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1.
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 Mössbauer 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. Mössbauer 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.  相似文献   

2.
Zinclipscombite, a new mineral species, has been found together with apophyllite, quartz, barite, jarosite, plumbojarosite, turquoise, and calcite at the Silver Coin mine, Edna Mountains, Valmy, Humboldt County, Nevada, United States. The new mineral forms spheroidal, fibrous segregations; the thickness of the fibers, which extend along the c axis, reaches 20 μm, and the diameter of spherulites is up to 2.5 mm. The color is dark green to brown with a light green to beige streak and a vitreous luster. The mineral is translucent. The Mohs hardness is 5. Zinclipscombite is brittle; cleavage is not observed; fracture is uneven. The density is 3.65(4) g/cm3 measured by hydrostatic weighing and 3.727 g/cm3 calculated from X-ray powder data. The frequencies of absorption bands in the infrared spectrum of zinclipscombite are (cm?1; the frequencies of the strongest bands are underlined; sh, shoulder; w, weak band) 3535, 3330sh, 3260, 1625w, 1530w, 1068, 1047, 1022, 970sh, 768w, 684w, 609, 502, and 460. The Mössbauer spectrum of zinclipscombite contains only a doublet corresponding to Fe3+ with sixfold coordination and a quadrupole splitting of 0.562 mm/s; Fe2+ is absent. The mineral is optically uniaxial and positive, ω = 1.755(5), ? = 1.795(5). Zinclipscombite is pleochroic, from bright green to blue-green on X and light greenish brown on Z (X > Z). Chemical composition (electron microprobe, average of five point analyses, wt %): CaO 0.30, ZnO 15.90, Al2O3 4.77, Fe2O3 35.14, P2O5 33.86, As2O5 4.05, H2O (determined by the Penfield method) 4.94, total 98.96. The empirical formula calculated on the basis of (PO4,AsO4)2 is (Zn0.76Ca0.02)Σ0.78(Fe 1.72 3+ Al0.36)Σ2.08[(PO4)1.86(AsO4)0.14]Σ2.00(OH)1. 80 · 0.17H2O. The simplified formula is ZnFe 2 3+ (PO4)2(OH)2. Zinclipscombite is tetragonal, space group P43212 or P41212; a = 7.242(2) Å, c = 13.125(5) Å, V = 688.4(5) Å3, Z = 4. The strongest reflections in the X-ray powder diffraction pattern (d, (I, %) ((hkl)) are 4.79(80)(111), 3.32(100)(113), 3.21(60)(210), 2.602(45)(213), 2.299(40)(214), 2.049(40)(106), 1.663(45)(226), 1.605(50)(421, 108). Zinclipscombite is an analogue of lipscombite, Fe2+Fe 2 3+ (PO4)2(OH)2 (tetragonal), with Zn instead of Fe2+. The mineral is named for its chemical composition, the Zn-dominant analogue of lipscombite. The type material of zinclipscombite is deposited in the Mineralogical Collection of the Technische Universität Bergakademie Freiberg, Germany.  相似文献   

3.
The Fe3+/Fetot of all Fe-bearing minerals has been analysed by Mössbauer spectroscopy in a suite of biotite-rich to biotite-free graphitic metapelite xenoliths, proxies of an amphibolite-granulite transition through progressive biotite melting. Biotite contains 9 to 16% Fe3+/Fetot, whereas garnet, cordierite and ilmenite are virtually Fe3+ -free (0–1% Fe3+/Fetot) in all samples, regardless of biotite presence. Under relatively reducing conditions (graphite-bearing assemblages), biotite is the only carrier of Fe3+ during high-temperature metamorphism; therefore, its disappearance by melting represents an important event of iron reduction during granulite formation, because haplogranitic melts usually incorporate small amounts of ferric iron. Iron reduction is accompanied by the oxidation of carbon and the production of CO2, according to the redox reaction:
Depending on the nature of the peritectic Fe-Mg mineral produced (garnet, cordierite, orthopyroxene), the CO2 can either be present as a free fluid component, or be completely stored within melt and cordierite. The oxidation of graphite by iron reduction can account for the in situ generation of CO2, implying a consequential rather than causal role of CO2 in some granulites and migmatites. This genetic model is relevant to graphitic rocks more generally and may explain why CO2 is present in some granulites although it is not required for their formation.  相似文献   

4.
Al-containing MgSiO3 perovskites of four different compositions were synthesized at 27 GPa and 1,873 K using a Kawai-type high-pressure apparatus: stoichiometric compositions of Mg0.975Si0.975Al0.05O3 and Mg0.95Si0.95Al0.10O3 considering only coupled substitution Mg2+ + Si4+ = 2Al3+, and nonstoichiometric compositions of Mg0.99Si0.96Al0.05O2.985 and Mg0.97Si0.93Al0.10O2.98 taking account of not only the coupled substitution but also oxygen vacancy substitution 2Si4+ = 2Al3+ + VO¨. Using the X-ray diffraction profiles, Rietveld analyses were performed, and the results were compared between the stoichiometric and nonstoichiometric perovskites. Lattice parameter–composition relations, in space group Pbnm, were obtained as follows. The a parameters of both of the stoichiometric and nonstoichiometric perovskites are almost constant in the X Al range of 0–0.05, where X Al is Al number on the basis of total cation of two (X Al = 2Al/(Mg + Si + Al)), and decrease with further increasing X Al. The b and c parameters of the stoichiometric perovskites increase linearly with increasing Al content. The change in the b parameter of the nonstoichiometric perovskites with Al content is the same as that of the stoichiometric perovskites within the uncertainties. The c parameter of the nonstoichiometric perovskites is slightly smaller than that of the stoichiometric perovskites at X Al of 0.10, though they are the same as each other at X Al of 0.05. The Si(Al)–O1 distance, Si(Al)–O1–Si(Al) angle and minimum Mg(Al)–O distance of the nonstoichiometric perovskites keep almost constant up to X Al of 0.05, and then the Si(Al)–O1 increases and both of the Si(Al)–O1–Si(Al) angle and minimum Mg(Al)–O decrease with further Al substitution. These results suggest that the oxygen vacancy substitution may be superior to the coupled substitution up to X Al of about 0.05 and that more Al could be substituted only by the coupled substitution at 27 GPa. The Si(Al)–O1 distance and one of two independent Si(Al)–O2 distances in Si(Al)O6 octahedra in the nonstoichiometric perovskites are always shorter than those in the stoichiometric perovskite at the same Al content. These results imply that oxygen defects may exist in the nonstoichiometric perovskites and distribute randomly.  相似文献   

5.
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. Mössbauer and infrared spectroscopy have been used to study the samples at room temperature, and Mössbauer 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.  相似文献   

6.
Three natural lawsonites from Syke Rock, Mendocino Co., Reed Ranch, Marin Co., and Blake Gardens, Sonoma Co., all from the Coast Range Region in California, were studied by 57Fe Mössbauer spectroscopy, electron microprobe analysis, and X-ray powder diffraction. The samples contain about 0.6, 1.0, and 1.4 wt% of total iron oxide, respectively. 57Fe Mössbauer spectra are consistent with the assumption that high-spin Fe3+ substitutes for Al in the octahedrally coordinated site. The Mössbauer spectrum of lawsonite from Syke Rock exhibits a second doublet with 57Fe hyperfine parameters typical for octahedrally coordinated high-spin Fe2+. Electronic structure calculations in the local spin density approximation yield quadrupole splittings for Fe3+ in quantitative agreement with experiment indicating, however, that substitution of Al by Fe3+ must be accompanied by local distortion around the octahedral site. Model calculations also reproduce the room temperature hyperfine parameters of ferrous high-spin iron assuming the substitution of Ca by Fe2+. However, it cannot be excluded that Fe2+ may occupy a more asymmetric site within the microstructural cavity occupied by Ca and a H2O molecule.  相似文献   

7.
Pumpellyite of the general formula W8X4Y8-Z12O56-n(OH)n contains Fe, Al and Mg in two crystallographically different octahedral sites. Three different pumpellyite samples covering the known compositional field from Al- to Fe-rich have been studied to determine the valence state and intracrystalline partitioning of the Fe cations between the two independent octahedral sites. Fe+2 and Fe+3 cation partitioning is interpreted on the basis of results obtained by 57Fe Mössbauer spectroscopy at 293 and 77 K and from Rietveld structure analysis performed on powder X-ray diffraction data. Pumpellyite from low-grade metamorphic rocks typically contains a majority of iron in the Fe+3 oxidation state, which is found in the smaller and less symmetrical octahedral Y-site. Fe+2 was also present in all pumpellyite samples studied and it is located in the larger and more symmetrical octahedral X-site.  相似文献   

8.
Phase equilibria in the join CaMgSi2O6-CaFeAlSiO6-CaTiAl2O6 have been determined in air at 1 atm by the ordinary quenching method. Clinopyroxeness, forsterite, perovskite, magnetitess, spinelss, hibonite and an unknown phase X are present at liquidus temperatures (ss: solid solution). At subsolidus temperatures the following phase assemblages were encountered; clinopyroxeness+perovskite, clinopyroxeness +perovskite+spinelss, clinopyroxeness +perovskite+melilite (+anorthite), clinopyroxeness +perovskite+melilite+spinelss+anorthite, clinopyroxeness +perovskite+anorthite+spinelss, and clinopyroxeness +perovskite+anorthite+hibonite. At subsolidus temperatures the single phase field of clinopyroxeness extends up to 19 wt.% CaTiAl2O6. Even in the field of clinopyroxeness+perovskite, the TiO2 content in clinopyroxeness continues to increase and attains 9.2 wt.% TiO2 with 24.8 wt.% Al2O3. An interesting fact is that unusual clinopyroxenes which contain more AlIV than SiIV are present in the CaFe-AlSiO6-rich region. The liquid coexisting with pyroxene is richer in Ti, Al, and Fe3+ than the coexisting pyroxene. The clinopyroxenesss coexisting with liquid contain less TiO2, Al2O3 and Fe2O3 than those crystallized at subsolidus temperatures. The petrological significance of the join and the crystallization of Ti- and Al-rich clinopyroxenes are discussed on the basis of the experimental results of the join.  相似文献   

9.
The structure of CaGe2O5 between room temperature and 923 K has been determined by X-ray powder diffraction. A continuous phase transition from triclinic C1¯ to monoclinic C2/c symmetry at Tc=714±3 K is observed. The transition is accompanied by a weak heat capacity anomaly. This anomaly and the strain analysis based on the measured lattice parameters indicate a classical second-order phase transition. The order parameter, as measured by the strain component e23, is associated with the displacement of the Ca cation. Electronic structure optimization by density functional methods is used to verify the centric space group of the low-temperature structure of CaGe2O5.  相似文献   

10.
In the olivine crystal structure, cations are distributed over two inequivalent octahedral sites, M1 and M2. Kinetics of cation exchange between the two octahedral sites in (Co0.1Mg0.9)2SiO4 single crystal have been studied in the temperature range from 600 to 800°C by monitoring the time evolution of the absorbance of Co2+ ions in M1 or M2 sites using optical spectroscopy after rapid temperature jumps. It was found from such temperature-jump induced relaxation experiments that with increasing temperature the absorbance of Co2+ ions in the M1 site decreases while that in the M2 site increases. This indicates a tendency of Co2+ cations to populate the M2 site with increasing temperatures and vice versa. The experimental relaxation data can be modeled using a triple exponential equation based on theoretical analysis. Activation energies of 221 ± 4 and 213 ± 10 kJ/mol were derived from relaxation experiments on the M2 site and M1 site, respectively, for the cation exchange processes in (Co0.1Mg0.9)2SiO4 olivine. Implications for cation diffusion at low temperatures are discussed.  相似文献   

11.
An equation of state for Mg(OH)2 brucite under high-pressure and high-temperature conditions has been obtained by measuring temperature dependence of volume up to 600 K at ambient pressure and pressure dependence of volume up to 16 GPa at 300, 473, 673, and 873 K with in situ X-ray diffraction. Pressure dependence of entropy of brucite has been calculated with thermal expansion coefficient and volume which are derived from the present EoS. This dependence indicates that generation of secondary OH dipoles affects entropy. The OH dipoles probably appear around 2 GPa and the number seems not to change over 8 GPa at 300 K.  相似文献   

12.
 The magnetic behavior of the Jahn-Teller structure braunite, (Mn2+ 1−yM y )(Mn3+ 6− x Mx)SiO12, is strongly influenced by the incorporation of elements substituting manganese. Magnetic properties of well-defined synthetic samples were investigated in dependence on the composition. The final results are presented in magnetic phase diagrams. To derive the necessary data, ac susceptibility and magnetization of braunites with the substitutional elements M = Mg, Fe, (Cu+Ti) and Cu were measured. Whereas the antiferromagnetic ordering temperature, T N , of pure braunite is hardly affected by the substitution of nonmagnetic Mg, it is rapidly suppressed by the substitution of magnetic atoms at the Mn positions. Typically for a concentration (x, y) ≥ 0.7 of the substituted elements, a spin glass phase occurs in the magnetic phase diagrams. Additionally, for the braunite system with Fe3+ substitutions, we observe in the concentration range 0.2 < x< 0.7 a double transition from the paramagnetic state, first to the antiferromagnetic state, followed by a transition to a spin glass state at lower temperatures. The unusual change of the magnetic properties with magnetic substitution at the Mn positions is attributed to the peculiar antiferromagnetic structure of braunite, which has been resolved recently. Received: 19 April 2001 / Accepted: 6 September 2001  相似文献   

13.
Pyrrhotite (Fe7S8) is a natural iron sulphide that can participate in rock magnetisation. Its electronic structure is not yet surely described. X-ray magnetic circular dichroism (XMCD) at Fe L2,3 edges on Fe7S8, coupled with multiplet calculations, shows that iron is present only as Fe2+ in this magnetic iron sulphide. It reveals a strong magnetic orbital moment. XMCD at Fe and S K edges shows the quite strong polarization of both Fe and S in Fe7S8.  相似文献   

14.
The thermal expansion, structural changes and the site partitioning of Co and Mg in synthetic CoMgSiO4 olivine have been studied by in situ time-of-flight neutron powder diffraction as a function of temperature, between 25 and 1,000°C. Thermal expansion of the unit cell dimensions and volume are linear within this temperature range and give no indications of a phase transition, although the thermoelastic behaviour indicates a slight strain minimum around 700°C. Co2+ shows a strong preference for the M1 site throughout this temperature range with an oscillatory behaviour; it decreases slightly at about 300°C, climbing up to nearly its original value at around 800°C and then decreasing by about 30% at 1,000°C. This behaviour is in contrast with that of (Fe, Mg)2SiO4 olivine, in which the initial Fe2+ site preference for the M1 site switches to the M2 site beyond a cross-over temperature. The oscillatory site preference in (CoMg)-olivine as a function of temperature is reflected in the M–O polyhedral volume changes and M–O bond lengths, as well as, thermoelastic strain and atomic thermal displacement parameters. The imbalance between the increasing vibrational and decreasing configurational entropy contributions, together with covalent bonding effects rather than crystal field contributions, seem to drive the cation partitioning in (CoMg)-olivine.  相似文献   

15.
Mn2+Sb2S4, a monoclinic dimorph of clerite, and benavidesite (Mn2+Pb4Sb6S14) show well-individualized single chains of manganese atoms in octahedral coordination. Their magnetic structures are presented and compared with those of iron derivatives, berthierite (Fe2+Sb2S4) and jamesonite (Fe2+Pb4Sb6S14). Within chains, interactions are antiferromagnetic. Like berthierite, MnSb2S4 shows a spiral magnetic structure with an incommensurate 1D propagation vector [0, 0.369, 0], unchanged with temperature. In berthierite, the interactions between identical chains are antiferromagnetic, whereas in MnSb2S4 interactions between chains are ferromagnetic along c-axis. Below 6 K, jamesonite and benavidesite have commensurate magnetic structures with the same propagation vector [0.5, 0, 0]: jamesonite is a canted ferromagnet and iron magnetic moments are mainly oriented along the a-axis, whereas for benavidesite, no angle of canting is detected, and manganese magnetic moments are oriented along b-axis. Below 30 K, for both compounds, one-dimensional magnetic ordering or correlations are visible in the neutron diagrams and persist down to 1.4 K.  相似文献   

16.
The behaviour of synthetic Mg-ferrite (MgFe2O4) has been investigated at high pressure (in situ high-pressure synchrotron radiation powder diffraction at ESRF) and at high temperature (in situ high-temperature X-ray powder diffraction) conditions. The elastic properties determined by the third-order Birch–Murnaghan equation of state result in K0=181.5(± 1.3) GPa, K=6.32(± 0.14) and K= –0.0638 GPa–1. The symmetry-independent coordinate of oxygen does not show significant sensitivity to pressure, and the structure shrinking is mainly attributable to the shortening of the cell edge (homogeneous strain). The lattice parameter thermal expansion is described by a0+a1*(T–298)+a2/(T–298)2, where a0=9.1(1) 10–6 K–1, a1=4.9(2) 10–9 K–2 and a2= 5.1(5) 10–2 K. The high-temperature cation-ordering reaction which MgFe-spinel undergoes has been interpreted by the ONeill model, whose parameters are = 22.2(± 1.8) kJ mol–1 and =–17.6(± 1.2) kJ mol–1. The elastic and thermal properties measured have then been used to model the phase diagram of MgFe2O4, which shows that the high-pressure transition from spinel to orthorombic CaMn2O4-like structure at T < 1700 K is preceded by a decomposition into MgO and Fe2O3.  相似文献   

17.
The successful synthesis of nanoparticles of Fe-bearing kuramite, (Cu,Fe)3SnS4, is reported in this study. Nanocrystalline powders were obtained through a mild, environmentally friendly and scalable solvothermal approach, in a single run. The sample was the object of a multidisciplinary investigation, including X-ray diffraction and absorption, scanning electron microscopy and microanalysis, electron paramagnetic resonance, diffuse reflectance and Mössbauer spectroscopy as well as SQUID magnetometry. The nanoparticles consist of pure Fe-bearing kuramite, exhibiting tetragonal structure. The valence state of the metal cations was assessed to be Cu+, Sn4+ and Fe3+. The material presents a band gap value of 1.6 eV, which is fully compatible with solar cell applications. The uptake of Fe by nanokuramite opens a compositional field where the physical properties can be tuned. We thus foster the application of Fe-bearing nanokuramite for photovoltaics and energy storage purposes.  相似文献   

18.
The crystal structure of a new compound [Mg(H2O)4(SeO4)]2(H2O) (monoclinic, P2 1/a, a = 7.2549(12), b = 20.059(5), c = 10.3934(17) Å, β = 101.989(13), V = 1479.5(5) Å3) has been solved by direct methods and refined to R 1 = 0.059 for 2577 observed reflections with |F hkl | ≥ 4σ|F hkl |. The structure consists of [Mg(H2O)4(SeO4)]0 chains formed by alternating corner-sharing Mg octahedrons and (SeO4)2? tetrahedrons. O atoms of Mg octahedrons that are shared with selenate tetrahedrons are in a trans orientation. The heteropoly-hedral octahedral-tetrahedral chains are parallel to the c axis and undulate within the (010) plane. The adjacent chains are linked by hydrogen bonds involving H2O molecules not bound with M2+ cations.  相似文献   

19.
A pyroxene with composition LiNiSi2O6 was synthesized at T = 1,473 K and P = 2.0 GPa; the cell parameters at T = 298 K are a = 9.4169(6) Å, b = 8.4465(7) Å, c = 5.2464(3) Å, β = 110.534(6)°, V = 390.78(3) Å3. TEM examination of the LiNiSi2O6 pyroxene showed the presence of h + k odd reflections indicative of a primitive lattice, and of antiphase domains obtained by dark field imaging of the h + k odd reflections. A HT in situ investigation was performed by examining TEM selected area diffraction patterns collected at high temperature and synchrotron radiation powder diffraction. In HTTEM the LiNiSi2O6 was examined together with LiCrSi2O6 pyroxene. In LiCrSi2O6 the h + k odd critical reflections disappear at about 340 K; they are sharp up to the transition temperature and do not change their shape until they disappear. In LiNiSi2O6 the h + k odd reflections are present up to sample deterioration at 650 K. A high temperature synchrotron radiation powder diffraction investigation was performed on LiNiSi2O6 between 298 and 773 K. The analysis of critical reflections and of changes in cell parameters shows that the space group is P-centred up to the highest temperature. The comparative analysis of the thermal and spontaneous strain contributions in P21/c and C2/c pyroxenes indicates that the high temperature strain in P-LiNiSi2O6 is very similar to that due to thermal strain only in C2/c spodumene and that a spontaneous strain contribution related to pre-transition features is not apparent in LiNiSi2O6. A different high-temperature behaviour in LiNiSi2O6 with respect to other pyroxenes is suggested, possibly in relation with the presence of Jahn–Teller distortion of the M1 polyhedron centred by low-spin Ni3+.  相似文献   

20.
The investigation of the NH3 loss in the NH4+-vermiculite (Santa Olalla) by thermogravimetry, evolved gas analysis, chemical analysis, X-ray diffraction and IR spectroscopy is reported here. The mass loss during heating takes place in two steps at about 650 and 825 °C. Additionally, the releases of H2O and NH3 occurs simultaneously. The experimental results indicate that the protons remaining in the interlayer space after NH3 removal trigger the H2O release. X-ray diffraction shows that during the decomposition of NH4+-vermiculite there are two domains with different interlayer spaces at ~9 and ~10 Å. As the decomposition proceeds, the intensity of the 9 Å peak increases at the expense of the second one. The change in the IR-stretching modes of the structural OH groups during heating indicates that the OH groups surrounded by 3Mg2+ or 2Mg2+Fe2+ are released at lower temperatures than those with environments like 2Mg2+Fe3+, 2Mg2+Al3+ or more complex ones.  相似文献   

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