published by Schweizerbart
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  • 8
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    In situ observation of the breakdown of magnetite (Fe3O4) to Fe4O5 and hematite at high pressures and temperatures (2012)
    Mineralogical Society of America
    Details
    Publication Date: 2012-10-01
    Description: In situ synchrotron X-ray powder diffraction measurements using a Paris-Edinburgh pressure cell were performed to investigate the nature of the high-pressure breakdown reaction of magnetite (Fe 3 O 4 ). Refinement of diffraction patterns reveals that magnetite breaks down via a disproportionation reaction to Fe 4 O 5 and hematite (Fe 2 O 3 ) rather than undergoing an isochemical phase transition. This result, combined with literature data indicates (1) that this reaction occurs at ~9.5–11 GPa and 973–1673 K, and (2) these two phases should recombine at yet higher pressures to produce an h -Fe 3 O 4 phase.
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
    Published by Mineralogical Society of America
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  • 9
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    Equations of state for Fe32+Fe23+Si3O12“skiagite” garnet and Fe2SiO4-Fe3O4spinel solid solutions (1999)
    American Geophysical Union
    Details
    Publication Date: 1999-09-10
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
    Published by American Geophysical Union
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  • 10
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    Iron speciation in minerals and glasses probed by $$hbox{M}_{2/3}$$ M 2 / 3 -edge X-ray Raman scattering spectroscopy (2014)
    Springer
    Details
    Publication Date: 2014-05-01
    Print ISSN: 0010-7999
    Electronic ISSN: 1432-0967
    Topics: Geosciences
    Published by Springer
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