<sup>57</sup>Fe-Mössbauer investigation on garnets from the Ivrea-Verbano Zone

Quartieri, Simona; Artioli, Gilberto; Deriu, A.; Lottici, Pier Paolo; Antonioli, G.

doi:10.1180/minmag.1993.057.389.11

Cited by 4 publications

(4 citation statements)

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“…Preliminary M6ssbauer results (Quartieri et al, 1993) confirm the low Fe 3+ content in our samples, indicating that no more than 2-5% of iron is in the octahedrally coordinated trivalent state. These percentages are below the limit of the XANES sensitivity, expecially when the absorbing element is present in the mineral with more than one oxidation state and coordination number (Brown et al, 1988;Artioli and Geiger, 1991).…”

Section: Discussionsupporting

confidence: 77%

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X-ray absorption study at the Fe K-edge of garnets from the Ivrea-Verbano zone

et al. 1993

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K-edge X-ray absorption spectra of Fe in garnet samples from the Ivrea-Verbano zone were collected using synchrotron radiation. From XANES analysis, the prevalent oxidation state of Fe has been determined as 2+ in all studied samples. Coordination numbers and Fe-O bond lengths derived from the EXAFS analysis are compatible with a dodecahedral environment of Fe atoms and seem to be nearly independent of the variable Fe/Ca ratio of the cations sharing the dodecahedral site in these garnets. This suggests that, since at least up to 0.5 Ca atoms p.f.u, no sensible deformation of the dodecahedron geometry is sensed by the Fe cations, iron might strongly compete with Ca atoms in controlling the entry of rare earth elements in the dodecahedral site of garnets falling within this compositional range. Comparison of the EXAFS results with the data from single crystal X-ray diffraction structure refinements indicates a first shell neighbour distance accuracy of +0.02 ~, using theoretical EXAFS phases and amplitudes. The Debye-Waller factors derived from the EXAFS analysis indicate a higher degree of disorder on the four longer Fe-O bond distances, in comparsion with the other four shorter distances of the height-coordinated cation; this could be related to the nonrigid polyhedral behaviour of the dodecahedral site.

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Section: Discussionsupporting

confidence: 77%

“…These authors measured the Mrssbauer spectra of a number of natural Fe-rich garnets with variable Ca contents and found virtually invariable isomer shift values of the Fe 2 § dodecahedral doublet. The Mrssbauer experiments performed on our garnets (Quartieri et al, 1993) should give more insight on this subject.…”

Section: Discussionmentioning

confidence: 99%

X-ray absorption study at the Fe K-edge of garnets from the Ivrea-Verbano zone

et al. 1993

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“…The measurable effect of "impurity" atoms in small concentrations on T N in garnet is apparently confirmed. This is of note because small concentrations of octahedral Fe 3+ occur in many synthetic and natural almandine crystals (Murad and Wagner 1987;Geiger et al 1988;Quartieri et al 1993;Woodland et al 1995). Furthermore, at high pressure there is complete solid solution between almandine and skiagite, ideally Fe 3 2+ Fe 2 3+ Si 3 O 12 (Woodland and O'Neill 1993), and, here, the magnetic interactions can be expected to be highly complex.…”

Section: Effect Of "Impurity" Atoms On T Nmentioning

confidence: 91%

An analysis of the magnetic behavior of olivine and garnet substitutional solid solutions

Geiger

Grodzicki

Dachs

2019

American Mineralogist

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The low-temperature magnetic properties and Néel temperature, TN, behavior of four silicate substitutional solid solutions containing paramagnetic ions are analyzed. The four systems are: fayaliteforsterite olivine [Fe22+SiO4-Mg2SiO4], and the garnet series, grossular-andradite [Ca3(Alx,Fe1−x3+)2Si3O12], grossular-spessartine [(Cax,Mn1−x2+)3Al2Si3O12], and almandine-spessartine [(Fex2+,Mn1−x2+)3Al2Si3O12]. Local magnetic behavior of the transition-metal-bearing end-members is taken from published neutron diffraction results and computational studies. TN values are from calorimetric heat capacity, CP, and magnetic susceptibility measurements. These end-members, along with more transition-metal-rich solid solutions, show a paramagnetic to antiferromagnetic phase transition. It is marked by a CP λ-anomaly that decreases in temperature and magnitude with increasing substitution of the diamagnetic component. For olivines, TN varies between 65 and 18 K and TN for the various garnets is less than 12 K. Local magnetic behavior can involve one or more superexchange interactions mediated through oxygen atoms. TN behavior shows a quasi-plateau-like effect for the systems fayalite-forsterite, grossular-andradite, and grossular-spessartine. More transition-metal-rich crystals show a stronger TN dependence compared to transition-metal-poor ones. The latter may possibly show superparamagnetic behavior. (Fex2+,Mn1−x2+)3Al2Si3O12 garnets show fundamentally different magnetic behavior. End-member almandine and spessartine have different and complex interacting local superexchange mechanisms and intermediate compositions show a double-exchange magnetic mechanism. For the latter, TN values show negative deviations from linear interpolated TN values between the end-members. Double exchange seldom occurs in oxides, and this may be the first documentation of this magnetic mechanism in a silicate. TN behavior may possibly be used to better understand the nature of macroscopic thermodynamic functions, CP and S°, of both end-member and substitutional solid-solution phases.

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“…characterize the Ca environment in the (Py, Alm)-like structure, their Ca content ranging from 8 to 17%. These samples were previously studied by means of EXAFS (Extended X-ray Absorption Fine Structure) and XANES techniques at Fe K-edge (Quartieri et al 1993 a) and by 5VFe-M6ssbauer spectroscopy (Quartieri et al 1993b) and information was gained on the oxidation state and the local geometric environment of the iron, sharing with Ca and Mg the dodecahedral site. In particular, both spectroscopic techniques suggest that, in the compositional range of these samples, the average geometric environment of Fe 2 + is not significantly modified, with respect to that assumed in the end-member almandine, by the presence of the larger Ca cations.…”

Section: Samples Studiedmentioning

confidence: 99%

Local structural environment of calcium in garnets: A combined structure-refinement and XANES investigation

Quartieri¹,

Chaboy

Merli³

et al. 1995

Phys Chem Minerals

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Abstract.Comparative crystal-chemical analysis of natural garnets allowed the compositional gap at Gr 45+70% in the (Py, Alm)-Gr join to be related to the presence of two different structural arrangements around the X site in pyralspitic and ugranditic composition, respectively. A combined investigation (single-crystal Xray structure-refinement (SREF), Ca K-edge XANES spectroscopy and multiple-scattering calculation of XANES spectra) carried out on a series of natural garnets in the (Py, Alm)-Gr join, allowed the structural modifications of the Ca local environment at increasing grossular content to be followed.The differences in the experimental XANES spectra as a function of the Ca content were shown to depend only on the configuration of the ligand shells surrounding the absorbing atom, and not on the nature of adjacent dodecahedral cations. Both experimental XANES spectra and their theoretical simulations confirm that Ca at the X site, when less abundant than (Fe + Mg), is not in the same structural configuration as in grossular, but adapts itself to that imposed by the dominant (Fe, Mg) cations. In the same way, (Fe, Mg) adapt themselves to the X-site configuration observed in grossular when Ca is the dominant cation. The deformation of the X site due to progressive Ca-~(Fe, Mg) substitution is gradual but not linear along the solid solution; it shows different slopes on the two sides of the observed (Py, Alm)-Gr compositional gap.Best performances in reproducing the XANES spectra were obtained in the framework of the one-electron fullmultiple-scattering theory, using the real HedinLundqvist exchange-correlation potential and clusters of 83 atoms (i.e. for a coordination sphere with 7 It radius from the central Ca absorber); smaller clusters resulted inadequate in simulating some XANES features, indicating that a high number of single and multiple scatteringpaths contribute in garnets to the first part of the absorption spectrum.

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⁵⁷Fe-Mössbauer investigation on garnets from the Ivrea-Verbano Zone

Cited by 4 publications

References 11 publications

X-ray absorption study at the Fe K-edge of garnets from the Ivrea-Verbano zone

X-ray absorption study at the Fe K-edge of garnets from the Ivrea-Verbano zone

An analysis of the magnetic behavior of olivine and garnet substitutional solid solutions

Local structural environment of calcium in garnets: A combined structure-refinement and XANES investigation

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57Fe-Mössbauer investigation on garnets from the Ivrea-Verbano Zone

Cited by 4 publications

References 11 publications

X-ray absorption study at the Fe K-edge of garnets from the Ivrea-Verbano zone

X-ray absorption study at the Fe K-edge of garnets from the Ivrea-Verbano zone

An analysis of the magnetic behavior of olivine and garnet substitutional solid solutions

Local structural environment of calcium in garnets: A combined structure-refinement and XANES investigation

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⁵⁷Fe-Mössbauer investigation on garnets from the Ivrea-Verbano Zone