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Acta crystallographica / Section E 76(5), 715 - 719 (2020). doi:10.1107/S2056989020005411, The crystal structure of MgCO$_3$-II has long been discussed in the literature where DFT-based model calculations predict a pressure-induced transition of the carbon atom from the sp$^2$ to the sp$^3$ type of bonding. We have now determined the crystal structure of iron-bearing MgCO$_3$-II based on single-crystal X-ray diffraction measurements using synchrotron radiation. We laser-heated a synthetic (Mg$_{0.85}$Fe$_{0.15}$)CO$_3$ single crystal at 2500 K and 98 GPa and observed the formation of a monoclinic phase with composition (Mg$_{2.53}$Fe$_{0.47}$)C$_3$O$_9$ in the space group C2/m that contains tetra­hedrally coordinated carbon, where CO$_4$$^{4-}$ tetra­hedra are linked by corner-sharing oxygen atoms to form three-membered C$_3$O$_3$$^{6-}$ ring anions. The crystal structure of (Mg$_{0.85}$Fe$_{0.15}$)CO$_3$ (magnesium iron carbonate) at 98 GPa and 300 K is reported here as well. In comparison with previous structure-prediction calculations and powder X-ray diffraction data, our structural data provide reliable information from experiments regarding atomic positions, bond lengths, and bond angles., Published by National Center for Biotechnology Information, U.S. National Library of Medicine15046, Bethesda, MD

Extreme pressures and temperatures are known to drastically affect the chemistry of iron oxides, resulting in numerous compounds forming homologous series nFeOmFe2O3 and the appearance of FeO2. Here, based on the results of in situ single-crystal x-ray diffraction, Mössbauer spectroscopy, x-ray absorption spectroscopy, and density-functional theory+dynamical mean-field theory calculations, we demonstrate that iron in high-pressure cubic FeO2 and isostructural FeO2H0.5 is ferric (Fe3+), and oxygen has a formal valence less than 2. Reduction of oxygen valence from 2, common for oxides, down to 1.5 can be explained by a formation of a localized hole at oxygen sites. © 2021 American Physical Society. We acknowledge the Deutsches Elektronen-Synchrotron (DESY, PETRA III), the European Synchrotron Radiation Facility (ESRF), and the Advance Photon Source (APS) for provision of beamtime. N. D. and L. D. thank the Federal Ministry of Education and Research, Germany (BMBF, Grant No. 05K19WC1) and the Deutsche Forschungsgemeinschaft (DFG Projects No. DU 954–11/1, No. DU 393–9/2, and No. DU 393–13/1) for financial support. N. D. and I. A. A. thank the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009 00971). Electronic structure calculations were supported by the Russian Science Foundation (Project No. 18-12-00492). Theoretical analysis of chemical bonding was supported by the Ministry of Science and Higher Education of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISIS” (No. K2-2019-001) implemented by a governmental decree, No. 211. Support from the Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant No. KAW-2018.0194), the Swedish Government Strategic Research Areas and SeRC, and the Swedish Research Council (VR) Grant No. 2019-05600 is gratefully acknowledged.

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Single crystals of δ-AlOOH, δ-(Al 0.953 ,Fe 0.047 )OOH, and δ-(Al 0.878 ,Fe 0.122 )OOH with dimensions up to ~0.4–0.6 mm were synthesized by the high-pressure hydrothermal method. Synthesis experiments were performed at 21 GPa and 1470 K for 4 h using a Kawai-type multi-anvil apparatus. The crystals of δ-AlOOH, δ-(Al 0.953 ,Fe 0.047 )OOH, and δ-(Al 0.878 ,Fe 0.122 )OOH were colorless, yellowish green, and brown, respectively. Mossbauer spectra showed 95–100% Fe 3+ /∑Fe at the octahedral site in δ-(Al,Fe)OOH. Chemical compositions of δ-(Al 0.953 ,Fe 0.047 )OOH and δ-(Al 0.878 ,Fe 0.122 )OOH are homogeneous with Fe/(Al+Fe) of 0.0469(8) and 0.122(3), respectively. Unit-cell parameters of δ-AlOOH are consistent with those of previous studies, and they increase with Fe/(Al+Fe). These results confirm that δ-AlOOH can form a solid solution with ɛ-FeOOH. The crystals contained a small number of fluid inclusions. The syntheses of large single crystals of δ-(Al,Fe)OOH will facilitate investigation of their phase stability, physical properties including elasticity and elastic anisotropy, behavior of hydrogen bonding, and spin state of Fe, which will improve models of the water and oxygen cycles in the deep Earth.

Magnesite (MgCO3), calcite (CaCO3), dolomite [(Ca, Mg)CO3], and siderite (FeCO3) are among the best-studied carbonate minerals at high pressures and temperatures. Although they all exhibit the calcite-type structure ( $${\text{R}}\bar{3}{\text{c}}$$ ) at ambient conditions, they display very different behavior at mantle pressures. To broaden the knowledge of the high-pressure crystal chemistry of carbonates, we studied spherocobaltite (CoCO3), which contains Co2+ with cation radius in between those of Ca2+ and Mg2+ in calcite and magnesite, respectively. We synthesized single crystals of pure spherocobaltite and studied them using Raman spectroscopy and X-ray diffraction in diamond anvil cells at pressures to over 55 GPa. Based on single crystal diffraction data, we found that the bulk modulus of spherocobaltite is 128 (2) GPa and K′ = 4.28 (17). CoCO3 is stable in the calcite-type structure up to at least 56 GPa and 1200 K. At 57 GPa and after laser heating above 2000 K, CoCO3 partially decomposes and forms CoO. In comparison to previously studied carbonates, our results suggest that at lower mantle conditions carbonates can be stable in the calcite-type structure if the radius of the incorporated cation(s) is equal or smaller than that of Co2+ (i.e., 0.745 A).

Angewandte Chemie / International edition 56(25), 7302-7306 (2017). doi:10.1002/anie.201702440, Published by Wiley-VCH, Weinheim

The new mineral ferrorhodonite, a Mn2+–Fe2+ ordered analogue of rhodonite with the idealized formula CaMn3Fe[Si5O15], was found in the manganese-rich metamorphic rocks of the Broken Hill Pb–Zn–Ag deposit, Yancowinna Co., New South Wales, Australia. Ferrorhodonite occurs as brownish red coarsely crystalline aggregates in association with galena, chalcopyrite, spessartine, and quartz. The mineral is brittle. Its Mohs hardness is 6. Cleavage is perfect on {201} and good on {021} and {210}. The measured and calculated values of density are 3.71 (2) and 3.701 g cm−3, respectively. Ferrorhodonite is optically biaxial positive, with α = 1.731 (4), β = 1.736 (4), γ = 1.745 (5) and 2 V (meas.) = 80 (10)°. The average chemical composition of ferrorhodonite is (electron-microprobe data, wt%): CaO 7.09, MgO 0.24, MnO 32.32, FeO 14.46, ZnO 0.36, SiO2 46.48, and total 100.95. The empirical formula calculated on 15 O apfu (Z = 2) is Ca0.81Mn2.92Fe1.29Mg0.04Zn0.03Si4.96O15. The Mossbauer and IR spectra are reported. The strongest reflections in the powder X-ray diffraction pattern [(d, A (I, %) (hkl)] are: 3.337 (32) (−1–13), 3.132 (54) (−210), 3.091 (41) (0–23), 2.968 (100) (−2–11), 2.770 (91) (022), 2.223 (34) (−204), 2.173 (30) (−310). Ferrorhodonite is isostructural with rhodonite. The crystal structure was solved based on single-crystal X-ray diffraction data and refined to R 1 = 4.02% [for 3114 reflections with I > 2σ(I)]. The mineral is triclinic, space group P $$\bar{1}$$ , a = 6.6766 (5), b = 7.6754 (6), c = 11.803 (1) A, α = 105.501 (1)°, β = 92.275 (1)°, γ = 93.919 (1)°; V = 580.44 (1). The crystal-chemical formula of ferrorhodonite inferred to be: M5(Ca0.81Mn0.19) M1−3(Mn2.52Fe0.48) M4(Fe 0.81 2+ Mn0.12Mg0.04Zn0.03) [Si5O15]. .

Coupled substitution of Fe 3+ and H + for Si in wadsleyite was studied by polarized infrared and Mossbauer spectroscopies and single-crystal X-ray diffraction. Single crystals of Fe-bearing hydrous wadsleyite were synthesized at 16 GPa and 1870 K using a Kawai-type multi-anvil apparatus. Water and Fe contents of the sample were 0.19–0.26 wt% H 2 O and Fe/(Mg+Fe) of 0.099(2), respectively. Mossbauer spectra showed 13(4)% Fe 3+ /∑Fe and Fe 3+ at the tetrahedral site with 5(3)% IV Fe 3+ /∑Fe. Crystal structure refinement by single-crystal X-ray diffraction indicated that Fe (presumably Fe 3+ ) occupied 4.9(5)% of the tetrahedral site. Infrared light polarized with the electric vector E // a and c was absorbed at 3477(2) cm −1 while no absorption was observed at the region in spectra with E // b . The pleochroic behavior of the 3477 cm −1 band can be interpreted as protonation of silicate oxygen O3 in Fe-bearing hydrous wadsleyite. The protonation of O3 together with the presence of Fe 3+ at the tetrahedral site confirms the coupled substitution of Fe 3+ and H + for Si in Fe-bearing hydrous wadsleyite.

Nature Communications 9(1), 4142 (2018). doi:10.1038/s41467-018-06457-x, A Verwey-type charge-ordering transition in magnetite at 120 K leads to the formation of linear units of three iron ions with one shared electron, called trimerons. The recently-discovered iron pentoxide (Fe$_4$O$_5$) comprising mixed-valent iron cations at octahedral chains, demonstrates another unusual charge-ordering transition at 150 K involving competing formation of iron trimerons and dimerons. Here, we experimentally show that applied pressure can tune the charge-ordering pattern in Fe$_4$O$_5$ and strongly affect the ordering temperature. We report two charge-ordered phases, the first of which may comprise both dimeron and trimeron units, whereas, the second exhibits an overall dimerization involving both the octahedral and trigonal-prismatic chains of iron in the crystal structure. We link the dramatic change in the charge-ordering pattern in the second phase to redistribution of electrons between the octahedral and prismatic iron chains, and propose that the average oxidation state of the iron cations can pre-determine a charge-ordering pattern., Published by Nature Publishing Group UK, [London]

We present a high-pressure study of $\mathrm{M}{\mathrm{g}}_{1--x}\mathrm{F}{\mathrm{e}}_{x}\mathrm{O}$ (ferropericlase, Fp) single crystals $0.04(1)\ensuremath{\le}x\ensuremath{\le}0.33(1)$ with a focus on ferrous iron spin-state crossover and the material behavior preceding it. Using Vegard's law and highly accurate high-pressure single-crystal experimental data, we extract the lattice parameter ${a}_{\mathrm{FeO}}^{\mathrm{HS}}$ of the ``FeO high spin lattice contribution'' in the MgO-FeO solid solution. We find that ferropericlases with a wide range of compositions share the same critical parameter ${a}_{C}$ (defined as the minimum ${a}_{\mathrm{FeO}}^{\mathrm{HS}}$ after which spin crossover starts). Furthermore, we discuss the effect of composition on spin crossover in ferrous iron in ferropericlase in the limits of low and moderate concentrations of $\mathrm{F}{\mathrm{e}}^{2+}$.

Green vesuvianite crystals occur with garnet and calcite in a hand specimen from the Nedvědice marble near Kozlov (near Stěpanov nad Svratkou, Svratka Crystalline Complex) in the Czech Republic. The average electron microprobe composition of the vesuvianite shows 12.10 wt% Fe 2 O 3 (4.66 Fe pfu), 2.77 wt% B 2 O 3 (2.45 B pfu), 1.71 wt% As 2 O 5 (0.46 As pfu), and 1.40 wt% F (2.26 F pfu). The Fe concentration is the highest ever recorded for a vesuvianite-group mineral. The boron contents are extremely variable and two of the five compositions show more than the 2.50 B pfu needed for wiluite, and the average is only slightly less than this. The crystal structure [ a = 15.7250(4), c = 11.7736(3) A] was refined in space group P 4/ nnc to an R 1 value of 0.0221. The site refinement and Mossbauer spectroscopy results show Fe 2+ substituting for Ca at the X3 site and filling the Y1 position, and Fe 3+ substituting for Al at the Y3 position. Most of the Fe (70% from the site refinements and 78% from the Mossbauer interpretation) is ferric. The main effect of the high-Fe concentration is to increase the mean Y3-O distance to an unusually large 2.018 A. Boron occurs at the T1 site, where it is coordinated by oxygen atoms at two O7B and two O11 positions, and at the T2 sites where it is coordinated by O atoms at one O10 and two O12A sites. When the nearby X3 site contains Fe, the T2 position is either vacant or [3]-coordinated by some combination involving an O10 site and two O12B positions, in which case the B atom is likely offset from the T2 site to reduce the B-O12B distance. Fluorine and OH occupy the O11 positions when there is a vacancy at the adjacent T1 position. Pentavalent As substitutes for Si at the Z2 site and Al at the Y2 site. The P 4/ nnc symmetry indicates that this vesuvianite formed at high temperatures (400–800 °C) and the predominance of Fe 3+ and As 5+ suggests under oxidizing conditions. The results showing Fe at three different sites with three different coordinations attests to the flexibility of the vesuvianite crystal structure. The incorporation of As at two different sites in the structure shows that rock-forming silicate minerals such as vesuvianite can be a reservoir for this heavy element.

Ferri-kaersutite, ideally NaCa2(Mg3TiFe3+)(Si6Al2)O22O2, is a new oxo-amphibole from Harrow Peaks, Northern Victoria Land, Antarctica. It occurs as brown prismatic crystals, up to 200 μm in length, with a vitreous luster, and a perfect {110} cleavage. Ferri-kaersutite is associated with forsterite, diopside, and Cr-bearing spinel. Chemical analyses, by a combination of electron microprobe, SIMS, and 57Fe Mossbauer spectroscopy, gave the following results (in wt%): SiO2 41.69, TiO2 5.30, Al2O3 13.65, Cr2O3 0.09, Fe2O3 4.52, MgO 15.54, CaO 11.03, MnO 0.11, FeO 2.83, Na2O 2.88, K2O 0.96, H2O 0.70, F 0.24, Cl 0.08, O=(F,Cl) −0.12, sum 99.50. On the basis of 24 anions per formula unit, the formula is (Na0.816K0.179)∑0.995(Ca1.726Fe2+0.258Mn0.014)∑1.998(Mg3.383Fe2+0.088Ti0.582Fe3+0.497Al0.439Cr0.011)∑5.00 (Si6.089Al1.911)∑8.00O22[O1.187(OH)0.682F0.111Cl0.020]∑2.00. Ferri-kaersutite is monoclinic with space group C 2/ m . Its unit-cell parameters are a = 9.8378(8), b = 18.0562(9), c = 5.3027(4) A, β = 105.199(9)°, V = 908.99(13) A3, Z = 2. The five strongest reflections in its X-ray powder diffraction pattern [ d in A (relative visual intensity, hkl )] are: 8.4 (s, 110), 3.379 (ms, 131), 3.115 (ms, 310), 2.707 (s, 151), 2.598 (ms, 061). The crystal structure of ferri-kaersutite has been refined on the basis of 1783 observed reflections [ F o > 4σ( F o)] with a final R 1 = 0.038. The relatively large equivalent isotropic displacement parameter at M (1), with respect to those at M (2) and M (3) sites, together with the short M (1)–O(3) distance, suggest the occurrence of Ti4+ at the M (1) site, whereas the small octahedral distortion at this site suggests a low Fe3+ occupancy. This element is mainly hosted at the M (2) and M (3) sites. The occurrence of amphiboles in the magma source region is notably relevant. The melting of Ti-rich amphibole in the lithosphere and subsequent degrees of melt/host peridotite reactions are able to produce melts that account for the compositional spectrum ranging from extreme alkaline lavas to the most common alkaline basalts. In particular, when this amphibole is formed by reaction between a peridotite matrix and metasomatic melts/fluids with high Fe3+/Fetot ratio, its subsequent melting can influence primary volatile contents and ultimately magma rheology.

We present 27 Al and 29 Si magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra of Al- and Fe-bearing, high-pressure pyroxene and perovskite samples, synthesized in a multi-anvil apparatus at 26 GPa and 1900 °C at targeted compositions of (Mg 1−x Fe x )(Si 1−x Al x )O 3 (x = 0.01, 0.025, and 0.05). 27 Al MAS-NMR spectra of the perovskite samples indicate that Al 3+ replaces both Si 4+ in the octahedral site and Mg 2+ in the larger 12-coordinated site. NMR signal loss caused by paramagnetic interactions is often a severe complication when performing NMR on materials containing Fe 2+,3+ ; however, careful measurement of signal loss and comparison to total Fe content in these samples sheds light on the nature of Al and Fe incorporation. NMR signal loss for the pyroxenes is linearly related to total Fe content as would be expected in the case of uncorrelated substitution of randomly distributed Al and Fe. However, 27 Al signal loss for the perovskite samples increases only slightly between samples with x = 0.01 and 0.025 indicating similar coordination of Al by Fe and non-random distribution. Complete signal loss at Fe/(Fe + Mg) = 0.05 suggests the upper limit of Fe 2+ and Fe 3+ concentration at which useful NMR data can be obtained for this system.

The structure of Bi0.81Pb0.19FeO2.905 was investigated on different length scales using a combination of electron diffraction, high-resolution scanning transmission electron microscopy, synchrotron X-ray powder diffraction, and Mössbauer spectroscopy. In the 80300 K temperature range, the average crystal structure of Bi0.81Pb0.19FeO2.905 is a cubic Pm3̅m perovskite with a = 3.95368(3) Å at T = 300 K. The (Pb2+, Bi3+) cations and O2 anions are randomly displaced along the 110 cubic directions, indicating the steric activity of the lone pair on the Pb2+ and Bi3+ cations and a tilting distortion of the perovskite framework. The charge imbalance induced by the heterovalent Bi3+ → Pb2+ substitution is compensated by the formation of oxygen vacancies preserving the trivalent state of the Fe cations. On a short scale, oxygen vacancies are located in anion-deficient (FeO1.25) layers that are approximately 6 perovskite unit cells apart and transform every sixth layer of the FeO6 octahedra into a layer with a 1:1 mixture of corner-sharing FeO4 tetrahedra and FeO5 tetragonal pyramids. The anion-deficient layers act as twin planes for the octahedral tilting pattern of adjacent perovskite blocks. They effectively randomize the octahedral tilting and prevent the cooperative distortion of the perovskite framework. The disorder in the anion sublattice impedes cooperative interactions of the local dipoles induced by the off-center displacements of the Pb and Bi cations. Magnetic susceptibility measurements evidence the antiferromagnetic ordering in Bi0.81Pb0.19FeO2.905 at low temperatures.

A combined powder and single-crystal X-ray diffraction analysis of dolomite [CaMg(CO3)2] heated to 1,200°C at 3 GPa was made to study the order–disorder–reorder process. The order/disorder transition is inferred to start below 1,100°C, and complete disorder is attained at approximately 1,200°C. Twinned crystals characterized by high internal order were found in samples annealed over 1,100°C, and their fraction was found to increase with temperature. Evidences of twinning domains combined with probable remaining disordered portions of the structure imply that reordering processes occur during the quench. Twin domains are hereby proposed as a witness to thermally induced intra-layer-type cation disordering.

We report results from high-pressure single-crystal X-ray diffraction and Mossbauer absorption experiments on magnetite. Based on high-quality diffraction data, we have obtained accurate information on the crystal structure of magnetite below 25 GPa, which enables an unambiguous interpretation of the Mossbauer data using constrained area ratios and a full transmission integral fit that avoids area distortion due to thickness effects. Based on our analysis, all aspects of the electronic and magnetic properties of magnetite reported previously below 25 GPa at ambient temperature can be explained solely by the enhanced delocalization of 3 d electrons of iron atoms. For instance, we present evidence that the compression-induced metallization changes the sign of the charge carrier spin polarization at 15 GPa.

The phase fcc FeHx (x ~ 1) was synthesized at pressures over 30 GPa and temperatures over 1600(50) K. At room temperature this iron hydride is stable at pressures from 19(1) GPa up to at least 68(2) GPa (the highest pressure achieved in this study). A fit of the PV data collected for fcc FeHx at room temperature gives the following parameters for the equation of state: V0 = 53.8(3) A3, K0 = 99(5) GPa, K′ = 11.7(5). Using this data the amount of H required to match the density of the Earth's core was estimated to be 0.5–1 wt.% hydrogen in the outer core and 0.08–0.16 wt.% hydrogen in the inner core. Our results also suggest that hydrogen and carbon do not occur together in the Earth's core.

Olivines from the meteorite Northwest Africa 2737 (NWA 2737) show unique characteristics such as strong brownish staining, the occurrence of perpendicular sets of bright lamellae and the exsolution of metallic nanoparticles. These features were investigated by transmission electron microscopy and spectroscopic techniques to understand their formation and consequent implications for the shock history of NWA 2737. Areas showing optically an intense brown staining are characterised by the occurrence of finely dispersed metallic nanoparticles, an extreme high density (10 16 m − 2 ) of polygonised dislocations with Burgers vector [001] and a high abundance of trivalent iron (16%) as determined by electron energy loss spectroscopy. In contrast, the bright lamellae contain individual dislocation lines with Burgers vector [001] of a slightly lower density (10 14 m − 2 ), almost no metallic nanoparticles and significantly lower trivalent iron content. Model mass balance calculations suggest that the high content of trivalent iron can be incorporated as a laihunite component into the cores of polygonised dislocations. This is charge balanced by the formation of metallic nanoparticles. Trivalent iron is likely responsible for the intense brown colour, whereas nanoparticles darken these areas considerably and cause an intense red slope in the infrared range. The proposed diffusion of trivalent iron into the cores of polygonised dislocations and the associated exsolution of metallic iron nanoparticles are likely fundamental processes for intense olivine staining during meteoritic impacts in general. Detailed analyses of variously stained regions suggest that the bright lamellae have experienced slightly higher shock-induced strain causing a recrystallisation of olivine, which occurred simultaneously with the polygonisation and exsolution processes resulting in the olivine staining of adjacent areas. The existence of individual dislocations can be interpreted as the result of a two-stage shock history for NWA 2737. After a first impact, the material of NWA 2737 was likely embedded in a hot ejecta blanket. This would ensure sufficient time at elevated temperatures for the polygonisation of dislocations and the recrystallisation of bright lamellae. A second impact resulted in the emission of individual dislocations into recrystallised olivine and the ejection of that meteorite from Mars.

Iron valence state and local environment in a set of fibrous diamonds from Brazilian and Congolese placers were investigated using X-ray absorption and Mössbauer spectroscopies. It is shown that the diamonds could be divided into two main groups differing in the type of dominant Fe-bearing inclusions. In the first group Fe is mostly trivalent and is present in octahedral coordination; diamonds from the second group contain a mixture of Fe2+ and Fe3+, most likely with Fe2+ in dodecahedral coordination. A few other diamonds contain iron in a more reduced state: The presence of metallic Fe and Fe3O4 is inferred from XAS measurements. Spatially resolved XANES and Mössbauer measurements on polished diamond plates show that in some cases the Fe valence state may change considerably between the core and rim, whereas in other cases Fe speciation and valence remain constant. It is shown that Fe valence does not correlate with water and/or carbonate content or ratio, suggesting that iron is a minor element in the growth medium of fibrous diamonds and plays a passive role. This study suggests that, when present, evolution of the C isotopic composition with diamond growth is largely due to changes in chemistry of the growth medium and not due to variations of fO2.

We investigated the concentration and site occupation of ferric iron (Fe3+) in (Mg,Fe)O to understand the influence of point defects on transport properties such as atomic diffusion, electrical conductivity and viscosity. We conducted Mossbauer spectroscopy of (Mg0.8Fe0.2)O single crystals synthesized at temperatures from 1673 to 2273 K and pressures from 5 to 15 GPa with Re–ReO2 and Mo–MoO2 oxygen fugacity buffers. The isomer shift of the Mossbauer spectra suggests that Fe3+ occupies mostly the tetrahedral site at reduced conditions and both the octahedral and tetrahedral sites at oxidized conditions. We formulate a thermodynamic model of point defect dissolution in (Mg,Fe)O which suggests that unassociated tetrahedral Fe3+ is more stable than unassociated octahedral Fe3+ at high-pressure and low oxygen fugacity due to the effect of configurational entropy. The pressure dependence of Fe3+ concentration indicates a change in the dominant site occupancy of Fe3+: (1) Fe3+ in the tetrahedral site, (2) Fe3+ in the octahedral site, and (3) defect clusters of Fe3+ and cation vacancy, in the order of increasing oxygen fugacity and decreasing pressure. This is in reasonable agreement with previously reported experiments on Fe3+ concentration, Mg–Fe interdiffusivity and electrical conductivity. We consider it plausible that (Mg,Fe)O accommodates Fe3+ in the tetrahedral site down to the lower mantle. Based on our results and available experimental data, we discuss the solubility competition between Fe3+ and protons (H+), and its implications for transport properties in the lower mantle.

We present in situ measurements of the unit-cell volume of a natural terrestrial ilmenite (Jagersfontein mine, South Africa) and a synthetic reduced ilmenite (FeTiO_3) at simultaneous high pressure and high temperature up to 16 GPa and 1273 K. Unit-cell volumes were determined using energy-dispersive synchrotron X-ray diffraction in a multi-anvil press. Mössbauer analyses show that the synthetic sample contained insignificant amounts of Fe^(3+) both before and after the experiment. Results were fit to Birch-Murnaghan thermal equations of state, which reproduce the experimental data to within 0.5 and 0.7 GPa for the synthetic and natural samples, respectively. At ambient conditions, the unit-cell volume of the natural sample [V_0 = 314.75 ± 0.23 (1 ) Å^3] is significantly smaller than that of the synthetic sample [V_0 = 319.12 ± 0.26 Å^3]. The difference can be attributed to the presence of impurities and Fe^(3+) in the natural sample. The 1 bar isothermal bulk moduli K_(T0) for the reduced ilmenite is slightly larger than for the natural ilmenite (181 ± 7 and 165 ± 6 GPa, respectively), with pressure derivatives K_0' = 3 ± 1. Our results, combined with literature data, suggest that the unit-cell volume of reduced ilmenite is significantly larger than that of oxidized ilmenite, whereas their thermoelastic parameters are similar. Our data provide more appropriate input parameters for thermo-chemical models of lunar interior evolution, in which reduced ilmenite plays a critical role.

The influence of concurrent plastic deformation on a continuous Fe–Mg exchange reaction in ferropericlase/periclase was examined as a function of strain and changing microstructure in a torsion experiment performed at 300 MPa and 1523 K. The measured Fe–Mg interdiffusion coefficient increases non-linearly by a factor of 1.2 to 1.4 (depending on composition) with increasing shear strain (up to a γ of 5.2), strain rate (up to 2.9 × 10− 4 s− 1) and decreasing grain size (from 200 μm to 80 μm). The change in diffusion is attributed to the increase in density of large-angle (> 15°) grain boundaries, whereas both subgrain boundary density and dislocation density do not correlate well with the changing diffusion coefficients. The increased diffusivity due to grain boundary diffusion does not follow any of the proposed equations for combined volume and boundary diffusion, indicating that either the effect of grain boundary diffusion has been underestimated or that there is an additional effect on diffusivity due to intracrystalline deformation (movement of dislocations and/or point defects, grain boundary migration during dynamic recrystallization) even if there is no net stress or strain in the direction of diffusion.

Plimerite, ideally Zn (PO4)3(OH)5, is a new mineral from the Block 14 Opencut, Broken Hill, New SouthWales. It occurs as pale-green to dark-olive-green, almost black, acicular to prismatic and bladed crystals up to 0.5 mm long and as hemispherical aggregates of radiating acicular crystals up to 3 mm across. Crystals are elongated along [001] and the principal form observed is {100} with minor {010} and {001}. The mineral is associated with hinsdalite-plumbogummite, pyromorphite, libethenite, brochantite, malachite, tsumebite and strengite. Plimerite is translucent with a pale-greyish-green streak and a vitreous lustre. It shows an excellent cleavage parallel to {100} and {010} and distinct cleavage parallel to {001}. It is brittle, has an uneven fracture, a Mohs’ hardness of 3.5–4, D(meas.) = 3.67(5) g/cm3 and D(calc.) = 3.62 g/cm3 (for the empirical formula). Optically, it is biaxial negative with α = 1.756(5), β = 1.764(4), γ = 1.767(4) and 2V(calc.) of –63º; pleochroism is X pale-greenish-brown, Y pale-brown, Z pale-bluish-green; absorption Z > X > Y; optical orientation XYZ = cab. Plimerite is orthorhombic, space group Bbmm, unit-cell parameters: a = 13.865(3) Å, b = 16.798(3) Å, c = 5.151(10) Å, V = 1187.0(4) Å3 (single-crystal data) and Z = 4. Strongest lines in the X-ray powder diffraction pattern are [d (A˚ ), I, hkl]: 4.638, (50), (111); 3.388, (50), (041); 3.369, (55), (131); 3.168, (100), (132); 2.753, (60), (115); 2.575, (90), (200); 2.414, (75), (220); 2.400, (50), (221); 1.957, (40), (225). Electron microprobe analysis yielded (wt.%): PbO 0.36, CaO 0.17, ZnO 20.17, MnO 0.02, Fe2O3 29.82, FeO 2.98, Al2O3 4.48, P2O5 32.37, As2O5 0.09, H2O (calc) 6.84, total 97.30 (Fe3+/Fe2+ ratio determined by Mössbauer spectroscopy). The empirical formula calculated on the basis of 17 oxygens is Ca0.02Pb0.01Zn1.68Al0.60P3.09As0.01O17.00H5.15. The crystal structure was solved by direct methods and refined to an R1 index of 6.41% for 1332 observed reflections from single-crystal X-ray diffraction data (Mo-Kα radiation, CCD area detector). The structure of plimerite is isotypic with that of rockbridgeite and is based on face-sharing trimers of (Mϕ6) octahedra which link by sharing edges to form chains, that extend in the b-direction. Chains link to clusters comprising pairs of corner-sharing (Mϕ6) octahedra that link to PO4 tetrahedra forming sheets parallel to (001). The sheets link via octahedra and tetrahedra corners into a heteropolyhedral framework structure. The mineral name honours Professor Ian Plimer for his contributions to the study of the geology of ore deposits.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Magnetic properties of six Ca-free P 2 1 / c clinopyroxenes along the clinoenstatite-clinoferrosilite join were studied by transmission 57 Fe Mossbauer spectroscopy (0.3–4.2 K) and SQUID magnetization measurements (2–300 K). The Fe x Mg 1− x SiO 3 members were synthesized in a multi-anvil press, and samples with x = 1.00, 0.91, 0.87, 0.78, 0.61, and 0.09 have been considered in this study. The magnetic order-disorder transition temperatures T N were determined by Mossbauer thermoscanning with zero source velocity. The magnetic Mossbauer spectra were refined using a full hyperfine-interaction Hamiltonian approach assuming different Fe 2+ -Mg 2+ next-nearest-neighbor configurations of the probe nuclei to give a distinct spectral component. The different strengths of the M1 and M2 hyperfine fields can be related to the different 5 D electronic level schemes as earlier determined from the paramagnetic Mossbauer spectra. The magnetic susceptibility measurements indicate a positive paramagnetic Curie temperature 𝛉 P for FeSiO 3 ( x = 1) above 50 K and a negative 𝛉 P for x ≤ 0.91 above 200 K. The Neel temperatures as determined from the susceptibility curves are equal to those obtained from the thermoscanning, indicating that spin relaxation effects can be ruled out. The macroscopic magnetic results further suggest ferromagnetic ordering of Fe 2+ ions within ribbons consisting of two linear bands of M2 sites that enclose chains of M1 sites, and antiferromagnetic coupling between neighboring ribbons. Generally spoken the magnetic behavior of Ca-free P 2 1 / c clinopyroxenes are similar to that of orthopyroxenes with similar (Mg,Fe) compositions.

Polyakovite-(Ce), ideally REE 4 Mg Cr3+2 Ti2 Si4 O22, monoclinic, a 13.398(1), b 5.6974(5), c 11.042(1) A, β 100.539(2)°, V 828.6(2) A3, a:b:c = 2.3516:1:1.9381, space group C 2/ m , Z = 2, is a new mineral species from the Ilmen Mountains, southern Urals, Russia. It is of hydrothermal origin; associated minerals are calcite, dolomite, fluororichterite, phlogopite, forsterite, monazite-(Ce), clinohumite, chromite and davidite-(Ce). It occurs as irregular grains up to 3 cm in diameter and euhedral crystals elongate along \[010\] (up to 2 mm) and flattened on {001}. The main forms are a {100}, c {001}, t {201}, κ{201}, m {110}, o {111}, n {111}, p {112}, ρ{112} and x {302}. It is translucent in thin fragments, black with a brown streak, has a vitreous luster, and does not fluoresce under ultraviolet light. Polyakovite-(Ce) has a Mohs hardness of 5½–6, is brittle with a conchoidal fracture, and has no cleavage or parting. The measured density is 4.75(7) g/cm3, D calc is 5.05 g/cm3. It is highly metamict. The infrared absorption spectrum has maxima at 473, 970 and 1115 cm−1. In reflected light, polyakovite-(Ce) is gray, non-pleochroic, isotropic, n in the range 1.931–1.935. The strongest five reflections in the X-ray powder-diffraction pattern of annealed material [ d in A(I)( hkl )] are: 2.715(100)(004), 3.18(50)(311), 5.44(40)(002), 3.15(40)(312), and 2.849(40)(020). An electron-microprobe analysis gave SiO2 19.08, TiO2 9.49, FeO 1.09, Fe2O3 4.30, MnO 0.05, MgO 2.61, CaO 1.06, Cr2O3 7.42, Nb2O5 3.98, ThO2 2.79, UO2 0.03, Y2O3 0.38, Ce2O3 24.24, La2O3 15.94, Nd2O3 4.76, Pr2O3 2.01, Sm2O3 0.38, H2O 0.14, sum 99.75 wt.%; the Fe2+/Fe3+ ratio was derived by Mossbauer spectroscopy. The corresponding chemical formula is (Ce1.87La1.24Nd0.36Pr0.16Sm0.03Y0.04Ca0.24Th0.13)∑4.07 (Mg0.82Fe2+0.19Mn0.01)∑1.02 (Cr1.24Fe3+0.68)∑1.92(Ti1.51Nb0.38)∑1.89 Si4.03 O22 [based on O = 22 apfu (atoms per formula unit)]. The mineral is named for Vladislav Olegovich Polyakov (1950–1993), who contributed greatly to our knowledge of the mineralogy of the Urals. The crystal structure of polyakovite-(Ce) (annealed single crystal) has been solved by direct methods and refined by least-squares to an R value of 4.6% using 1074 unique observed (| F o| > 4σ F ) reflections collected with a single-crystal diffractometer fitted with a CCD detector and Mo K α X-radiation. Polyakovite-(Ce) is a sorosilicate of Cr3+, Ti4+, Fe2+ and REE 3+, and the first titanosilicate where Cr3+ is a species-forming element. In the crystal structure of polyakovite-(Ce), there are four octahedrally coordinated M sites: M (1), 0.80 Mg + 0.20 Fe2+, 2.108 A; M (2), 1.52 Ti + 0.32 Nb + 0.16 □, 1.984 A; M (4) and M (3) are occupied by 0.64 Cr3+ + 0.36 Fe3+, 1.995 A. Two distinct Si sites are occupied solely by Si, and the resulting (SiO4) tetrahedra share one vertex to form an [Si2O7] group: 1.614 A, Si(1)–O–Si(2) 173.2(8)°. There are two types of chains of octahedra: (1) [ M (3) and M (4)] octahedra, and (2) M (2) octahedra, parallel to the b axis, form together an octahedral layer parallel to (001). In a chain, every two octahedra have a common edge. M (2–4) octahedral layers, M (1) octahedra, and [Si2O7] groups form a three-dimensional framework. There are two A sites (CN = [8] and [10], respectively) in which Ce3+ is dominant over other REE 3+, Ca and Th4+. Polyakovite-(Ce) differs from chevkinite-(Ce), ( REE ,Ca)4 (Fe2+,Mg) Fe3+2 (Ti,Nb)2 Si4 O22, by the prevalence of Mg at the M (1) site and Cr3+ at the M (3) and M (4) sites.

The crystal structure of gladiusite, (Fe2+,Mg)4 Fe3+2 (PO4) (OH)11 (H2O), monoclinic, a 16.950 (2), b 11.650 (1), c 6.2660 (6) A, β 90.000 (4)°, V 1237.3(4) A3, space group P 21/ n, Z = 2, D (calc.) = 3.08 g cm−3, was solved by direct methods for a twinned crystal and refined to an R 1 index of 5.4% based on 1214 observed [ F o > 4σ F ] unique reflections measured with Mo K α X-radiation on a CCD diffractometer. There is one P site tetrahedrally coordinated by four O atoms. There are six distinct octahedrally coordinated M sites containing Fe3+, Fe2+, Mg and Mn2+; the octahedra are coordinated by O atoms, OH and H2O groups. The ( M ϕ6) (ϕ = unspecified anion) octahedra each share two trans edges to form a rutile-like [ M ϕ4] chain along [001]. Pairs of chains, shifted relative to each other by a half-octahedron, link together through common edges to form a ribbon extending along c ; thus, each octahedron shares four edges with other octahedra. These ribbons are connected through common vertices of ( M ϕ6) octahedra into a framework with channels along the c axis. The (PO4) tetrahedra share each of two vertices with three ( M ϕ6) octahedra, and the remaining two vertices are connected to octahedra through hydrogen bonds. There is a system of hydrogen bonds in the crystal structure of gladiusite ( D – A < 3.50 A), some of them bifurcated. The Mossbauer spectrum of gladiusite shows three doublets, two of which may be assigned to Fe2+ in octahedral coordination and one of which may be assigned to Fe3+ in octahedral coordination. The intensity ratios of the various doublets indicate that Fe2+/(Fe2+ + Fe3+) = 0.49.

The crystal structure of ferrian winchite (□0.87K0.13)∑1.00 (Ca0.65Na1.34)∑1.99 (Mg2.78Fe2+0.12Mn0.07Ti0.01)∑2.98 (Fe2+0.88Fe3+0.92Al0.20)∑2.00 (Si7.88Al0.12)∑8.00 O22 [(OH)1.77F0.23]∑2.00, a 9.8339(2), b 18.0357(2), c 5.2974(1) A, β 104.195(1)°, V 910.9(2) A3, C 2/ m , Z = 2, D (calc) = 3.12 g/cm3, has been refined using the Rietveld method to the following values: R p = 4.1 and R F = 3.6%, s = 2.74. The Fe3+/(Fe2+ + Fe3+) value, 0.48, was determined by Mossbauer spectroscopy. Site populations were assigned on the basis of the refined site-scattering values at the A -, B - and C -group sites. T Al was assigned to the T (1) site of the tetrahedral double chain on the basis of the observed distances. In the structure of ferrian winchite, all Fe3+ occurs at the M (2) site, in accord with the observed distance of 2.05 A. Fe2+ is partitioned between the M (2) and M (3) sites. The high amount of Fe3+ (0.92 apfu ) at the M (2) site focuses attention on some defects of the current system of amphibole nomenclature.

Study of the Mössbauer effect over a range of both temperature and pressure on polycrystalline Fe-doped åkermanite, and X-ray measurements of spontaneous strain on single crystals of åkermanite under pressure, have been used to study the nature of the order parameters, and the characteristics for the transition of the normal to incommensurate structure in this compound. A recent study implies that the known atomic displacements in this phase must satisfy the symmetry of a space group irreducible representation induced from the representations A2 and B1 of the group associated with the vector 000 of the space group P¯421m, the high temperature space group of melilite. The Mössbauer effect deals with local order, and may be explained directly in terms of the existence of two types of 57Fe2+ containing Z tetrahedra, one of which is simply rotated on the basis of the A2 representation (η) and the other distorted on the basis of the diad symmetry of B1,(Ξ). In contrast the single crystal strain effects observed by single crystal X-ray diffraction must be explained in terms of both local strains associated with the A2 and B1 distortions, and a macroscopic strain field which couples the local distortions under the full symmetry of the tetragonal phase.

The crystal structure of potassic-ferrisadanagaite, (K 0.66Na0.32)S0.98 (Ca1.73Na0.23Mn0.04)S2.00 (Mg0.55Fe 2+ 2.16Mn0.25Fe 3+ 1.11 Al0.78Ti0.19)S5.00 (Si5.33Al2.67)S8.00 O22 [(OH)1.21F0.41O0.38]S2.00, a 9.9309(1), b 18.0949(3), c 5.3681(1) A, b 105.19(2)°, V 930.9(1) A 3 , C2/m, Z = 2, D(calc) = 3.36 g/cm 3 , has been refined using the Rietveld method to the following values: Rp = 3.5, RB = 2.6%. Mossbauer spectroscopy was used to characterize the Fe 3+ /(Fe 2+ + Fe 3+ ) value, and site populations were assigned on the basis of the refined site-scattering values at the A-, B -, and C-group sites, the unit formula calculated from the results of the chemical analysis, and the observed mean bond-lengths. The T(1) site is approximately half-occupied by [4] Al (2.15 apfu: atoms per formula unit) and the T(2) site is significantly occupied by [4] Al (0.66 apfu according to the distance). This distribution is in accord with no T Al‐O‐ T Al linkages in the structure (in accord with the absence of Ca at the A site). In the presence of large amounts of T Al (>> 2 apfu), linkage between the octahedron strip and the double-chain of tetrahedra is maintained by incorporation of large cations at the M(1), M(2) and M(3) sites (Fe 2+ ! Mg, Fe 3+ ! Al) and maximal kinking of the double-chain of tetrahedra (the latter being facilitated by K ! Na at the A site).