Hans Annersten

Magnetic dipolar and electric quadrupolar effects on the Mssbauer spectra of magnetite above the Verwey transition

Mössbauer spectroscopic studies (57Fe) of powdered magnetite have been undertaken between 120 K and 880 K. Below the magnetic transition temperature (TC=839.5 K) three six-line patterns have been fitted to our experimental spectra. The broadening of the B-pattern is explained by two magnetically non-equivalent B-site irons, suggesting broadening due to electron hopping to be negligible. In the paramagnetic state the electric quadrupole splittings of iron at A-and B-sites are found to be constant, independent of temperature, having the values zero and 0.16 mm/s, respectively. The centroid shifts, on the other hand, show above 700 K large deviations from the calculated second order Doppler shift. It is proposed that the deviations arise from a variation in band overlap. The temperature variation of the magnetic fields is found to be proportional to the sublattice magnetization. The difference in the magnetic fields at the two non-equivalent B-sites is measured to be 1.1 T at 310 K.

The Mössbauer spectrum of 57Fe in titanium-bearing andradites

Mössbauer spectra of 57Fe in 2 schorlomite garnets reveal 5 distinct quadrupole-split doublets: dodecahedral Fe2+, octahedral Fe2+ and Fe3+, and tetrahedral Fe2+ and Fe3+. The isomer shifts and nuclear quadrupole splittings of the 5 doublets were studied between 15 and 500 K. The site occupancies for iron were determined. Reference of the chemical analyses to a basis of 12 oxygens and the Mössbauer data show that in the 2 schorlomites titanium is exclusively quadruvalent within the experimental error.The isomer shift of tetrahedral Fe2+ between 15 and 295 K seems to be rather small. The shift is interpreted in terms of localized chemical bonding. Above 295 K the shift cannot be evaluated because of overlapping peaks. If electronic transfer processes (e.g. “electron hopping”) between cations are present their relaxation times must be longer than ∼10−7 s.

Stability of the assemblage orthopyroxene-sillimanite-quartz in the system MgO-FeO-Fe2O3-Al2O3-SiO2-H2O

The stability of coexisting orthopyroxene, sillimanite and quartz and the composition of orthopyroxene in this assemblage has been determined in the system MgO-FeO-Fe2O3-Al2O3-SiO2-H2O as a function of pressure, mainly at 1,000° C, and at oxygen fugacities defined mostly by the hematite-magnetite buffer. The upper stability of the assemblage is terminated at 17 kbars, 1,000° C, by the reaction opx+Al-silicate →gar+qz, proceeding toward lower pressures with increasing Fe/(Fe+Mg) ratio in the system. The lower stability is controlled by the reaction opx+sill+qz→ cord, which occurs at 11 kbars in the iron-free system but is lowered to 9 kbars with increasing Fe/(Fe+Mg). Spinel solid solutions are stabilized, besides quartz, up to 14 kbars in favour of garnet in the iron-rich part of the system (Fe/(Fe+Mg)≧0.30). Ferric-ferrous ratios in orthopyroxene are increasing with increasing ferro-magnesian ratio. At least part of the generally observed increase in Al content with Fe2+ in orthopyroxene is not due to an increased solubility of the MgAlAlSiO6 component but rather of a MgFe3+AlSiO6 component. The data permit an estimate of oxygen fugacity from the composition of orthopyroxene in coexistence with sillimanite and quartz.

Ferric iron in orthopyroxene: a Mössbauer spectroscopic study

Abstract Ferric iron solid solution in synthetic orthopyroxene has been studied along the joins MgSiO 3 -Al 2 O 3 · Fe 2 O 3 and MgSiO 3 -Fe 2 O 3 . The partially reduced synthetic orthopyroxenes showed that major incorporation of ferric iron can only occur together with a concomitant incorporation of Al. Maximum solid solution of ferric iron along the join MgSiO 3 -Fe 2 O 3 was found to be only 0.63 wt% Fe 2 O 3 at 1000°C and 2 kb total pressure. From the observed Mossbauer parameters octahedral ferric iron can be assigned to the MI position in orthopyroxene. Incorporation of Fe 3+ and/or Al will increase the disorder of Fe 2+ and Mg between the M1 and M2 sites, which is also observed in a ferric iron-containing aluminous orthopyroxene of metamorphic origin. In the assemblage orthopyroxene + sillimanite + quartz the ferric iron content of orthopyroxene is directly related to oxygen fugacity.

Mössbauer study of iron-rich biotites

It has been possible by detailed studies of the Mossbauer spectra for biotite, to distinguish between the two iron sites.

A mineral chemical study of a metamorphosed iron formation in northern Sweden

The composition of coexisting biotite, Ca-amphiboles, and iron oxides in a metamorphic iron formation in Northern Sweden, analysed by micro-probe technique, is correlated with the oxygen pressure as indicated by the presence of distinct iron oxide minerals. A close approach to chemical equilibrium during metamorphism is indicated by regular element distribution between the phases and the restriction of the Mg/Fe ratio and Ti/Mg+Fe+Ti ratio of the ferromagnesian silicates in presence of magnetite and hematite. The use of iron-titanium oxides as indicators of oxygen pressure in rocks is discussed. There is an increase of the Mg/Mg+Fe 2+ and the ferric-ferrous ratios, and a decrease of the Ti content in the silicates with increasing oxygen pressure. Different ways in which biotite and Ca-amphiboles stabilize at increasing oxygen pressure are investigated. The role of water in the rocks during metamorphism is discussed.

In situ pressure Raman spectroscopy and mechanical stability of superhard boron suboxide

In situ Raman spectroscopy was used to explore the mechanical stability and electronic properties of superhard boron suboxide (B6O) under nonhydrostatic pressure. B6O stabilizes in a rhombohedral structure (R3¯m) to the peak pressure of 90GPa. The icosahedra-chain (B–O) phonon at 274cm−1 exhibits a negative pressure dependent profile of −0.21cm−1∕GPa, while the intraicosahedral vibrations (B–B) between 400 and 1100cm−1 and the intericosahedral phonon at 1141cm−1 display 0.7–2.3 and 3.8cm−1∕GPa, respectively. The small pressure dependencies of intraicosahedral modes resulting from the crystal lattice are significant indications of the low compressibility of B6O. Upon decompression to 3.3GPa, crystalline B6O transforms to the amorphous boron oxide and glassy boron. The structure instability of superhard B6O upon release of pressure implies its critical weakness for applications in harsh environments of dynamic impacts and high stress concentrations.

Structural investigations of natural and synthetic chlorite minerals by X-ray diffraction, Mossbauer spectroscopy and solid-state nuclear magnetic resonance

The structures of one synthetic and two natural chlorites of the chlinochlore type were explored using X-ray diffraction, magic-angle spinning nuclear magnetic resonance (NMR) and Mössbauer spectroscopy. Rietveld refinements indicated that all structures are of the trioctahedral ordered IIb polytype. Mössbauer spectra provided the ratio IIFe/IIIFe but gave no evidence for the presence of IIIFe in the brucite-like sheet. We also report unit-cell parameters, Mössbauer isomeric shifts, Si NMR chemical shifts as well as 27Al isotropic shifts and quadrupolar coupling parameters. Very broad 29Si NMR peaks from the natural samples prevented us from obtaining accurate information on the Si-Al ordering in the tetrahedral sheets; the limitations of 29Si NMR as applied to natural chlorites are discussed. High-resolution 3QMAS NMR resolved the 27Al signal of the M4 octahedral site in the brucite-like sheet from the other three Al signals of crystallographically inequivalent octahedral positions.

Distribution of major and minor elements in coexisting minerals from a metamorphosed iron formation

Abstract A mineral chemical investigation has been made on coexisting biotite, Caamphibole and iron oxides from the metamorphosed iron formation in Grangesberg, Sweden. The distribution of major and trace elements among these minerals is investigated. The distribution for most of the elements was found to be regular, indicating a close approach to chemical equilibrium between the coexisting phases. The mineral compositions are shown to be restricted by the pressure of oxygen which varies within the iron formation. Deviations in the distribution coefficients for Ti and Mn are found to be related to the content of tetrahedral Al in Ca-amphibole and Mn in Ca-amphibole respectively. Fractionation of the transition elements was found to be dependent on their respective crystal field stabilization in octahedral environments.

The high-pressure phase transformation and breakdown of MgFe2O4

Abstract The high-pressure transformation of MgFe2O4 was studied by Mössbauer and Raman spectroscopy and synchrotron X-ray diffraction using the DAC technique and laser annealing at temperatures of 1500-2000 K. The high-pressure phase of MgFe2O4 was observed from in situ Mössbauer spectra at 17 ± 1 GPa after laser annealing by the appearance of two quadrupole doublets. This indicates a disordered distribution of Mg and Fe in an early stage. The displacive nature of the transformation of the spinel into its high-pressure polymorph was shown at increasing pressure by the redistribution of iron into only one site. After decompression Mössbauer spectroscopy revealed the presence of Fe2O3 in the sample. This was further confirmed by Raman spectroscopy at ambient conditions and by in situ high-pressure XRD, indicating a partial breakdown of the spinel into its constituent oxides MgO and Fe2O3. The XRD pattern of the high-pressure phase of MgFe2O4 can be indexed in agreement with the CaMn2O4-type structure, with cell parameters a = 2.775(2), b = 9.283(16), and c = 9.446(5) Å at 23 ± 2 GPa. The multiphase spectra from all three analytical methods suggests that inhomogeneous conditions prevailed in the DAC experiments, resulting in two different reactions at high pressure and temperature, i.e., T < 1800 K: MgFe2O4 → Fe2O3 + MgO and T > 1800 K: MgFe2O4 → hp-MgFe2O4.