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Between 1993 and 2007, an estimated 2500–3000 individual moldavite pieces have been found in the Tertiary Cheb Basin, Western Bohemia. This identifies the area as the third most prominent source of Central European tektites, next to the South Bohemian and West Moravian strewn subfields. Basic macroscopic physical properties (weight, shape, color and sculpture) were evaluated for over 350 individual finds of tektites from 4 different localities in the Cheb Basin. All these properties are similar to those observed for the South Bohemian moldavites, particularly with respect of color and weight distribution. In total, 24 tektites from the Cheb Basin have been characterized chemically using electron microprobe. For comparison, a set of 17 moldavites from the South-Bohemian and Moravian strewn subfields was measured as well. Contents of major elements overlap between the two sample sets; the largest variation was observed for iron. The trends observed in the Harker plots, however, seem to differentiate several partial subgroups, some of them characteristic for Cheb tektites only. These results are also substantiated by cluster analysis, which reveals a tight group for most of the tektites from the Cheb Basin, forming two partial clusters. The rest of the Cheb moldavites cluster with the South Bohemian samples. Minor and trace elements were measured with an LA-ICP-MS technique; CI-normalized REE patterns compare well with those for other moldavites. Many tektites, both from Cheb and South Bohemia or Moravia, display considerable heterogeneity: they frequently show schlieren and fluidal fabric. Two samples of this kind from the Cheb Basin showed considerable enrichment in volatile elements (e.g., Zn and Cu), which is typical for Muong Nong-type Australasian tektites. Mossbauer spectroscopy confirmed the highly reducing character of 5 studied moldavites. Discovery of a new moldavite strewn subfield around Cheb substantiates the theory that moldavites were ejected from the Ries impact structure in a fan-shaped jet, although it is not clear yet if it was continuous or composed of individual rays. In addition, the chemistry of the Cheb moldavites indicates significant precursor material heterogeneity.

aB stra C t The Pt-graphite double-capsule technique is a very commonly used method in high-temperature, high-pressure experimental petrology, particularly for anhydrous experiments relevant to primitive basaltic magmas and mantle melting. We have performed a series of experiments that place better constraints on the range of oxygen fugacity imposed by this capsule material, on the Fe 3+ /Fe 2+ ratios in experimentally produced melts and minerals, and on the temperature reproducibility in Pt-graphite capsules. Oxygen fugacity in our piston-cylinder experiments using Pt-graphite capsules is CCO-0.7 (IW+1.5, QFM-2.2) at 1.5 GPa and 1360 °C. Comparison with other estimates and thermodynamic calculations indicate that a value of CCO-0.8 ± 0.3 can be used as a first approximation at least over the P-T range relevant for MORB and OIB magma generation (0.5–3.0 GPa, 1100–1500 °C). Under those conditions, the amount of Fe 3+ in silicate phases (pyroxenes, olivine, glass) and spinel is negligible (Fe 3+ / ΣFe < 0.05) and would not significantly affect thermodynamic properties. Significantly higher values of fO2 cannot be achieved using Pt-graphite or graphite only capsules, but fO2 can be tuned to lower values by using small pieces of PtFe alloys. The potential range of fO2 that can be reached in graphite or Pt-graphite capsules is CCO to CCO-4. Temperature reproducibility in piston-cylinder experiments has been examined and can be as low as ±10 °C. Finally, unless capsules are dried overnight at 400 °C before the experiment, small amounts of H 2O are always present in nominally dry experiments. These small amounts of H 2O should not, however, significantly change phase relations.

The oxidation state, reflected in the oxygen fugacity (fO2), of the subcratonic lithospheric mantle is laterally and vertically heterogeneous. In the garnet stability field, the Kaapvaal lithospheric mantle becomes progressively more reducing with increasing depth from Δlog fO2 FMQ-2 at 110 km to FMQ-4 at 210 km. Oxidation accompanying metasomatism has obscured this crystal-chemical controlled depth-fO2 trend in the mantle beneath Kimberley, South Africa. Chondrite normalized REE patterns for garnets, preserve evidence of a range in metasomatic enrichment from mild metasomatism in harzburgites to extensive metasomatism by LREE-enriched fluids and melts with fairly unfractionated LREE/HREE ratios in phlogopite-bearing lherzolites. The metasomatized xenoliths record redox conditions extending up to Δlog fO2 = FMQ, sufficiently oxidized that magnesite would be the stable host of carbon in the most metasomatized samples. The most oxidized lherzolites, those in or near the carbonate stability field, have the greatest modal abundance of phlogopite and clinopyroxene. Clinopyroxene is modally less abundant or absent in the most reduced peridotite samples. The infiltration of metasomatic fluids/melts into diamondiferous lithospheric mantle beneath the Kaapvaal craton converted reduced, anhydrous harzburgite into variably oxidized phlogopite-bearing lherzolite. Locally, portions of the lithospheric mantle were metasomatized and oxidized to an extent that conversion of diamond into carbonate should have occurred.

The Earth’s lowermost mantle displays an important mineralogical transition from perovskite to post-perovskite but the spin state of iron in these phases remains poorly known. Experimental results suggest that iron occurs in the intermediate spin state in both of these phases, which implies that changes in physical properties of the lower mantle must be governed by factors other than spin transitions in iron.

A high-pressure quasi-single crystal X-ray diffraction study of a synthetic iron oxide Fe0.95O sample was performed up to 80 GPa using the diamond anvil cell technique. At a pressure of about 75 GPa after laser annealing we observed a new high-pressure monoclinic modification of FeO with P21/m space group. The structure is closely related to that of cubic sodium chloride as a distortion of the lattice. The sequence of transitions cubic-trigonal-monoclinic for FeO follows group-subgroup relations.

Oxygen thermobarometry measurements on spinel peridotite rocks indicate that the oxygen fugacity at the top of the upper mantle falls within ±2 log units of the fayalite-magnetite-quartz (FMQ) oxygen buffer. Measurements on garnet peridotites from cratonic lithosphere reveal a general decrease in fo2 with depth, which appears to result principally from the effect of pressure on the controlling Fe3+/Fe2+ equilibria. Modeling of experimental data indicates that at approximately 8 GPa, mantle fo2 will be 5 log units below FMQ and at a level where Ni-Fe metal becomes stable. Fe-Ni alloy and an Fe2O3-garnet component will be formed as a result of the disproportionation of FeO, which is experimentally demonstrated through observations of high Fe3+/ΣFe ratios in minerals in equilibrium with metallic Fe. In the lower mantle, the favorable coupled substitution of Al and Fe3+ into (Fe,Mg)SiO3 perovskite results in very high perovskite Fe3+/ΣFe ratios in equilibrium with metallic Fe. As a result, the lower mantle should contain approximately 1 weight% metallic Fe formed through FeO disproportionation, if the bulk oxygen content is the same as the upper mantle. Loss of disproportionated metallic Fe from the lower mantle during core formation could explain the higher Fe3+/ΣFe ratio of the present-day upper mantle when compared to that expected during core formation. The influence of pressure on mantle fo2 has important implications for the speciation of C-O-H-S volatile phases in Earth today and during its early evolution.

Li contents [Li] and isotopic composition (δ 7 Li) of mafic minerals (mainly amphibole and clinopyroxene) from the alkaline to peralkaline Ilimaussaq plutonic complex, South Greenland, track the behavior of Li and its isotopes during magmatic differentiation and final cooling of an alkaline igneous system. [Li] in amphibole increase from b10 ppm in Caamphiboles of the least differentiated unit to N3000 ppm in Na-amphiboles of the highly evolved units. In contrast, [Li] in clinopyroxene are comparatively low (b85 ppm) and do not vary systematically with differentiation. The distribution of Li between amphibole and pyroxene is controlled by the major element composition of the minerals (Ca-rich and Na-rich, respectively) and changes in oxygen fugacity (due to Li incorporation via coupled substitution with ferric iron) during magmatic differentiation. δ 7 Li values of all minerals span a wide range from +17 to −8‰, with the different intrusive units of the complex having distinct Li isotopic systematics. Amphiboles, which dominate the Li budget of whole-rocks from the inner part of the complex, have constant δ 7 Li of +1.8±2.2‰ (2σ, n=15). This value reflects a homogeneous melt reservoir and is consistent with their mantle derivation, in agreement with published O and Nd isotopic data. Clinopyroxenes of these samples are consistently lighter, with Δ 7 Liamph-cpx as large as 8‰ and are thus not in Li isotope equilibrium. These low values probably reflect late-stage diffusion of Li into clinopyroxene during final cooling of the rocks, thus enriching the clinopyroxene in 6 Li. At the margin of the complex δ 7 Li in the syenites increases systematically, from +2 to high values of +14‰. This, coupled with the observed Li isotope systematics of the granitic country rocks, reflects post-magmatic open-system processes occurring during final cooling of the intrusion. Although the shape and magnitude of the Li isotope and elemental profiles through syenite and country rock are suggestive of diffusion-driven isotope fractionation, they cannot be modeled by one-dimensional diffusive transport and point to circulation of a fluid having a high δ 7 Li value (possibly seawater) along the chilled contact. In all, this study

The crystal structures of a suite of amphiboles from the Kola Superdeep Borehole, Russia, have been refined to R values of ∼3% using single-crystal Mo-Kα X-ray diffraction data. The same crystals used in the collection of the intensity data were subsequently analysed by electron microprobe (EMP) and milliprobe Mössbauer spectroscopy. Site populations were assigned from the results of site-scattering refinement and stereochemical analysis, taking into account the unit formula determined for each crystal. The Fe3+/(Fe2++Fe3+) values were derived (1) by least-squares refinement of the Mössbauer spectra, and (2) SREF (Structure REFinement) by careful analysis of the mean bond lengths at the M(2) site using two possible models for the behaviour of Ti4+: (i) Ti4+ at M(2); and (ii) Ti4+ at M(1). The agreement between the SREF and Mössbauer/EMP values for Fe3+ is very close for Ti4+ assigned to M(2). This result indicates that the calculation of Fe3+ contents in amphiboles from refined site populations and distances are accurate. This paper presents the first confirmation of this result.

Magnetic and structural phase relations in FeO and MnO were studied in situ using Mossbauer spectroscopy, X-ray and neutron powder diffraction at high pressures and over a broad temperature range. Two principal transition boundaries were mapped: antiferromagnetic ordering (the Neel transition) and the structural distortion from the cubic B1 to a rhombohedral rB1 lattice. These two transitions are not necessarily coupled and the relations between the structural and magnetic properties of transition metal monoxides are discussed.

The high-pressure elastic behaviour and the pressure-induced structural evolution of synthetic magnetite were investigated up to 11.11(5) GPa by means of in situ single-crystal X-ray diffraction with a diamond anvil cell, using the mix methanol:ethanol:water = 16:3:1 as pressure-transmitting medium and the ruby-fluorescence method for pressure-calibration. The evolution of the ruby R1-fluorescence line with P, with a drastic increase of the full-width-at-half-maximum (FWHM) of the Lorentzian profile at P > 9 GPa, shows that the P-medium is not hydrostatic above 9 GPa. Such a condition is well reflected by the drastic increase (by 20–22%) of the FWHM of the diffraction peak profiles of magnetite, by the behaviour of the Eulerian finite strain versus normalized pressures plot (f e –F e plot) and by the slight EoS-misfit. However, the diffraction data collected during decompression showed a reversible/complete restoration of the diffraction profiles and of the elastic behaviour in the f e –F e plot. The reflection conditions dictated by the $$ Fd\bar 3m $$ space group confirm that symmetry of magnetite is maintained within the P-range investigated. The structural refinements performed at 0.0001, 4.99(3) and 9.21(8) GPa show that the evolution of the oxygen u-parameter is almost constant within the P-range investigated. A weighted linear regression through the data points gives only a slight negative slope and no discontinuity is observed within the P-range investigated. A similar continuous behaviour is also observed in the evolution of the T- and M-polyhedral volumes, bond distances and angles with P. On the basis of data reported in this study, it appears that the elastic behaviour and the structural evolution of magnetite is drastically influenced by the experimental conditions (i.e., hydrostatic or non-hydrostatic), and diffraction data of magnetite collected under non-hydrostatic conditions are unusable for a reliable description of the elastic behaviour and for the P-induced structural rearrangements. Comparisons are carried out between the experimental findings of this study and those reported in the previous ones, in which an inverse-to-direct spinel phase transition at P > 8 GPa is suggested.

The transition zone of the Earth's upper man- tle is characterized by three discontinuities in seismic wave velocity profiles. One of these at about a depth of 520 km is assigned to the transformation of wadsleyite (β-) to ring- woodite (γ-(Mg, Fe) 2 SiO 4 ) (e.g., Shearer in J Geophys Res 101:3053-3066, 1996). The exact location, width, and other properties of that discontinuity are affected by a mul- titude of parameters. The present study specifically focuses on the effect of water, iron content, and redox environment on the depth of the phase transition. We performed high- pressure experiments in a multi-anvil apparatus at 1200 °C with variation in Mg-Fe compositions (0.10 < xFe < 0.24), water contents (0 < xH2O < 2 wt%), and the redox envi- ronment (using different buffers: Fe/FeO (reducing), Re/ ReO 2 (oxidizing)). Run products were investigated using electron microprobe and Fourier transform infrared spec- troscopy to obtain the composition including the hydroxyl

Ferropericlase (fPer) inclusions from kimberlitic lower-mantle diamonds recovered in the Juina area, Mato Grosso State, Brazil were analyzed with transmission electron microscopy, electron energy-loss spectroscopy and the flank method. The presence of exsolved non-stoichiometric Fe3+-enriched clusters, varying in size from 1–2 nm to 10–15 nm and comprising ∼3.64 vol.% of fPer was established. The oxidation conditions necessary for fPer formation within the uppermost lower mantle ( P = 25 GPa , T = 1960 K ) vary over a wide range: Δ log ⁡ f O 2 (IW) from 1.58 to 7.76 ( Δ = 6.2 ) , reaching the fayalite–magnetite–quartz (FMQ) oxygen buffer position. This agrees with the identification of carbonates and free silica among inclusions within lower-mantle Juina diamonds. On the other hand, at the base of the lower mantle Δ log ⁡ f O 2 values may lie at and below the iron–wustite (IW) oxygen buffer. Hence, the variations of Δ log ⁡ f O 2 values within the entire sequence of the lower mantle may reach ten logarithmic units, varying from the IW buffer to the FMQ buffer values. The similarity between lower- and upper-mantle redox conditions supports whole mantle convection, as already suggested on the basis of nitrogen and carbon isotopic compositions in lower- and upper-mantle diamonds. The mechanisms responsible for redox differentiation in the lower mantle may include subduction of oxidized crustal material, mechanical separation of metallic phase(s) and silicate–oxide mineral assemblages enriched in ferric iron, as well as transfer of fused silicate–oxide material presumably also enriched in ferric iron through the mantle.

The oxygen fugacity (fO2) at which carbonate-bearing melts are reduced to either graphite or diamond in synthetic eclogite compositions has been measured in multi-anvil experiments performed at pressures between 3 and 7 GPa and temperatures between 800 and 1,300 °C using iron–iridium and iron–platinum alloys as sliding redox sensors. The determined oxygen fugacities buffered by the coexistence of elemental carbon and carbonate-bearing melt are approximately 1 log unit below thermodynamic calculations for a similar redox buffering equilibrium involving only solid phases. The measured oxygen fugacities normalized to the fayalite–magnetite–quartz oxygen buffer decrease with temperature from ~−0.8 to ~−1.7 log units at 3 GPa, most likely as a result of increasing dilution of the carbonate liquid with silicate. The normalized fO2 values also decrease with pressure and show a similar decrease with temperature at 6 GPa from ~−1.5 log units at 1,100 °C to ~−2.4 log units at 1,300 °C. In contrast to previous arguments, the stability field of the carbonate-bearing melt extends to lower oxygen fugacity in eclogite rocks than in peridotite rocks, which implies a wider range of conditions over which carbon remains mobile in natural eclogites. The raised prevalence of diamonds in eclogites compared to peridotites may, therefore, reflect more effective scavenging of carbon by melts in these rocks. The ferric iron contents of monomineralic layers of clinopyroxene and garnet contained in the same experiments were also measured using Mossbauer spectroscopy. A preliminary model was derived for determining the fO2 of eclogitic rocks from the compositions of garnet and clinopyroxene, including the Fe3+/ΣFe ratio of garnet, using the equilibrium, $$\mathop { 5 {\text{CaFeSi}}_{2} {\text{O}}_{6} }\limits_{\text{cpx}} + \mathop {1/3{\text{Ca}}_{3} {\text{Al}}_{2} {\text{Si}}_{3} {\text{O}}_{12} }\limits_{\text{garnet}} + {\text{O}}_{2} = \mathop { 2 {\text{Ca}}_{3} {\text{Fe}}_{2} {\text{Si}}_{3} {\text{O}}_{12} }\limits_{\text{garnet}} + \mathop {1/3{\text{Fe}}_{3} {\text{Al}}_{2} {\text{Si}}_{3} {\text{O}}_{12} }\limits_{\text{garnet}} + \mathop {4{\text{SiO}}_{2} }\limits_{\text{coesite}}$$ The model, which reproduces the independently determined fO2 of the experimental data to within 0.5 log units, can be used to estimate the fO2 of ultrahigh-pressure metamorphic eclogites and cratonic eclogitic xenoliths. Although there are very few analyses of garnet Fe3+/ΣFe ratios from eclogite samples, the range in fO2 recorded by available eclogitic xenoliths is similar to that reported for peridotitic xenoliths and generally within the graphite/diamond stability field. Estimates for the average bulk Fe3+/ΣFe ratio of modern basaltic oceanic crust, however, are higher than the values for most of these xenoliths, and upon subduction, crustal carbon is likely to remain in the carbonate stability field to depths of at least 250 km. If eclogite xenoliths originated from subducted oceanic crust, then their generally lower fO2 most likely reflects either lower initial basaltic Fe3+/ΣFe ratios, loss of Fe2O3 through partial melting or the initial presence of organic carbon.

Podiform chromitites enclosed in depleted harzburgites of the Luobusa massif (southeastern Tibet) contain diamond and a highly reduced trace-mineral association. Exsolution of diopside and coesite from chromite suggests inversion from the Ca-ferrite structure in the upper part of the mantle transition zone (>400 km). However, the trace-element signatures of the chromites are typical of ophiolitic chromitites, implying primary crystallization at shallow depths. Os-Ir nuggets in the chromitites have Re-Os model ages (T RD ) of 234 ± 3 Ma, while T RD ages of in situ Ru-Os-Ir sulfides range from 290 to 630 Ma, peaking at ca. 325 Ma. Euhedral zircons in the chromitites give U-Pb ages of 376 ± 7 Ma, e Hf = 9.7 ± 4.6, and d 18 O = 4.8‰–8.2‰. The sulfide and zircon ages may date formation of the chromitites from boninite-like melts in a supra-subduction-zone environment, while the model ages of Os-Ir nuggets may date local reduction in the transition zone following Devonian subduction. Thermo-mechanical modeling suggests a rapid (≲10 m.y.) rise of the buoyant harzburgites from >400 km depth during the early Tertiary and/or Late Cretaceous rollback of the Indian slab. This process may occur in other collision zones; mantle samples from the transition zone may be more widespread than currently recognized.

Skiagite-rich garnet was synthesized as single crystals at 9.5 GPa and 1100 °C using a multi-anvil apparatus. The crystal structure [cubic, space group Ia 3 d , a = 11.7511(2) A, V = 1622.69(5) A3, D calc = 4.4931 g/cm3] was investigated using single-crystal synchrotron X-ray diffraction. Synchrotron Mossbauer source spectroscopy revealed that Fe2+ and Fe3+ predominantly occupy dodecahedral (X) and octahedral (Y) sites, respectively, as expected for the garnet structure, and confirmed independently using nuclear forward scattering. Single-crystal X-ray diffraction suggests the structural formula of the skiagite-rich garnet to be Fe32+(Fe2+0.234(2)Fe3+1.532(1)Si4+0.234(2))(SiO4)3, in agreement with electron microprobe chemical analysis. The formula is consistent with X-ray absorption near-edge structure spectra. The occurrence of Si and Fe2+ in the octahedral Y-site indicates the synthesized garnet to be a solid solution of end-member skiagite with ~23 mol% of the Fe-majorite end-member Fe32+(Fe2+Si4+)(SiO4)3.

Fe-bearing carbonates have been proposed as possible candidate host minerals for carbon inside the Earth’s interior and hence their spectroscopic properties can provide constraints on the deep carbon cycle. Here we investigate high-pressure spin crossover in synthetic FeCO 3 (siderite) using a combination of Mossbauer, Raman, and X-ray absorption near edge structure spectroscopy in diamond-anvil cells. These techniques sensitive to the short-range atomic environment show that at room temperature and under quasi-hydrostatic conditions, spin crossover in siderite takes place over a broad pressure range, between 40 and 47 GPa, in contrast to previous X-ray diffraction data that described the transition as a sharp volume collapse at approximately 43 GPa. Based on these observations we consider electron spin pairing in siderite to be a dynamic process, where Fe atoms can be either high spin or low spin in the crossover region. Mode Gruneisen parameters extracted from Raman spectra collected at pressures below and above spin crossover show a drastic change in stiffness of the Fe-O octahedra after the transition, where they become more compact and hence less compressible. Mossbauer experiments performed on siderite single crystals as well as powder samples demonstrate the effect of differential stress on the local structure of siderite Fe atoms in a diamond-anvil cell. Differences in quadrupole splitting values between powder and single crystals show that local distortions of the Fe site in powder samples cause spin crossover to start at higher pressure and broaden the spin crossover pressure range.

We have investigated the partitioning of Fe3+ between orthopyroxene (Opx) and garnet (Grt) in well-equilibrated mantle xenoliths using Mossbauer spectroscopy. The samples cover a wide range of P–T conditions (2.1–6.6 GPa, 690–1,412 °C) and geothermal gradients, and are thus representative for Earth’s upper mantle in both on-craton and off-craton continental settings. Garnet has Fe3+/Fetot ratios of 0.03–0.13 and Fe2O3 contents of 0.24–1.00 wt%. Orthopyroxene has, on average, lower Fe3+/Fetot ratios (0.01–0.09) and Fe2O3 contents (0.05–0.63 wt%). In low-pressure, high-temperature samples, however, Opx is systematically richer in Fe2O3 than the coexisting Grt. The Fe3+ Opx/Grt partition coefficient $$\left( {D_{{{\text{Fe}}^{ 3+ } }}^{\text{Opx/Grt}} } \right)$$ shows no obvious relationship with temperature, but increases with decreasing pressure and with increasing NaOpx. The observed Opx/Grt Fe3+ systematics imply that the Opx–Grt Fe–Mg exchange thermometer is not robust against redox changes if total Fe is treated as Fe2+. An approximate evaluation of errors on T estimates due to redox effects predicts negligible deviations for strongly reduced conditions ( to ≫100 °C) for strongly oxidized conditions, especially at very high pressure and when both P and T are calculated by iteration.

Hydrogenation of C60 at 2 GPa and 723–823 K was performed with thermal decomposition of LiAlH4 as a hydrogen source. Analysisof hydrogenation products showed that the method can produce not only C60H36, but also hydrofullerides C60Hx with 44 < x < 52.Unlike other hydrogenation methods in which pristine C60 reacts with hydrogen, the high pressure/high temperature conditions resultfirst in C60 polymerization into a tetragonal phase which later reacts with hydrogen. It is suggested that hydrogen first attacks intermolecularcarbon–carbon bonds, resulting in complete depolymerization of C60 at a later stage of the hydrogenation process. This modelsuggests the possibility to select starting points for hydrogenation by choosing specific polymeric structures of C60. Hydrogenated polymericC60 was identified as an intermediate reaction product. Characterization of highly reduced material was performed by Raman andIR spectroscopy, X-ray diffraction, and atmospheric pressure photoionization Fourier transform ion cyclotron resonance massspectrometry.

Combined X-ray powder diffraction, Mossbauer, and XANES spectroscopy in situ experiments revealed the transformation of cubic (Mg0.8Fe0.2)O ferropericlase to a rhombohedrally distorted phase at 35(1) GPa and room temperature. The Mossbauer spectroscopy results show that the rhombohedral distortion does not involve magnetic ordering. Combined with data from the literature, our results imply that the cubic to rhombodedral transition occurs in (Mg,Fe)O under conditions of non-hydrostatic stress over a wide range of composition (0.2≤x Fe≤1).

We report Mossbauer spectroscopy and electron paramagnetic resonance spectroscopy (EPR) studies of a number of New Zealand minerals that can be classified under the non‐specific names “pounamu” or “greenstone”. Mossbauer measurements allowed the distribution of iron in its +2 and +3 oxidation states to be assigned to the M1 to M4 sites of the constituent minerals, tremolite‐actinolite in the case of nephrite jades. EPR at two frequencies, 9.4 GHz (X‐band) and 94.1 GHz (W‐band), allowed the determination of the distribution of Mn2+ in these sites. The results are apparently contrary to the usual assignments made for the parent minerals tremolite and actinolite. In two cases, one a nephrite jade and the other a serpentine, significant quantities of magnetically ordered minerals chalcopyrite and magnetite were identified by Mossbauer spectroscopy. These minerals could not be found in the X‐ray diffraction patterns of the same minerals leading to the conclusion that they are very finely divided in th...

Examination of schorlomite from ijolite at Magnet Cove (USA) and silicocarbonatite at Afrikanda (Russia), using electron-microprobe and hydrogen analyses, X-ray diffraction and Mossbauer spectroscopy, shows the complexity of substitution mechanisms operating in Ti-rich garnets. These substitutions involve incorporation of Na in the eightfold-coordinated X site, Fe2+ and Mg in the octahedrally coordinated Y site, and Fe3+, Al and Fe2+ in the tetrahedrally coordinated Z site. Substitutions Ti4+Fe3+Fe3+−1Si−1 and Ti4+Al3+Fe3+−1Si−1 are of major significance to the crystal chemistry of schorlomite, whereas Fe2+ enters the Z site in relatively minor quantities (

Germanate garnets are often used as isostructural analogues of silicate garnets to provide insight into the crystal chemistry and symmetry of the less accessible natural garnet solid solutions. We synthesised two series of germanate garnets at 3 GPa along the joinVIIICa 3 VI (CaGe)IVGe3O12–VIIICa 3 VI Fe 2 IV Ge3O12 at 900 °C and 1,100 °C. Samples with compositions close to the CaGeO3 end-member consist of tetragonal garnet with a small amount of triclinic CaGe2O5. Samples with nominal compositions between XFe=0.4 and 1.0 consist of a mixture of tetragonal and cubic garnets; whereas, single-phase cubic garnets were obtained for compositions with XFe>1.2 (XFe gives the iron content expressed in atoms per formula unit, and varies between 0 and 2 along the join). Run products which were primarily single-phase garnet were investigated using Mossbauer spectroscopy. Spectra from samples synthesised at 1,100°C consist of one well-resolved doublet that can be assigned to Fe3+ in the octahedral site of the garnet structure. A second doublet, present primarily in samples synthesised at 900°C, can be assigned to Fe2+ at the octahedral sites of the garnet structure. The relative abundance of Fe2+ decreases with increasing iron content. Transmission electron microscopy analyses confirm this tendency and show that the garnets are essentially defect-free. The unit-cell parameters of tetragonal VIIICa 3 VI (CaGe)IVGe3O3 garnet decrease with increasing synthesis temperature, and the deviation from cubic symmetry becomes smaller. Cubic garnets show a linear decrease of unit-cell parameter with increasing iron content. The results are discussed in the context of iron incorporation into VIIIMg 3 VI (MgSi)IVSi3O3 majorite.

We have heated ferropericlase (Mg0.80Fe0.20)O to temperatures of over 2300 K at pressures of above 100 GPa to investigate the stability of the solid solution at elevated P, T conditions. The in situ X-ray study in laser-heated diamond anvil cells, as well as X-ray powder diffraction and chemical analysis of recovered samples, show that ferropericlase dissociates to magnesium oxide and ferrous oxide rich components. We demonstrate the importance of the combination of different analytical techniques for studies of solid-state chemical processes at extreme conditions.

Wadsleyite II is a variably hydrous magnesium-iron silicate phase similar to spinelloid IV and a potential host for H in the Transition Zone of the Earth’s mantle. Two separate samples of wadsleyite II synthesized at 17.5 GPa and 1400°C and at 18 GPa and 1350°C have been characterized by electron microprobe, single-crystal X-ray diffraction, visible, IR, Raman, and Mossbauer spectroscopies, and transmission electron microscopy including electron energy-loss spectroscopy. The two samples have the following chemical formulae: Mg1.71Fe0.18Al0.01H0.33Si0.96O4 and Mg1.60Fe0.22Al0.01 H0.44Si0.97O4. Mossbauer spectroscopy and electron energy loss spectroscopy (EELS) indicate that about half of the iron present is ferric. Refinement of the structures shows them to be essentially the same as spinelloid IV. Calculated X-ray powder diffraction patterns show only subtle differences between wadsleyite and wadsleyite II. The hydration mechanism appears to be protonation of the non-silicate oxygen (O2) and possibly the oxygens surrounding the partially vacant tetrahedral site Si2, charge-balanced by cation vacancies in Si2, M5 and M6. The unit cell volume of this phase and its synthesis conditions indicate that it may be an intermediate phase occurring between the fields of wadsleyite and ringwoodite, if sufficient trivalent cations are available. The unit cell parameters have been refined at pressures up to 10.6 GPa by single-crystal X-ray diffraction in the diamond anvil cell. The refined bulk modulus for the sample containing 2.8 wt% H2O is 145.6 ± 2.8 GPa with a K’ of 6.1 ± 0.7. Similar to wadsleyite and ringwoodite, hydration has a large effect on the bulk modulus. The presence of this phase in the mantle could serve to obscure the seismic expression of the phase boundary between wadsleyite and ringwoodite near 525 km. The large apparent effect of hydration on bulk modulus is consistent with hydration having a larger effect on seismic velocities than temperature in the Transition Zone.

Metamorphic Fe-bearing dravite from a glaucophane schist from Syros, Greece, associated with omphacite, has been characterized by chemical analyses (EMPA, SIMS, Mossbauer study) and by crystal structure determination. The optimized formula, calculated using chemical and structural data (including from Mossbauer spectroscopy) is x (Na 0.96 Ca 0.02 □ 0.02 ) Y (Mg 1.29 Al 0.99 Fe 2+ 0.44 Fe 3+ 0.19 Ti 0.05□ 0.04 ) Z (Al 4.90 Mg 1.10 ) T (Si 5.83 B 0.17 ) B 3 O 27 [(OH) 3.93 F 0.07 ], with a = 15.9443(3), c = 7.2094(3) A, R = 0.017. The OH- content is nearly 4 apfu , thus there is no significant O 2− at the V and W site. The X site is nearly completely filled with Na, contrary to most natural tourmalines. We conclude that tourmaline samples from the dravite-schorl series where the O1 site ( W site) is mainly occupied by OH- and/or F, but not by O 2− , favour an X -site occupation with Na. Mg and Al are strongly disordered in this tourmaline sample. This Mg-rich tourmaline is an unusual example of Al-Mg disorder which is not driven by the short-range requirements of O 2− at the O 1 site. Surprisingly, we found small amounts of [4] B in this Mg-rich dravite. This is confirmed by the chemical analysis (including light elements), by the refinement (∼ 0.26 [4] B apfu ), as well as by the relatively small T -O> distance of 1.6179 A. This is the first known example of a Mg-rich tourmaline which contains significant amounts of [4] B. The formation of this dravite (at PT conditions of ∼ 6 to 7 kbar/∼ 400°C) took place in a subduction-exhumation environment at undersaturated SiO 2 -conditions.

A room temperature high pressure in situ Mossbauer study of wustite Fe 0.94 O was carried out in a diamond anvil cell under hydrostatic conditions at pressures up to 13.5(1) GPa. At room temperature magnetic ordering (Neel transition) occurs at 5.0(5) GPa, which differs significantly from the pressure reported in the literature (ca. 15 GPa) for the phase transformation from the sodium chloride structure (B1) to a rhombohedral structure.

The authors report a redox profile based on Mossbauer data of spinel and garnet to a depth of 210 km from mantle xenoliths of the northern (N) and southeastern (SE) Slave craton (northern Canada). The profile transects three depth facies of peridotites that form segments of different bulk composition, represented by spinel peridotite, spinel–garnet peridotite, low-temperature garnet peridotite, high-temperature garnet peridotite, and pyroxenite. The shallow, more depleted N Slave spinel peridotite records lower oxygen fugacities compared to the deeper, less depleted N Slave spinel–garnet peridotite, consistent with their different spinel Fe3+ concentrations. Garnet peridotites show a general reduction in Δlog fO2 (FMQ)s with depth, where values for garnet peridotites are lower than those for spinel–garnet peridotites. There is a strong correlation between depletion and oxygen fugacity in the spinel peridotite facies, but little correlation in the garnet peridotite facies. The strong decrease in Δlog fO2 (FMQ) with depth that arises from the smaller partial molar volume of Fe3+ in garnet, and the observation of distinct slopes of Δlog fO2 (FMQ) with depth for spinel peridotite compared to spinel–garnet peridotite strongly suggest that oxygen fugacity in the cratonic peridotitic mantle is intrinsically controlled by iron equilibria involving garnet and spinel.

Samples of olivine, wadsleyite, ringwoodite and majorite with Mg/(Mg+Fe)=0.85–0.90 were synthesised at high pressure and temperature using a multianvil press under nominally anhydrous conditions as well as hydrous conditions with varying conditions of oxygen fugacity, and examined ex situ using Mossbauer spectroscopy to determine Fe3+/ΣFe, and either Fourier Transform Infrared Spectroscopy or Secondary Ion Mass Spectrometry to determine H concentration. Under oxidising conditions, the relative concentration of Fe3+ increases with water content in olivine, wadselyite and ringwoodite. Majorite does not incorporate significant water, but shows high Fe3+/ΣFe values when synthesised under oxidising conditions in the presence of water. Fe3+/ΣFe is strongly influenced by oxygen fugacity for all phases studied, and the mechanism of hydration is likely coupled to Fe3+ incorporation in the crystal structure. Enhanced Fe3+/ΣFe in mantle-derived olivine and orthopyroxene has a negligible effect on oxygen fugacities estimated using olivine–orthopyroxene–garnet oxybarometers, while increased Fe3+/ΣFe in garnet due to dehydrogenation could lead to an overestimation of oxygen fugacity in exhumed samples. Subduction zones are likely oxidised relative to the rest of the mantle, and hence represent regions where not only water is concentrated, but also Fe3+.

Inclusions of ferropericlase and former (Mg,Fe)(Si,Al)O3 perovskite in diamonds from Kankan, Guinea believed to originate in the lower mantle were studied using Mossbauer spectroscopy to determine Fe3+/ΣFe. Fe3+ concentration in the (Mg,Fe)(Si,Al)O3 inclusion is consistent with empirical relations relating Fe3+/ΣFe to Al concentration, supporting the inference that it crystallised in the perovskite structure at lower mantle conditions. In ferropericlase there is a nearly linear variation of trivalent cation abundance with monovalent cation abundance, suggesting a substitution of the form Na0.5M0.53+O (M=Fe3+, Cr3+, Al3+). Excess positive charge is likely balanced by cation vacancies, where their abundance is observed to increase with increasing iron concentration, consistent with high-pressure experiments. The abundance of cation vacancies is related to oxygen fugacity, where ferropericlase inclusions from Kankan and Sao Luiz (Brazil) are inferred to have formed at conditions more oxidising than Fe–(Mg,Fe)O equilibrium, but more reducing than Re–ReO2 equilibrium. Fe2+/Mg partition coefficients between (Mg,Fe)(Si,Al)O3 and ferropericlase were calculated for inclusions co-existing in the same diamond using Mossbauer data and empirical relations based on high-pressure experimental work. Most values are consistent with high-pressure experiments, suggesting that these inclusions equilibrated at lower mantle conditions. The measured ferropericlase Fe3+ concentrations are consistent with diamond formation in a region of redox gradients, possibly arising from the subduction of oxidised material into reduced lower mantle. Reduction of carbonate to form ferropericlase and diamond is consistent with a slight shift of Kankan δ13C values to isotopically heavy compositions compared to the worldwide dataset, and could supply the oxygen necessary to satisfy the high Fe3+ concentration in (Mg,Fe)(Si,Al)O3 perovskite, as well as account for the high proportion of ferropericlase in the lower mantle paragenesis. The heterogeneity of lower mantle diamond sources indicates that the composition of lower mantle diamonds do not necessarily reflect those of the bulk mantle.

The oxidation state of iron can significantly influence the physical and chemical properties of lower mantle minerals. To improve methods for estimation of Fe3+/∑Fe, synthetic assemblages of (Mg,Fe)(Si,Al)O3 perovskite and (Mg,Fe)O ferropericlase were synthesised from oxide starting mixtures in Re or Fe capsules at 26 GPa and 1650–1850 °C using a multianvil press. (Mg,Fe)(Si,Al)O3 majorite was also present in some of the run products. Both electron energy loss spectra (EELS) and Mossbauer spectra were measured for each run product, and a robust fitting method was developed for Mossbauer spectra using EELS results as a standard that enabled Fe3+/∑Fe of (Mg,Fe)(Si,Al)O3 perovskite to be determined from Mossbauer spectra of multiphase assemblages. There is a close to linear variation between Fe3+/∑Fe and Al concentration in (Mg,Fe)(Si,Al)O3 perovskite, independent of oxygen fugacity. The concentration of Fe3+ in (Mg,Fe)O increases with increasing iron concentration along curves of constant oxygen fugacity, where higher oxygen fugacity stabilises greater Fe3+ concentrations. Fe2+/Mg partition coefficients calculated from chemical composition data corrected for measured Fe3+/∑Fe showed values nearly identical within experimental error for all samples, and independent of Al concentration and oxygen fugacity. Simple empirical relations were derived to calculate Fe3+/∑Fe in (Mg,Fe)(Si,Al)O3 perovskite and (Mg,Fe)O ferropericlase samples for which no Mossbauer or EELS data were available, and tested by applying them to calculation of Fe2+/Mg partition coefficients from literature data for (Mg,Fe)(Si,Al)O3 perovskite-(Mg,Fe)O assemblages where only total iron concentrations had been measured. Results showed Fe2+/Mg partition coefficients that were equal to existing values within experimental error, hence confirming the validity of the empirical relations.

The relatively low-variance mineral assemblage of a talc-kyanite eclogite from Dabie Shan enabled application of both conventional geothermobarometers (garnet-clinopyroxene geothermometer and the garnet-omphacite-phengite geobarometer) and a multi-equilibrium method to determine peak P-T conditions (THERMOCALC, “average PT”). The results were highly discrepant: 840 °C / 31.1 kbar vs. 590 °C / 29.8 kbar. Mossbauer spectroscopy showed that Fe3+/Fetotal in omphacite was significantly higher than the value obtained from standard formula recalculation. When the activities were corrected for Fe endmembers, geothermobarometry gave consistent results (606 °C / 31.3 kbar vs. 585 °C / 30.8 kbar). These are close to those obtained earlier by “average PT”, confirming the robustness of the multi-equilibrium approach. The high Fe3+ concentration in omphacite is best explained using a Ca-eskola endmember Ca0.5[]0.5AlSi2O6 and allowing corresponding vacancies in the omphacite structure.

One of the most widely used methods to estimate magmatic oxygen fugacity involves the use of empirical equations relating fO2 to the iron redox state in quenched silicate liquids; however none of the equations have been calibrated experimentally under subliquidus conditions at temperatures and oxygen fugacities that are relevant to natural magmas. To address this problem, we tested two empirical relationships [Eq. (1) in Kress and Carmichael 1991; Eq. (6) in Nikolaev et al. 1996] on synthetic glasses synthesized from a ferrobasaltic and a transitional alkali-basaltic composition at sub- to superliquidus temperatures (1,132–1,222°C) and controlled oxygen fugacities (ΔFMQ=−2 to +1.4). Fe3+/ΣFe was determined using conventional and milliprobe Mossbauer spectroscopy, and verified using wet chemical analysis on selected samples. For the ferrobasaltic bulk composition “SC1-P”, both empirical models reproduce the Fe3+/ΣFe ratio of the quenched liquids generally within 0.03 for sub- as well as superliquidus temperatures, although agreement is worse at higher oxygen fugacities (ΔFMQ>+1) at subliquidus temperatures. For the transitional alkali-basaltic composition “7159V-P”, both models reproduce the Fe3+/ΣFe ratio of the quenched liquids generally within 0.04, although agreement is worse for both models at high oxygen fugacities (ΔFMQ>+1). Such behaviour may be related to a change in melt structure, where a progressive change in Fe3+ coordination is inferred to occur as a function of Fe3+/ΣFe based on Mossbauer center shifts. Recasting the data in terms of oxygen fugacity shows that calculated oxygen fugacities deviate from those actually maintained during the equilibration of the sample material by generally no more than 0.5 log-bar unit, with maximum deviations that only rarely exceed one log-bar unit.

At ambient pressure the solubility of magnesium in iron is very limited (of the order of 0.025 at.% in the liquid state), and vice versa. In a series of experiments using the piston–cylinder and multi-anvil apparatus at pressures up to 20 GPa and temperatures up to 2000 °C, we found an increase in the solid solubility of magnesium in iron up to 4 at.% Mg. The solution of magnesium in the bcc-structured Fe-based alloy increases the lattice parameter by approximately 2.4% with respect to that of pure iron.

The oxidation state recorded by rocks from the Earth's upper mantle can be calculated from measurements of the distribution of Fe3+ and Fe2+ between the constituent minerals1,2,3. The capacity for minerals to incorporate Fe3+ may also be a significant factor controlling the oxidation state of the mantle4,5, and high-pressure experimental measurements of this property might provide important insights into the redox state of the more inaccessible deeper mantle. Here we show experimentally that the Fe3+ content of aluminous silicate perovskite, the dominant lower-mantle mineral, is independent of oxygen fugacity. High levels of Fe3+ are present in perovskite even when it is in chemical equilibrium with metallic iron. Silicate perovskite in the lower mantle will, therefore, have an Fe3+/total Fe ratio of at least 0.6, resulting in a whole-rock ratio of over ten times that of the upper mantle5,6. Consequently, the lower mantle must either be enriched in Fe3+ or Fe3+ must form by the disproportionation of Fe2+ to produce Fe3+ plus iron metal. We argue that the lower mantle contains approximately 1 wt% of a metallic iron-rich alloy. The mantle's oxidation state and siderophile element budget have probably been influenced by the presence of this alloy.

We have synthesized samples in the system BaTi 1− x Fe x O 3− x/ 2 with x =0.1−0.6 at temperatures of 1200–1300°C under reducing conditions of oxygen fugacity. After drop quenching, samples were characterized using the electron microprobe, X-ray diffraction and Mossbauer spectroscopy. All samples were hexagonal with a 6H-BaTiO 3 type structure. Mossbauer spectroscopy showed all iron to be present as Fe 3+ , occurring in octahedral and pentahedral sites. Analysis of area ratios indicates that oxygen vacancies are distributed randomly over O1 sites, and that a random distribution of Fe and Ti cations over M 1 and M 2 sites is consistent with the data. No evidence for ordering of oxygen vacancies was found. Results are consistent with conductivity results, which show generally increasing ionic conductivity with increasing oxygen vacancy concentration.

The chemical potential of oxygen (lO2 )i n equilibrium with magnesiowustite solid solution (Mg, Fe)O and metallic Fe has been determined by gas-mixing experiments at 1,473 K supplemented by solid-cell EMF experiments at lower temperatures. The results give

The structure of the metallic high-pressure Fe3O4 polymorph : experimental and theoretical study

Seven separate samples of hydrous ringwoodite with compositions ranging from Fo100 to Fo89 and hydrogen contents from 0.2 to 1.1 wt% were synthesized in the 5000 ton multi-anvil press at the Bayerisches Geoinstitut. Synthesis conditions ranged from 18 to 22 GPa and 1400 to 1500 rC. The crystals were characterized by single-crystal X-ray diffraction, electron microprobe, IR and Mossbauer spectroscopy, and by analytical and high-resolution transmission electron microscopy. The crystals are optically isotropic, and the Fe-bearing samples are deep blue in color. Mossbauer spectroscopy and ELNE spectroscopy applied to the Fe-bearing samples indicates about 10% of the iron is in the ferric state. High-resolution TEM examination of one of the Fe-bearing samples indicates that the crystals are homogeneous and free of significant inclusions or exsolution features. Infrared spectra show a broad absorption band extending from about 2500 to 3600 cm –1 with maxima ranging from 3105 for the pure magnesian samples to 3150 cm –1 for the Fo89 samples. The crystal structures of the seven ringwoodite samples were refined by X-ray single-crystal diffraction. Refinement of cation site occupancies indicates full occupancy of the tetrahedral site for all samples, whereas the occupancy of the octahedral site appears to decrease systematically with H content. The principal hydration mechanism involves octahedral cation vacancies. The IR spectra are consistent with protonation of the short O-O approach on the tetrahedral edge, which would imply partial Mg-Si disorder.

Infrared, optical absorption spectroscopy, and Mossbauer spectroscopy were used to investigate the color of jade from New Zealand. Spectroscopic results were supplemented by chemical analyses and petrological examination. Infrared spectra gave a quick identification of the matrix, optical absorption spectra gave information on color in relation to observed absorption bands, and Mossbauer spectra gave the distribution of Fe2+ and Fe3+ at the cation sites and also show how the Fe3+/Fe2+ ratio increases due to oxidative weathering. The development of the attractive flecking in the gem-quality jade is due to agglomerations of colloidally dispersed magnetite or chromite that can also lead to the formation of black spots. Darker samples are generally high in total iron, although not all lightly colored (or pale) samples are low in iron—cream or white unweathered nephrite can also contain high iron concentrations. Weathering under the climatic conditions where the samples occur can produce either a brown, hydrated iron oxide, or a whitish outer rind if the acidity is high enough to remove the oxide. In either case the nephrite matrix is unaltered. Two quite rare variations were found and ascribed to: (1) incomplete nephrite formation in samples developed in association with an unusual ultramafic protolithology and (2) the formation of chromian margarite giving rise to a bluish-green (pseudo) jade.

A kinetic model for the rate of iron–hydrogen redox exchange in silicate glasses and melts has been derived from time-series experiments performed on natural rhyolitic obsidians. Cylinders of the starting glasses were exposed to reducing mixtures composed of H2-Ar-CO2-CO in 1-atm furnaces and H2-Ar in a cold seal pressure vessel. Overall, runs covered the temperature range 300 to 1000°C. The progression of a front of ferric iron reduction within the quenched melt was observed optically through a change of color. For all run conditions, the advancement of the front (ξ) was proportional to the square root of time, revealing the reaction as a diffusion-limited process. Iso-fO2 runs performed in CO2-CO, H2-Ar, and H2-CO2 gases have shown that fH2 rather than fO2 is the dominant parameter controlling the reaction rate. The fH2 dependence of the rate constant was characterized in the range 0.02 to 70 bar. The growth of the reduced layer, which is accompanied by an increase in reaction-derived OH-group content, was fitted considering that the reaction rate is controlled by the migration of a free mobile species (H2) immobilized in the form of OH subsequent to reaction with ferric iron. The reaction rate is thus a function of both solubility and diffusivity of H2 weighted by the concentration of its sink (ferric iron). We extracted a single law for both solubility and diffusivity of H2 in amorphous silicates that applies over a range of temperatures below and above the glass transition temperature. Melt/glass structure (degree of polymerization) does not seem to significantly affect both solubility and diffusivity of H2. We therefore provide a model that allows the prediction of oxidation–reduction rates in the presence of hydrogen for a wide range of compositions of amorphous glasses and melts. Comparisons with previous work elucidating rate of redox exchange in dry systems allow us to anticipate the fH2-T domains where different redox mechanisms may apply. We conclude that equilibration of redox potential in nature should be dominated by H2 transfer at a rate controlled by both H2 solubility and diffusion. Numerical applications of the model illustrate redox exchanges in natural magmas and in glasses exposed to weathering under near surface conditions. We show that crustal events such as magmas mixing should not modify the iron redox state of magmas. In the case of nuclear-waste-bearing glasses, the fH2 conditions in the host terrain are clearly a parameter that must be taken into account to predict glass durability.

Fifteen samples of (Mg,Fe)SiO3 majorite with varying Fe/Mg composition and one sample of (Mg,Fe)(Si,Al)O3 majorite were synthesized at high pressure and temperature under different conditions of oxygen fugacity using a multianvil press, and examined ex situ using X-ray diffraction and Mossbauer and optical absorption spectroscopy. The relative concentration of Fe3+ increases both with total iron content and increasing oxygen fugacity, but not with Al concentration. Optical absorption spectra indicate the presence of Fe2+–Fe3+ charge transfer, where band intensity increases with increasing Fe3+ concentration. Mossbauer data were used in conjunction with electron microprobe analyses to determine the site distribution of all cations. Both Al and Fe3+ substitute on the octahedral site, and charge balance occurs through the removal of Si. The degree of Mg/Si ordering on the octahedral sites in (Mg,Fe)SiO3 majorite, which affects both the c/a ratio and the unit cell volume, is influenced by the thermal history of the sample. The Fe3+ concentration of (Mg,Fe)(Si,Al)O3 majorite in the mantle will reflect prevailing redox conditions, which are believed to be relatively reducing in the transition zone. Exchange of material across the transition boundary to (Mg,Fe) (Si,Al)O3 perovskite would then require a mechanism to oxidize sufficient iron to satisfy crystal-chemical requirements of the lower-mantle perovskite phase.

Kinetics and reaction paths of Fe 3+ reduction by H 2 in high-Fe and low-H 2 O silicate melts have been investigated at 800 °C. Time-series experiments were performed in cold-seal pressure vessels at 50 bars of pure H 2 using rapid-heating and rapid-quench strategies. Within the first minutes of the experiments, a fast partitioning of Na occurred between the gas and the melt due to the reducing conditions. Kinetically decoupled from the Na partitioning, the progression of a front of Fe 3+ reduction within the quenched melt was observed and was identified as a diffusion-limited process. The growth of the reduced layer is accompanied by an increase in concentration of OH-groups suggesting that reduction operates through proton incorporation within the melt. As this growth rate is slightly faster than predicted from the diffusion of molecular H 2 O, a different and mobile water-derived species seems likely. One possible mechanism is the reduction of Fe 3+ by the transport of molecular H 2 . As this process is limited by the flux of H 2 , it will depend on both diffusivity and solubility of H 2 in the melt. Alternatively, migration of protons (H + ) and electronic species within the melt could control the velocity of the reduction front. The increase in concentration of the reaction-derived OH groups produces a water over saturation followed by partial dehydration of the melt. This dehydration leads to a change in the redox conditions within the gas that influences the Na partitioning between gas and melt.

The programMossAprovides a straightforward approach to the fitting of57Fe conventional and synchrotron energy-domain Mössbauer spectra. Sites can be defined simply by mouse clicks and hyperfine parameters can be constrained to constant values, within specific ranges, and can be coupled linearly between different subspectra. The program includes a full transmission integral fit with Lorentzian line shape (conventional source) or Lorentzian-squared line shape (synchrotron source). The fitting process is graphically displayed in real time while fitting and can be interrupted at any time. Gaussian-shaped quadrupole splitting distributions for analyzing nonmagnetic amorphous materials are included.MossAis designed especially for the rapid and comprehensive analysis of complex Mössbauer spectra, made possible by its native graphical user input.

La ferriallanite-(Ce), de composition ideale CaCeFe 3 + AlFe 2 + (SiO 4 )(Si 2 O 7 )O(OH), est un analogue de l'allanite-(Ce) a dominance de Fe 3 + dans le site octaedrique M(1). Cette nouvelle espece minerale, d'origine metasomatique, provient d'une pegmatite granitique alcaline au mont Ulyn Khuren, chaine de montagnes Altai (environ 48°32'N, 92°55'E), en Republique Populaire de Mongolie. Les cristaux de ferriallanite-(Ce) sont noirs, opaques ou translucides, avec une rayure brune; l'eclat est resineux. La durete de Mohs est 6; c'est un mineral cassant, sans clivage ou plan de separation observe. La fracture est conchoidale a inegale. Sa densite est 4.22 g/cm 3 . Elle est biaxe (-), a 1.825(2), β 1.855(5), γ 1.880(5), 2V c a l c 83°. Les cinq raies les plus intenses du spectre de diffraction (methode des poudres) [d(A)(I)(hkl)] sont: 2.93(65)(113), 2.72(80)(120,013), 2.63(60)(311), 2.18(100)(122,123,401) et 2.14(80)(221,403,223,014). Nous en avons affine la structure cristalline jusqu'a un residu R de 0.026 en utilisant 2296 reflexions observees prelevees avec un diffractometre a quatre cercles [a 8.962(2), b 5.836(2), c 10.182(2) A, β 115.02(1)°, groupe spatial P2 1 /m]. Nous precisons la position de l'atome d'hydrogene. D'apres les donnees chimiques, l'affinement de la structure, la spectroscopie de Mossbauer et les calculs de la distribution des charges, nous avons pu attribuer les cations aux sites cristallographiques independants, et nous obtenons la formule cristallochimique suivante: (Ca 0 . 9 7 Ce 0 . 0 3 )(Ce 0 . 8 9 Ca 0 . 1 1 )(Fe 3 + 0 . 8 0 Ti 4 + 0 . 1 4 Al 0 . 0 6 )(Al 0 . 5 6 Fe 3 + 0 . 4 4 )(Fe 2 + 0 . 9 3 Mn 0 . 0 7 )(Si 0 . 9 4 Al 0 . 0 6 O 4 )(Si 2 O 7 )O(OH). Dans le meme echantillon, une analyse ponctuelle indique une composition ideale CaCeFe 3 + 2 Fe 2 + (SiO 4 )(Si 2 O 7 )O(OH), mais dans un volume insuffisant pour permettre une caracterisation complete de ce que semble etre un nouveau membre du groupe de l'epidote.

The partitioning of Fe and Mn between the large M(4) site and the octahedral sites, M(1,2,3) in the amphibole structure has been investigated in two natural manganogrunerites of compositions Ca0.1Mn1.9 Mg1.25Fe2+ 3.56Fe3+ 0.38Si7.81O22(OH)2 and Ca0.24Mn1.57 Mg2.27 Fe2+ 2.76Fe3+ 0.32Si7.84O22(OH)2. The long-range cation distribution in the two samples has been elucidated by in situ neutron powder diffraction revealing that Mn is preferentially ordered onto M(4) ≫ M(2) >M(1) >M(3) in both samples. Partitioning of Mn from M(4) into the octahedral sites begins at 350 °C, with site exchange energies of −16.6 kJ mol−1 and −14.9 kJ mol−1, in samples containing 1.90 and 1.57 Mn apfu, respectively. Mossbauer and infrared spectroscopy have been used to study the samples at room temperature, and Mossbauer data agree well with the diffraction results, confirming that high-temperature cation distributions are retained during cooling. The fine structure in the hydroxyl-stretching region of the IR absorption spectra has been used to discuss qualitatively the site occupancies of the coordinating M(1)M(3)M(1) triplet, linked by O(3). On the basis of such modelling, we conclude that a degree of local clustering is present in both samples.

Neustadtelite and cobaltneustadtelite, two new minerals related to medenbachite, were found on samples from the dumps of the Guldener Falk mine near Schneeberg-Neustadtel, Saxony, Germany. The general appearance of the two new minerals is very similar: small tabular crystals up to 0.2 mm in diameter, transparent to translucent, with a brown color and a light brown streak; the lustre is adamantine. Both minerals are optically biaxial negative, 2 V = 65(5)°, n x = 2.02(2), n y = 2.09 (calc.), n z = 2.12(2); pleochroism is strong with X = brown to opaque, Y = yellow, Z = pale yellow. Mohs’ hardness is 4.5. The cleavage parallel to {001} is good. The chemical compositions were derived by means of electron-microprobe analyses. Average contents for neustadtelite/cobaltneustadtelite are (in wt%): Bi2O3 52.58/51.54, PbO 0.08/0.08, CaO 0.15/0.32, Fe2O3 13.92/10.90, Al2O3 0.29/0.07, CoO 3.35/5.47, NiO 0.34/1.61, ZnO 0.09/0.39, CuO 0.07/0.00, As2O5 26.82/25.91, P2O5 0.23/0.43, H2O (calc.) 2.56/3.01, total 100.48/99.73. Mossbauer spectra of cobaltneustadtelite and medenbachite confirmed that all of the iron is trivalent. Based on 12 O atoms, the empirical formulae for the neustadtelite and cobaltneustadtelite type materials are (Bi1.94Ca0.02)∑1.96Fe1.00(Fe0.50Co0.38Ni0.04Al0.05Zn0.01 Cu0.01)∑0.99[(OH)2.44O1.40]∑3.84[(AsO4)2.01(PO4)0.03]∑2.04 and (Bi1.91Ca0.05)∑1.96Fe1.02(Co0.63Fe0.16Ni0.19 Zn0.04Al0.01)∑1.03[(OH)2.88O1.12]∑4.00[(AsO4)1.95(PO4)0.05]∑2.00, respectively. As derived from chemical analyses and crystal-structure investigations the ideal end-member compositions are Bi2Fe3+Fe3+O2 …

A high-pressure Mossbauer spectroscopic study of [57Fe(PPIX)OH] is reported under conditions where the pressure is increased and then released. Data were recorded over the pressure range from room pressure to 7.1 GPa using a modified Merril-Bassett Diamond Anvil cell. [57Fe(PPIX)OH] exhibits an asymmetric quadrupole doublet at room temperature and pressure, caused by population of higher energy levels known as Kramer’s doublets. Under the application of pressure a S = 5/2 high spin to S = 5/2, 3/2 admixed spin state transition of the Fe(III) site is seen to occur above pressures of 2.2 GPa. This follows a general trend observed with other porphyrin compounds. This is thought to be evidence of movement of the ligand towards the iron atom and movement of the iron atom towards the porphyrin plane. Further evidence for this motion is found in the decrease in the asymmetry of the original site, which is caused by changes in population of the energy levels of the Kramer’s doublets. At the highest reported pressures a reversal in asymmetry is observed for the inner S = 5/2 quadrupole doublet.

Transmission Mossbauer spectra of synthetic Ca-free P2(1)/c Mg0.22Fe0.78SiO3 clinopyroxene were collected at temperatures in the range 4.2 to 745 K and in an external magnetic field of 60 kOe at 180 K. The magnetic order-disorder transition temperature was determined by Mossbauer thermoscanning to be 21 +/- 3 K. Above this temperature, all Mossbauer spectra consist of a superposition of two doublets, respectively produced by Fe2+ ions at an almost regular octahedral M I site and at a more distorted octahedral M2 site. The temperature variation of the Fe2+ center shifts were analyzed using the Debye model for the lattice vibrations. The characteristic Mossbauer temperatures were found to be 356 K +/- 35 K for M1 and 333 K +/- 25 K for M2. From the external field (60 kOe) Mossbauer spectrum recorded at 180 K, the principal component V-zz of the electric field gradient (EFG) was determined to be positive for both sites but precise values for the magnitudes of the asymmetry parameters eta of the EFG could not be determined. The temperature variations of the M1 and M2 quadrupole splittings DeltaE(Q)(T) are consistent with the higher distortion of the M2 octahedra. Using the crystal-field model to interpret DeltaE(Q)(T), the energy gaps delta (1) and delta (2) of the first excited electronic states within the D-5 orbital term were estimated to be 410 +/- 50 cm(-1) and 730 +/- 50 cm(-1) for M1 and delta (1) = 1050 +/- 75 cm(-1) for M2.

The crystal structure of baricite, the Mg-dominant analogue of vivianite, (Mg 1.70 Fe 2+ 1.30− x Fe 3+ x )(PO 4 ) 2 (OH) x ·(8− x )H 2 O, a 10.085(2), b 13.390(3), c 4.6713(9) A, β 104.96(3)°, V 609.4(1) A 3 , space group C 2/ m , Z = 2, ρ calc = 2.440 g/cm 3 , has been determined [automated single-crystal diffractometer, Mo K α, graphite monochromator, image-plate area-detector system, T = 193 K, 5714 reflections, w R 2 = 0.106 for all 1244 unique reflections, R = 0.043 for 965 observed reflections with I ≤ 2σ( I )]. A refinement of site occupancies shows that Fe and Mg are partially ordered in the structure. According to X-ray and Mossbauer data, the position at the center of symmetry ( M 1) is mainly occupied by Fe, whereas Mg prefers the position at the two-fold axis ( M 2); Fe atoms occupy two-thirds of the M 1 and one-third of M 2 octahedra. The positions of four independent H atoms were obtained from difference-Fourier syntheses and were refined under isotropic approximation. The crystal structure of baricite consists of two type of octahedral structural units: isolated M 1O 2 (H 2 O) 4 octahedra and clusters of two edge-sharing polyhedra M 2O 6 (H 2 O) 4 . These octahedral units are connected in the structure by orthophosphate tetrahedra to form layers two octahedra thick parallel to the ac plane. In the b direction, these layers are held together by hydrogen bonds. The occurrence in nature of an oxidized form, a triclinic symplesite-like phase, is considered to be highly probable.