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1.
Four synthetic endmember olivines (Mg,Mn,
Fe,Co) 2SiO 4 with space group Pbnm were loaded together in one diamond cell mount. Their unit-cell parameters were determined by single
crystal X-ray diffraction to 10 GPa. The linear compressibilities β a, β b, β c were 1.53, 2.90, 2.32; 1.45, 3.48, 1.98; 1.35, 3.29, 1.76; and 1.25, 2.82, 2.01×10 −3 GPa −1 for Mg 2SiO 4, Mn 2SiO 4, Fe 2SiO 4 and Co 2SiO 4, respectively. The b axis is the most compressible direction in all crystals studied. Bulk modulus K T0 and its first pressure derivative were simultaneously determined for Mg 2SiO 4, Fe 2SiO 4 and Co 2SiO 4 crystals respectively by fitting volume data to a third order Birch-Murnaghan equation of state. They are 127(4) and 4.2(8),
136(3) and 4.1(7), and 144(2) and 4.1(5). The K T0 and could not simultaneously be determined unambiguously for Mn 2SiO 4. Direct comparisons of unit-cell volumes at high pressure among pairs of olivines reveal anomalous compression behavior of
the Mg 2SiO 4 crystal regarding the bulk modulus-volume relationship. This behavior, however, could not be observed in the transition metal
olivines (Mn,Fe,Co) 2SiO 4. The distinct electronic configurations of Mg 2+ and the transition metal cations Mn 2+, Fe 2+, and Co 2+ result in the different compression behaviors of Mg 2SiO 4 and (Mn,Fe,Co) 2SiO 4.
Received: 14 April 1997 / Revised, accepted: 29 July 1997 相似文献
2.
Polarized infrared absorption spectra of thin single-crystal slabs parallel to (010) and (001) of a staurolite from Pizzo Forno, Ticino, with analyzed composition (Fe 2.9Mg 0.9Zn 0.1Mn 0.1)Al 17.5Ti 0.1(Si 7.7Al 0.3)O 48H 3 have been measured in the range of 3000–4000 cm ?1. From the pleochroitic behaviour of the OH-vibrations three groups of bands can be distinguished: the bands of group I, a strong band at 3445 cm ?1 plus a weak shoulder at 3358 cm ?1, and the bands of group II, a weak band centered at 3677 cm ?1 plus a shoulder at 3635 cm ?1, are assigned to the H1 and H2 protons, respectively. The bands of group III, a weak band at 3577 cm ?1 plus a shoulder, cannot be interpreted on the basis of the proton positions known so far. We assign them to an additional proton H3, which is bonded to O1 and shows a bifurcated hydrogen bridge to two O5 in a vacant T2 site. 相似文献
3.
Geochemical modeling was used to investigate downstream changes in coal mine drainage at Silver Creek Metro-park, Summit
County, Ohio. A simple mixing model identified the components that are undergoing conservative transport (Cl –, PO 4
3–, Ca 2+, K +, Mg 2+ and Na +) and those undergoing reactive transport (DO, HCO 3
–, SO 4
2–, Fe 2+, Mn 2+ and Si). Fe 2+ is removed by precipitation of amorphous iron-hydroxide. Mn 2+ are removed along with Fe 2+ by adsorption onto surfaces of iron-hydroxides. DO increases downstream due to absorption from the atmosphere. The HCO 3
– concentration increases downstream as a result of oxidation of organic material. The rate of Fe 2+ removal from the mine drainage was estimated from the linear relationship between Fe +2 concentration and downstream distance to be 0.126 mg/s. Results of this study can be used to improve the design of aerobic
wetlands used to treat acid mine drainage.
Received: 4 June 1996 · Accepted: 17 September 1996 相似文献
4.
The compression of synthetic pyrope Mg 3Al 2 (SiO 4) 3, almandine Fe 3Al 2(SiO 4) 3, spessartine Mn 3Al 2 (SiO 4) 3 grossular Ca 3Al 2(SiO 4) 3 and andradite Ca 3Fe 2 (SiO 4) 3 was studied by loading the crystals together in a diamond anvil cell. The unit-cell parameters were determined as a function
of pressure by X-ray diffraction up to 15 GPa using neon as a pressure transmitting medium. The unit-cell parameters of pyrope
and almandine were measured up to 33 and 21 GPa, respectively, using helium as a pressure medium. The bulk moduli, K
T
0, and their first pressure derivatives, K
T
0
′, were simultaneously determined for all five garnets by fitting the volume data to a third order Birch-Murnaghan equation
of state. Both parameters can be further constrained through a comparison of volume compressions between pairs of garnets,
giving for K
T
0 and K
T
0
′ 171(2) GPa and 4.4(2) for pyrope, 185(3) GPa and 4.2(3) for almandine, 189(1) GPa and 4.2 for spessartine, 175(1) GPa and
4.4 for grossular and 157(1) GPa and 5.1 for andradite, where the K
T
0
′ are fixed in the case of spessartine, grossular and andradite. Direct comparisons of the unit-cell volumes determined at
high pressures between pairs of garnets reveal anomalous compression behavior for Mg 2+ in the 8-fold coordinated triangular dodecahedron in pyrope. This agrees with previous studies concerning the compression
behaviors of Mg 2+ in 6-fold coordinated polyhedra at high pressures. The results show that simple bulk modulus–volume systematics are not obeyed
by garnets.
Received: 29 July 1998 / Revised, accepted: 7 April 1999 相似文献
5.
Single crystals of the garnet Mn 2+ 3Mn 3+ 2[SiO 4] 3 and coesite were synthesised from MnO 2-SiO 2 oxide mixtures at 1000°C and 9 GPa in a multianvil press. The crystal structure of the garnet [space group Ia3¯ d, a=11.801(2) Å] was refined at room temperature and 100 K from single-crystal X-ray data to R1=2.36% and R1=2.71%, respectively. In contrast to tetragonal Ca 3Mn 3+ 2[GeO 4] 3 (space group I4 1/ a), the high-pressure garnet is cubic and does not display an ordered Jahn-Teller distortion of octahedral Mn 3+. A disordered Jahn-Teller distortion either dynamic or static is evidenced by unusual high anisotropic displacement parameters. The room temperature structure is characterised by following bond lengths: Si-O=1.636(4) Å (tetrahedron), Mn 3+-O=1.995 (4) Å (octahedron), Mn 2+-O=2.280(5) and 2.409(4) Å (dodecahedron). The cubic structure was preserved upon cooling to 100 K [ a=11.788(2) Å] and upon compressing up to 11.8 GPa in a diamond-anvil cell. Pressure variation of the unit cell parameter expressed by a third-order Birch-Murnaghan equation of state led to a bulk modulus K 0=151.6(8) GPa and its pressure derivatives K′=6.38(19). The peak positions of the Raman spectrum recorded for Mn 2+ 3Mn 3+ 2[SiO 4] 3 were assigned based on a calderite Mn 2+ 3Fe 3+ 2[SiO 4] 3 model extrapolated from andradite and grossular literature data. 相似文献
6.
Polarized absorption spectra of natural piemontite (Ca1.802Mn
2+0.178
Mg0.025) (Mn
3+0.829
Fe
3+0.346
Al1.825) [(Si2.992Al0.008) O12OH], viridine (Al1.945Mn
3+0.033
Fe
3+0.063
Mg0.003) [O|Si0.970 O4], and kanonaite (Al1.291Mn
3+0.682
Fe
3+0.019
) [O|Si1.006 O4] were measured at 295 and ca. 100 K. For piemontite, lowering the temperature resulted in a sharpening of broad bands in the 10 000–25 000 cm−1 region supporting their assignment to single ion Mn3+ in M3 non-centrosymmetric sites. Alternatively, in kanonaite, temperature behaviour pointed to a slightly stronger influence of vibronic coupling on strong bands near 16 000 and 22 000 cm−1, which supported an interpretation of Mn3+ in nearly centrosymmetric M1 sites. Measurements at ca. 100 K show pronounced fine structure in the viridine spectra which is attributed to Fe3+. The ɛ values for Mn3+ spin-allowed bands in the three minerals lie in the range 18 to 227 [1·g-atom−1·cm−1]. For the same band and polarisation, ɛ values in Mn3+-bearing andalusite-type minerals viridine and kanonaite are the same, which indicates an absence of strong magnetic coupling effects between Mn3+ ions in the andalusite type structure down to ca. 100 K. In silicates, the high ɛ values for Mn3+ spin-allowed bands, in comparison to those obtained for Fe2+ spin-allowed bands from sites of “similar distortion”, is attributed to a higher degree of covalency in the Mn3+-O bonds compared to the Fe2+-O bonds, as a result of the higher valence state of manganese. 相似文献
7.
The topotactic oxidation and delithiation reaction from triphylite, Li(Fe,Mn)PO 4, leading to ferrisicklerite, Li <1(Fe 3+,Mn 2+)PO 4, was investigated under hydrothermal conditions. A cuboid cut from a triphylite single-crystal (Palermo Mine, New Hampshire, USA) with the composition Li 0.93(3)(Fe 2+ 0.733(6),Fe 3+ 0.015(1),Mn 2+ 0.210(4),Mg 0.063(2)) 1.021(8)P 1.00(2)O 4 in addition with ground bulk material were treated with KMnO 4 and 30 % H 2O 2(aq) as oxidizing agent in a 0.1 N hydrochloric acid solution in the temperature range between 60 and 200 °C. At 120 °C a rim of 0.1 mm thickness of ferrisicklerite had formed around the core of unreacted triphylite. The sharp reaction boundary was clearly visible, due to the reddish brown absorption colors of ferrisicklerite, compared to colorless triphylite. Using single-crystal X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS), electron probe micro-analysis (EPMA) and 57Fe-Mössbauer spectroscopy the product ferrisicklerite was characterized and its composition determined as Li 0.30(7)(Fe 2+ 0.049(1)Fe 3+ 0.65(2)Mn 2+ 0.218(5)Mg 0.062(2)) 0.98(1)P 1.01(3)O 4, with unit cell parameters a?=?4.795(1), b?=?9.992(4), and c?=?5.886(2) Å. EPMA investigations across the reaction boundary showed no changes in the concentrations of Fe, Mn, Mg, and P. In contrast, SIMS measurements clearly proved the delithiated state of the ferrisicklerite product. Polarization microscopy revealed that the orientation of the ferrisicklerite rim was the same as that of the original triphylite single-crystal, confirming the strictly topotactic character of the reaction. 相似文献
8.
The dielectric constants and dissipation factors of synthetic tephroite (Mn 2SiO 4), fayalite (Fe 3SiO 4) and a forsteritic olivine (Mg 1.80Fe 0.22SiO 4) were measured at 1 MHz using a two-terminal method and empirically determined edge corrections. The results are: tephroite, κ′ a= 8.79 tan δ a = 0.0006 κ′ b = 10.20 tan δ b = 0.0006 κ′ c= 8.94 tan δ c= 0.0008 fayalite, gk′ a = 8.80 tan δ a = 0.0004 gk′ b= 8.92 tan δ b = 0.0018 gk′ c = 8.58 tan δ c = 0.0010 olivine, gk′ a = 7.16 tan δ a = 0.0006 gk′ b = 7.61 tan δ b = 0.0008 gk′ c = 7.03 tan δ c = 0.0006 The low dielectric constant and loss of the fayalite indicate an exceptionally low Fe 3+ content. An FeO polarizability of 4.18 Å 3, determined from α D(FeO) = [α D (Fe 2SiO 4)-α D(SiO 2)]/2, is probably a more reliable value for stoichiometric FeO than could be obtained from Fe xO where x = 0.90–0.95. The agreement between measured dielectric polarizabilities as determined from the Clausius-Mosotti equation and those calculated from the sum of oxide polarizabilities according to α D(M 2M′X 2) = 2α D(MX) + α D(M′X 2) is ~+2.8% for tephroite and +0.2% for olivine. The deviation from additivity in tephroite is discussed. 相似文献
9.
The electrical conductivity of (Mg 0.93Fe 0.07)SiO 3 ilmenite was measured at temperatures of 500–1,200 K and pressures of 25–35 GPa in a Kawai-type multi-anvil apparatus equipped
with sintered diamond anvils. In order to verify the reliability of this study, the electrical conductivity of (Mg 0.93Fe 0.07)SiO 3 perovskite was also measured at temperatures of 500–1,400 K and pressures of 30–35 GPa. The pressure calibration was carried
out using in situ X-ray diffraction of MgO as pressure marker. The oxidation conditions of the samples were controlled by
the Fe disk. The activation energy at zero pressure and activation volume for ilmenite are 0.82(6) eV and −1.5(2) cm 3/mol, respectively. Those for perovskite were 0.5(1) eV and −0.4(4) cm 3/mol, respectively, which are in agreement with the experimental results reported previously. It is concluded that ilmenite
conductivity has a large pressure dependence in the investigated P–T range. 相似文献
10.
The thermoelastic behaviour of a natural gedrite having the crystal-chemical formula ANa 0.47
B(Na 0.03 Mg 1.05 Fe 0.862+ Mn 0.02 Ca 0.04) C(Mg 3.44 Fe 0.362+ Al 1.15 Ti 0.054+) T(Si 6.31 Al 1.69)O 22
W(OH) 2 has been studied by single-crystal X-ray diffraction to 973 K ( Stage 1). After data collection at 973 K, the crystal was heated to 1,173 K to induce dehydrogenation, which was registered by significant
changes in unit-cell parameters, M1–O3 and M3–O3 bond lengths and refined site-scattering values of M1 and M4 sites. These changes and the crystal-chemical formula calculated from structure refinement show that all Fe 2+ originally at M4 migrates into the ribbon of octahedrally coordinated sites, where most of it oxidises to Fe 3+, and there is a corresponding exchange of Mg from the ribbon into M4. The resulting composition is that of an oxo-gedrite with an inferred crystal-chemical formula ANa 0.47
B(Na 0.03 Mg 1.93 Ca 0.04) C(Mg 2.56 Mn 0.022+ Fe 0.102+ Fe 1.223+ Al 1.15 Ti 0.054+) T(Si 6.31 Al 1.69) O 22
W[O 1.122− (OH) 0.88]. This marked redistribution of Mg and Fe is interpreted as being driven by rapid dehydrogenation at the H3A and H3B sites, such that all available Fe in the structure orders at M1 and M3 sites and is oxidised to Fe 3+. Thermoelastic data are reported for gedrite and oxo-gedrite; the latter was measured during cooling from 1,173 to 298 K
( Stage 2) and checked after further heating to 1,273 K ( Stage 3). The thermoelastic properties of gedrite and oxo-gedrite are compared with each other and those of anthophyllite. 相似文献
11.
Classical atomistic simulation techniques have been used to investigate the energies of hydrogen defects in Mg 2SiO 4 and Mg 2GeO 4 spinels. Ringwoodite (γ-Mg 2SiO 4) is considered to be the most abundant mineral in the lower part of the transition zone and can incorporate large amounts
of water in the form of hydroxyls, whereas the germanate spinel (γ-Mg 2GeO 4) corresponds to a low-pressure structural analogue for ringwoodite. The calculated defect energies indicate that the most
favourable mechanisms for hydrogen incorporation are coupled either with the reduction of ferric iron or with the creation
of tetrahedral vacancies. Hydrogen will go preferentially into tetrahedral vacancies, eventually leading to the formation
of the hydrogarnet defect, before associating with other negatively charged point defects. The presence of isolated hydroxyls
is not expected. The same trend is observed for germanate, and thus γ-Mg 2GeO 4 could be used as a low-pressure analogue for ringwoodite in studies of water-related defects and their effect on physical
properties. 相似文献
12.
The crystal structure of Bi 2Al 4−x
Fe
x
O 9 compounds ( x = 0–4) has striking similarities with the crystal structure of mullite. A complete substitution of Al by Fe 3+ in both octahedral and tetrahedral sites is a particular structural feature. The infrared (IR) spectra of the Bi 2M 4O 9 compounds (M = Al, Fe 3+) are characterised by three band groups with band maxima in the 900–800, 800–600 and 600–400 cm −1 region. Based on the spectroscopic results obtained from mullite-type phases, the present study focuses on the composition-dependent analysis of the 900–800 cm −1 band group, which is assigned to Al(Fe 3+)–O stretching vibrations of the corner-sharing MO 4 tetrahedra. The Bi 2Al 4O 9 and Bi 2Fe 4O 9 endmembers display single bands with maxima centred at 922 and 812 cm −1, respectively. Intermediate Bi 2Al 4−x
Fe
x
O 9 compounds exhibit a distinct splitting into three relatively sharp bands, which is interpreted in terms of ordering effects within the tetrahedral pairs. Thereby the high-energy component band of the band triplet relates to Al–O–Al conjunctions and the low-energy component band to Fe–O–Fe conjunctions. The intermediate band is assigned to stretching vibrations of Al–O–Fe or Fe–O–Al configurations of the corner-sharing tetrahedral pairs. Bands in the 800–600 cm −1 range are assigned to low-energy stretching vibrations of the MO 4 tetrahedra and to M–O–M bending vibrations of the tetrahedral pairs. Absorptions in the 600–400 cm −1 range are essentially determined by M–O stretching modes of the M cations in octahedral coordination. 相似文献
13.
High-resolution core level and valence band (VB) X-ray photoelectron spectra (XPS) of olivine [(Mg 0.87Fe 0.13) 2SiO 4], bronzite [(Mg 0.8Fe 0.2) 2Si 2O 6] and diopside [Ca(Mg 0.8Fe 0.2)Si 2O 6] were collected before and after leaching in pH ∼2 solutions with the Kratos magnetic confinement charge compensation system which minimizes differential charge broadening. The leached samples yield Si 2p, Mg 2p, Ca 2p and O 1s XPS spectral linewidths and lineshapes similar to those collected from the respective pristine samples prior to leaching. As with previous XPS studies on crushed samples, our broadscan XPS spectra show evidence for initial, preferential leaching of cations (i.e., Ca 2+ and Mg 2+) from the near-surface of these minerals. The O 1s spectra of leached olivine and pyroxenes show an additional peak due to OH −, which arises from H + exchange with near-surface cations (Ca 2+ and Mg 2+) via electrophilic attack of H + on the M-O-Si moiety to produce the H 2Mg(M1)SiO 4(surf) complex at olivine surfaces, and two complexes, H 2Mg(M1)Si 2O 6(surf) and H 4Si 2O 6(surf) at diopside and enstatite surfaces. The olivine and pyroxene surface complexes H 2Mg(M1)SiO 4(surf) and H 2Mg(M1)Si 2O 6(surf) have been proposed previously, but the second pyroxene surface complex H 4Si 2O 6(surf) has not. Two electrophilic reactions occur in both olivine and pyroxene. For olivine, the more rapid attacks the M2-O-Si moiety producing H 2Mg(M1)SiO 4(surf); while the second attacks the M1-O-Si moiety ultimately producing H 4SiO 4 which is released to solution. For pyroxenes, the first electrophilic reaction produces H 2Mg(M1)Si 2O 6(surf), while the second produces.H 4Si 2O 6(surf). These two reactions are followed by a nucleophilic attack of H 2O (or H 3O +) on Si of H 4Si 2O 6(surf). This reaction is responsible for rupture of the brigding oxygen bond of the Si-O-Si moiety and release of H 4SiO 4 to solution. The intensity of the OH − peak for the leached pyroxenes is about double the OH − intensity for the leached olivine, consistent with the equivalent of about a monolayer of the above surface complexes being formed in all three minerals.Valence band XPS spectra and density functional calculations demonstrate the remarkable insensitivity of the valence band to leaching of Ca 2+ and Mg 2+ from the surface layers. This insensitivity is due to a dearth of Ca and Mg valence electron density in the valence band: the Ca-O and Mg-O bonds are highly ionic, with metal-derived s orbital electrons taking on strong O 2p character. The valence band spectrum of leached olivine shows an additional very weak peak at about 13.5 eV, which is assigned to Si 3s valence orbitals in the surface complex H 2Mg(M1)SiO 4, as indicated by high quality density functional calculations on an olivine where Mg 2+ in M2 is replaced by 2H +. The intensity of this new peak is consistent with formation of the equivalent of a monolayer of the surface complex. 相似文献
14.
Natural calcite from Kuerle, Xinjiang, China, shows orange-red fluorescence when exposed to short-wave ultraviolet (UV) light (Hg 253.7 nm). Photoluminescence (PL) emission and excitation spectra of the calcite are observed at room temperature in detail. The PL emission spectrum under 208 nm excitation consists of three bands: two UV bands at 325 and 355 nm and an orange-red band at 620 nm. The three bands are ascribed to Pb 2+, Ce 3+ and Mn 2+, respectively, as activators. The Pb 2+ excitation band is observed at 243 nm, and the Ce 3+ excitation band at 295 nm. The Pb 2+ excitation band is also observed by monitoring the Ce 3+ fluorescence, and the Pb 2+ and Ce 3+ excitation bands, in addition to six Mn 2+ excitation bands, are also observed by monitoring the Mn 2+ fluorescence. These indicate that four types of the energy transfer can occur in calcite through the following processes: (1) Pb 2+ → Ce 3+, (2) Pb 2+ → Mn 2+, (3) Ce 3+ → Mn 2+ and (4) Pb 2+ → Ce 3+ → Mn 2+. 相似文献
15.
Structural parameters and cation ordering are determined for four compositions in the synthetic MgGa 2O 4-Mg 2GeO 4 spinel solid solution (0, 8, 15 and 23 mol% Mg 2GeO 4; 1400 °C, 1 bar) and for spinelloid β-Mg 3Ga 2GeO 8 (1350 °C, 1 bar), by Rietveld refinement of room-temperature neutron diffraction data. Sample chemistry is determined by
XRF and EPMA. Addition of Mg 2GeO 4 causes the cation distribution of the MgGa 2O 4 component to change from a disordered inverse distribution in end member MgGa 2O 4, [4]Ga = x = 0.88(3), through the random distribution, toward a normal cation distribution, x = 0.37(3), at 23 mol% Mg 2GeO 4. An increase in a o with increasing Mg 2GeO 4 component is correlated with an increase in the amount of Mg on the tetrahedral site, through substitution of 2 Ga 3+⇄ Mg 2++Ge 4+. The spinel exhibits high configurational entropy, reaching 20.2 J mol −1 (four oxygen basis) near the compositional upper limit of the solid solution. This stabilizes the spinel in spite of positive
enthalpy of disordering over the solid solution, where Δ H
D
= α x + β x
2, α = 22(3), β = −21(3) kJ mol −1. This model for the cation distribution across the join suggests that the empirically determined limit of the spinel solid
solution is correlated with the limit of tetrahedral ordering of Mg, after which local charge-balanced substitution is no
longer maintained.
Spinelloid β-Mg 3Ga 2GeO 8 has cation distribution M1[Mg 0.50(2)Ga 0.50(2)] M2[Mg 0.96(2)Ga 0.04(2)] M3[Mg 0.77(2) Ga 0.23(2)] 2 (Ge 0.5Ga 0.5) 2O 8 (tetrahedral site occupancies are assumed). Octahedral site size is correlated to Mg distribution, where site volume, site
distortion, and Mg content follow the relation M1<M3<M2. The disordered cation distribution provides local electrical neutrality
in the structure, and stabilization through increased configurational entropy (27.6 J mol −1; eight oxygen basis). Comparison of the crystal structures of Mg 1+
N
Ga 2−2
N
Ge
N
O 4 spinel, β-Mg 3Ga 2GeO 8, and Mg 2GeO 4 olivine reveals β-Mg 3Ga 2GeO 8 to be a true structural intermediate. Phase transitions across the pseudobinary are necessary to accommodate an increasing
divergence of cation size and valence, with addition of Mg 2GeO 4 component. Octahedral volume increases while tetrahedral volume decreases from spinel to β-Mg 3Ga 2GeO 8 to olivine, with addition of Mg and Ge, respectively. Furthermore, M-M distances increase regularly across the join, suggesting
that changes in topology reduce cation-cation repulsion.
Received: 9 November 1998 / Revised, accepted: 3 August 1999 相似文献
16.
Majorite of bulk composition Mg 0.86Fe 0.15SiO 3 was synthesized at 19 GPa and 1900 °C at an oxygen fugacity close to the Re/ReO 2 buffer. Optical absorption spectra of polycrystalline samples were measured from 4000 to 25000cm ?1. The following features were observed: (1) Three bands at 4554, 6005 and 8093 cm ?1 due to the 5E g → 5T 2g transition of Fe 2+ in a distorted dodecahedral site. (2) A band at 9340 cm ?1 due to the transition 5T 2g → 5E g of octahedral Fe 2+. (3) A band at 22784 cm ?1 resulting from Fe 3+, probably in an octahedral site ( 6A 1g → 4A 1g, 4E g). (4) A very intense system of Fe 2+ → Fe 3+ intervalence charge transfer bands which can be modelled by two Gaussian components centered at 16542 and 20128 cm ?1. The existence of two components in the charge transfer spectrum could be related to the fact that the tetragonal majorite structure may contain Fe 3+ in two different octahedral sites. The crystal field splitting Δ of Fe 2+ in dodecahedral coordination is 5717 cm ?1. If a splitting of the ground state in the order of 1000 cm ?1 is assumed, this yields a crystal field stabilization energy (CFSE) of 3930 cm ?1, comparable to the CFSE of Fe 2+ in pyrope-rich garnet. However, the splitting of 5T 2g is significantly higher than in pyrope. This would be consistent with Fe 2+ preferentially occupying the more distorted one of the two dodecahedral sites in the majorite structure. For octahedral Fe 2+, Δ= 9340 cm ?1 and CFSE=3736 cm ?1, assuming negligible splitting of the ground state. 相似文献
17.
Polarized electronic absorption spectra of orthorhombic fayalite, Fe 2SiO 4, [ E || a(|| Z),E || b(|| X), E || c(|| Y)], space group Pbnm, have been studied in the temperature range 293 ≤ T/K ≤ 1273. The spectra were analysed into component bands originating from spin-allowed dd transitions of iron(II) at the different sites, M1 and M2, in the structure. The assignments of bands, made on the basis
of the polarization dependence of the spectra and considerations of transition energies, were confirmed by the analysis of
the temperature-dependent spectra. The temperature dependencies of integral intensities, half band widths and energy positions
of absorptions bands caused by Fe 2+ on the different octahedral sites, M1 and M2, were evaluated for the individual transitions. Independent of the site symmetry,
absorption bands shift to lower energies and half band widths increase on rising temperature. The temperature dependence of
band intensities depends on site symmetry. The integral intensities are found to increase with temperature for the transition
metal ion on a centrosymmetric site, or remain constant when the site is missing an inversion centre. This is consistent with
the general conclusion of Taran et al. (1994).
Received: 11 October 2001 / Accepted: 17 January 2002 相似文献
18.
The solubility and incorporation mechanisms of water in synthetic and natural MgAl 2O 4 spinel have been investigated in a series of high-pressure/temperature annealing experiments. In contrast to most other nominally anhydrous minerals, natural spinel appears to be completely anhydrous. On the other hand, non-stoichiometric Al-rich synthetic (defect) spinel can accommodate several hundred ppm water in the form of structurally-incorporated hydrogen. Infrared (IR) spectra of hydrated defect spinel contain one main O-H stretching band at 3343-3352 cm −1 and a doublet consisting of two distinct O-H bands at 3505-3517 cm −1 and 3557-3566 cm −1. IR spectra and structural refinements based on single-crystal X-ray data are consistent with hydrogen incorporation in defect spinel onto both octahedral and tetrahedral O-O edges. Fine structure of O-H bands in IR spectra can be explained by partial coupling of interstitial hydrogen with cation vacancies, or by the effects of Mg-Al disorder on the tetrahedral site. The concentration of cation vacancies in defect spinel is a major control on hydrogen affinity. The commercial availability of large single crystals of defect spinel coupled with high water solubility and similarities in water incorporation mechanisms between hydrous defect spinel and hydrous ringwoodite (Mg 2SiO 4) suggests that synthetic defect spinel may be a useful low-pressure analogue material for investigating the causes and consequences of water incorporation in the lower part of Earth’s mantle transition zone. 相似文献
19.
The assignment of spin-allowed Fe 2+-bands in orthopyroxene electronic absorption spectra is revised by studying synthetic bronzite (Mg 0.8 Fe 0.2) 2Si 2O 6, hypersthene (Mg 0.5 Fe 0.5) 2Si 2O 6 and ferrosilite (Fe 2Si 2O 6). Reheating of bronzite and hypersthene single crystals causes a redistribution of the Fe 2+-ions over the M1 and M2 octahedra, which was determined by Mössbauer spectroscopy and correlated to the intensity change of the spin-allowed Fe 2+ d-d bands in the polarized absorption spectra. The 11000 cm -1 band is caused by Fe 2+ in M1 ( 5B 2g→ 5A 1g) and Fe 2+ in M2 ( 5A 1→ 5A 1), the 8500 cm -1 band by Fe 2+ in M1 ( 5B 2g→ 5B 1g) and the 5000 cm -1 band by Fe 2+ in M2 octahedra ( 5A 1→ 5B 1). The Fe 2+-Fe 3+ charge transfer band is identified at 12500cm -1 in the spectra of synthetic Fe 3+ -Al bearing ferrosilite. This band shows a strong γ-polarization and therefore is caused by Fe 2+ -Fe 3+-ions in edge-sharing octahedra. 相似文献
20.
Phase transformations in a natural sample of hedenbergite ((Ca 0.93Fe 0.61Mn 0.34Mg 0.08Na 0.01Zn 0.02Al 0.003)Si 2O 6) have been studied by X-ray diffraction up to 40 GPa at ∼ 1200°C in a diamond anvil cell interfaced with a laser heating
system. The starting hedenbergite phase decomposed into a garnet plus γ-spinel and stishovite at ∼ 14 GPa; then into garnet plus stishovite and wüstite at ∼ 18 GPa; and finally into perovskite
plus stishovite and wüstite at pressures higher than ∼ 24 GPa. On decompression to 0.1 MPa, all the high pressure phases are
retained except for the cubic perovskite, which reverts back into the ɛ-CaSiO 3 phase, in accordance with previous reports. Energy-dispersive SEM analyses show that the garnet is present as a calcium-rich
ABO
3-type phase. As no garnet phase has been previously observed either in pure CaSiO 3 or in pure CaMgSi 2O 6, it appears that the observed calcium-rich garnet phase has been stabilized by the presence of other cations such as the
Na +, Zn 2+, Mn 2+, Fe 2+, Mn 3+, Fe 3+ and Al 3+. 相似文献
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