Polarized X-ray absorption spectra and electronic structure of molybdenite (2H-MoS2)

Polarized S K- and L-edge, Mo L₃- and L₂-edge x-ray absorption near-edge structure (XANES) of natural molybdenite (2H-MoS₂) have been measured with synchrotron radiation. These results are qualitatively interpreted using the energy band model of molybdenite and provide important information on the unoccupied states of molybdenite. The valence band (VB) maximum of molybdenite is characterized by fully occupied Mo 4d_z ², and the conduction band (CB) minimum of molybdenite is characterized by unoccupied Mo 4d states. The unoccupied Mo 4d band is split into two sub-bands, designated as t _2g ^– /t _2g ⁺ and e _g ^– /e _g ⁺ sets. Although the relative energy of these two sets are difficult to be evaluated, probably the former has the lower energy than the latter, both two sets have the combination wave functions of the other unoccupied Mo 4d components, rather than the simple 4d_x ² — y₂ and 4d_xy states. The unoccupied Mo 4d sub-bands contain significant DOS of both S 3 p- and 3 s-like states, indicating strong hybridization with S 3s and 3 p states. In the lower energy sub-band, the DOS of the S p_z- and p_x,y-like states are very similar. However, in the higher energy sub-band, the DOS of the S 3 p_x,y-like state is lower than that of the S 3p_z state. Polarized S K-edge XANES also reveal the features of antibonding S p_z- and p_x,y-like states in molybdenite. The feature assigned to the S 3 p_z-like states is stronger and sharper, and shifts to lower energy by about 2 eV relative to that for the S 3 p_x,y-like states.     

Polarized electronic absorption spectra of Cr2SiO4 single crystals

Polarized electronic absorption spectra, E∥a(∥X), E∥b(∥Y) and E∥c(∥Z), in the energy range 3000–5000?cm^–1 were obtained for the orthorhombic thenardite-type phase Cr₂SiO₄, unique in its Cr²⁺-allocation suggesting some metal-metal bonding in Cr²⁺Cr²⁺ pairs with Cr-Cr distance 2.75?Å along [001]. The spectra were scanned at 273 and 120?K on single crystal platelets ∥(100), containing optical Y and Z, and ∥(010), containing optical X and Z, with thicknesses 12.3 and 15.6?μm, respectively. Microscope-spectrometric techniques with a spatial resolution of 20?μm and 1?nm spectral resolution were used. The orientations were obtained by means of X-ray precession photographs. The xenomorphic, strongly pleochroic crystal fragments (X deeply greenish-blue, Y faint blue almost colourless, Z deeply purple almost opaque) were extracted from polycrystalline Cr₂SiO₄, synthesized at 35?kbar, above 1440?°C from high purity Cr₂O₃, Cr (10% excess) and SiO₂ in chromium capsules. The Cr₂SiO₄-phase was identified by X-ray diffraction (XRD). Four strongly polarized bands, at about 13500 (I), 15700 (II), 18700 (III) and 19700 (IV) cm^–1, in the absorption spectra of Cr₂SiO₄ single crystals show properties (temperature behaviour of linear and integral absorption coefficients, polarization behaviour, molar absorptivities) which are compatible with an assignment to localized spin-allowed transitions of Cr²⁺ in a distorted square planar coordination of point symmetry C₂. The crystal field parameter of Cr²⁺ is estimated to be 10?Dq?10700?cm^–1. A relatively intense, sharp band at 18400?cm^–1 and three other minor features can, from their small half widths, be assigned to spin-forbidden dd-transitions of Cr²⁺. The intensity of such bands strongly decreases on decreasing temperature. The large half widths, near 5000?cm^–1 of band III are indicative of some Cr-Cr interactions, i.e. δ-δ* transitions of Cr₂ ⁴⁺, whereas the latter alone would be in conflict with the strong polarization of bands I and II parallel [100]. Therefore, it is concluded that the spectra obtained can best be interpreted assuming both dd-transitions of localized d-electrons at Cr²⁺ as well as δ-δ* transitions of Cr₂ ⁴⁺ pairs with metal-metal interaction. To explain this, a dynamic exchange process 2 Cr_loc ²⁺?Cr_{2, cpl} ⁴⁺ is suggested wherein the half life times of the ground states of both exchanging species are significantly longer than those of the respective optically excited states, such that the spectra show both dd- and δ-δ*-transitions.     

Polarized electronic absorption spectra of synthetic (Mg,Fe)-orthopyroxenes,ferrosilite and Fe3+-bearing ferrosilite

The assignment of spin-allowed Fe²⁺-bands in orthopyroxene electronic absorption spectra is revised by studying synthetic bronzite (Mg_0.8 Fe_0.2)₂Si₂O₆, hypersthene (Mg_0.5 Fe_0.5)₂Si₂O₆ and ferrosilite (Fe₂Si₂O₆). Reheating of bronzite and hypersthene single crystals causes a redistribution of the Fe²⁺-ions over the M1 and M2 octahedra, which was determined by Mössbauer spectroscopy and correlated to the intensity change of the spin-allowed Fe²⁺ d-d bands in the polarized absorption spectra. The 11000 cm^-1 band is caused by Fe²⁺ in M1 (⁵B_2g→⁵A_1g) and Fe²⁺ in M2 (⁵A₁→⁵A₁), the 8500 cm^-1 band by Fe²⁺ in M1 (⁵B_2g→⁵B_1g) and the 5000 cm^-1 band by Fe²⁺ in M2 octahedra (⁵A₁→⁵B₁). The Fe²⁺-Fe³⁺ charge transfer band is identified at 12500cm^-1 in the spectra of synthetic Fe³⁺ -Al bearing ferrosilite. This band shows a strong γ-polarization and therefore is caused by Fe²⁺ -Fe³⁺-ions in edge-sharing octahedra.     

Polarized electronic absorption spectra of colourless chalcocyanite,CuSO4, with a survey on crystal fields in Cu2+ minerals

Polarized electronic absorption spectra of colourless chalcocyanite, CuSO₄, have been measured using microscope-spectrometric techniques. The spectra are characterized by a structured and clearly polarized band system in the near-infrared spectral range with components centred at 11,720, 10,545, 9,100, and 7,320 cm^?1, which have been assigned to crystal field d–d transitions of Cu²⁺ cations in pseudo-tetragonally elongated CuO₆ polyhedra with point symmetry C _i ( \(\bar{1}\) ). The polarization behaviour is interpreted based on a D ₂(C ₂″) pseudo-symmetry. Crystal field calculations were performed for the actual triclinic point symmetry by applying the Superposition Model of crystal fields, as well as in terms of a ‘classic’ pseudo-tetragonal crystal field approach yielding the parameters Dq _(eq) = 910, Dt = 395, and Ds = 1,336 cm^?1, corresponding to a cubically averaged Dq _cub = 679 cm^?1. A comparative survey on crystal fields in Cu²⁺ minerals shows that the low overall crystal field strength in chalcocyanite, combined with a comparatively weak pseudo-tetragonal splitting of energy levels, is responsible for its unique colourless appearance among oxygen-based Cu²⁺ minerals. The weak crystal field in CuSO₄ can be related to the lower position of the SO₄ ^2? anion compared to, e.g. the H₂O molecule in the spectrochemical series of ligands.     

Polarized optical absorption spectra of synthetic chromium doped Mg2SiO4 (forsterite)

Polarized optical absorption measurements were carried out on three single crystals of Mg₂SiO₄ (forsterite), differently doped with Cr. Two crystals containing average 0.013 and 0.027 weight% Cr, respectively, were pulled from the melt in air, whereas one crystal containing average 0.08 weight% Cr was pulled from the melt in an argon atmosphere. The absorption spectra of the three crystals agree with each other although the intensity of single absorption bands varies significantly. In all -polarized patterns a sharp absorption line around 18000cm^-1 (550 nm) appears. Conjectures are presented to assign this line to the lasing center in Cr doped forsterite which very likely exists as Cr⁴⁺ at the fourfold coordinated Si site.     

Polarized electronic absorption spectra of Co2+ ions in the kieserite-type compounds CoSO4 · H2O and CoSeO4 · H2O

Polarized electronic absorption spectra of the kieserite-type compounds CoSO₄ · H₂O and CoSeO₄ · H₂O have been obtained at room temperature (spectral range 35 000-5000 cm^-1) and at liquid nitrogen temperature (visible spectral region), using microscope-spectrometric techniques. The spectra are interpreted and evaluated in terms of a tetragonal crystal field formalism for the d⁷ configuration, in regard to the pseudotetragonal elongation of the CoO₄(H₂O)₂ octahedra, known from previous X-ray structure investigations, employing the tetragonal parameters Dq, Dt, and Ds, and the Racah parameters B and C. The observed and calculated energy levels are in good agreement for the following parameter sets: CoSO₄ · H₂O: Dq=826, Dt=40, Ds=350, B=856, C=3580 cm^-1; CoSeO₄· H₂O: Dq=817, Dt=44, Ds=406, B=841, C=3490 cm^-1; corresponding ‘cubic’ crystal field strengths Dq_cub are 803 and 792 cm^-1, respectively. The values of Dq_(cub), Racah B and C are in the common range for Co²⁺ ions in (pseudo) octahedral fields of oxygen ligands, and their differences in CoSO₄· H₂O compared to CoSeO₄ · H₂O are consistent with somewhat different mean Co-O bond lengths and with a slightly higher covalent contribution to Co-O bonding in the selenate compound. The values found for the parameter Dt, which is directly correlated to the extent of tetragonal distortion, are much lower than expected from purely geometrical considerations, thus confirming a significantly higher position of H₂O ligands in the spectrochemical series compared to oxygen ligands belonging to SO₄ or SeO₄ groups.     

Determination of quantitative cation distribution in orthopyroxenes from electronic absorption spectra

Electronic and Mössbauer absorption spectra and electron microprobe data are correlated for iron-bearing orthopyroxenes. The correlation provides a means of quantitatively determining the distribution of Fe²⁺ between the M(1) and M(2) sites of orthopyroxene crystals from electronic spectra and electron microprobe analysis. The electronic spectra are used to analyze the changes in the Fe²⁺ distribution produced during heating experiments and confirm earlier results from Mössbauer spectra. Two components of the spin-allowed transition of Fe²⁺ in the M(1) site are identified at about 13,000 cm^?1 and 8,500 cm^?1 in γ. Molar absorptivity (?) values for all spin-allowed Fe²⁺ absorption bands in the near-infrared region are determined. The M(2) Fe²⁺ band at ～5,000 cm^?1 in β is the analytically most useful for site occupancy determinations. It remains linear with concentration (?=9.65) over the entire compositional range. The band at ～10,500 cm^?1 in α is the most sensitive to M(2) Fe²⁺ concentration (?=40.8), but deviates from linearity at high iron concentrations. The origins of spin-forbidden transitions in the visible region are examined.     

Crystal structure and chemical formula of schmiederite,Pb2Cu2(OH)4(SeO3)(SeO4), with a comparison to linarite,PbCu(OH)2(SO4)

Summary Based on a X-ray structure analysis it was proved that the mineral schmiederite contains both selenite and selenate groups [a = 9.922(3)Å,b = 5.712(2)Å,c = 9.396(3)Å, = 101.96(3)°, space group P2₁/m,Z = 2 {Pb₂Cu₂(OH)₄(SeO₃)(SeO₄)},R _w = 0.055 for 1131 reflections up to sin / = 0.65 Å^–1]. The crystal structure is closely related to that of linarite [a = 9.701(2) Å,b = 5.650(2) Å,c= 4.690(2)Å, = 102.65(2)°, space group P2₁/m,Z = 2 {PbCu(OH)₂(SO₄)},R _w = 0.034 for 1991 reflections up to sin / = 1.0 Å^–1].The Pb atom in linarite and the Pb(1) atom in schmiederite have each three Pb-O bonds < 2.45 Å with trigonal pyramidal arranged ligands; the Pb(2) atom in schmiederite has only one such near O atom. The Cu atoms are approximately square planar coordinated by hydroxil groups. In addition two further O atoms complete the coordination figure to a strongly distorted octahedron. All the anion groups have the usual geometry.

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Kristallstruktur und chemische Formel von Schmiederit, Pb₂Cu₂(OH)₄(SeO₃)(SeO₄), mit einem Vergleich zu Linarit, PbCu(OH)₂(SO₄)

Zusammenfassung Basierend auf einer Röntgen-Strukturuntersuchung konnte das Vorliegen von Selenit-und Selenatgruppen im Mineral Schmiederit belegt werden [a=9,922(3) Å,b = 5,712(2) Å,c = 9,396(3) Å, = 101,96(3)°, Raumgruppe P2₁/m,Z=2 {Pb₂Cu₂(OH)₄(SeO₃)(SeO₄)},R _w = 0,055 für 1131 Reflexe bis sin /, = 0,65 Å^–1]. Die Kristallstruktur weist enge Beziehungen zu jener des Linarits auf [a = 9,701(2) Å,b = 5,650(2) Å,c = 4,690(2) Å, = 102,65(2)°, Raumgruppe P2₁/m,Z=2 {PbCu(OH)₂(SO₄)},R _w = 0,034 für 1991 Reflexe bis sin / = 1,0 Å^–1].Das Pb-Atom im Linarit sowie das Pb(1)-Atom im Schmiederit haben jeweils drei Pb-O-Bindungen <,45 Å, wobei die Liganden trigonal pyramidal angeordnet sind; das Pb(2)-Atom im Schmiederit hat hingegen nur ein derart nahes O-Atom. Die Cu-Atome sind etwa quatratisch planar von Hydroxilgruppen koordiniert; zwei weitere O-Atome ergänzen die Koordinationsfigur zur einem stark verzerrten Oktaeder. Die Aniongruppen haben die üblichen Dimensionen.

     

The crystal structure of trigonite,Pb3Mn(AsO3)2(AsO2OH)

Summary The mineral trigonite crystallizes in the monoclinic space groupPn–C _s ² witha ₀=7.26,b ₀=6.78,c ₀=11.09Å; =91.5°,Z=2. The structure was determined from 1250 X-ray intensities collected on an automatic two circle Weissenberg-type diffractometer. The final residual isR=6.5% using anisotropic temperature factors for Pb, Mn and As, and isotropic temperature factors for O.The structure consists of MnO₆ octahedra, sharing all six oxygens with arsenite groups to form a framework. The Pb atoms are attached to this framework with Pb–O distances2.23Å. One oxygen, bound only to an As atom, is interpreted as the donor for a hydrogen bond of 2.75Å.

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Die Kristallstruktur des Trigonits, Pb₃Mn(AsO₃)₂(AsO₂OH)

Zusammenfassung Das Mineral Trigonit kristallisiert monoklin, RaumgruppePn–C _s ² ,a ₀=7,26,b ₀=6,78,c ₀=11,09Å; =91,5°;Z=2. Die Strukturermittlung erfolgte anhand von 1250 Röntgenintensitäten, die auf einem automatischen Zweikreis-Weissenbergdiffraktometer gesammelt wurden. Mit anisotropen Temperaturfaktoren für Pb, Mn und As sowie isotropen für die O-Atome ergibt sich einR-Wert von 6,5%.Die MnO₆-Oktaeder werden über die sechs Sauerstoffe mit Arsenitgruppen zu einem dreidimensionalen Gerüst verknüpft. Über Pb-O-Abstände2,23 Å sind die Pb-Atome in dieses Gerüst eingebaut. Ein Sauerstoff, nur an ein As-Atom gebunden, wird als Donator einer H-Brücke von 2,75 Å interpretiert.

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With 2 Figures     

Oxidative Ce3+ sequestration by fungal manganese oxides with an associated Mn(II) oxidase activity

Sequestration of Ce³⁺ by biogenic manganese oxides (BMOs) formed by a Mn(II)-oxidizing fungus, Acremonium strictum strain KR21-2, was examined at pH 6.0. In anaerobic Ce³⁺ solution, newly formed BMOs exhibited stoichiometric Ce³⁺ oxidation, where the molar ratio of Ce³⁺ sequestered (Ce_seq) relative to Mn²⁺ released (Mn_rel) was maintained at approximately two throughout the reaction. A similar Ce³⁺ sequestration trend was observed in anaerobic treatment of BMOs in which the associated Mn(II) oxidase was completely inactivated by heating at 85 °C for 1 h or by adding 50 mM NaN₃. Aerobic Ce³⁺ treatment of newly formed BMO (enzymatically active) resulted in excessive Ce³⁺ sequestration over Mn²⁺ release, yielding Ce_seq/Mn_rel > 200, whereas heated or poisoned BMOs released a significant amount of Mn²⁺ with lower Ce³⁺ sequestration efficiency. Consequently, self-regeneration by the Mn(II) oxidase in newly formed BMO effectively suppressed Mn²⁺ release and enhanced oxidative Ce³⁺ sequestration under aerobic conditions. Repeated treatments of heated or poisoned BMOs under aerobic conditions confirmed that oxidative Ce³⁺ sequestration continued even after most Mn oxide was released from the solid phase, indicating auto-catalytic Ce³⁺ oxidation at the solid phase produced through primary Ce³⁺ oxidation by BMO. From X-ray diffraction analysis, the resultant solid phases formed through Ce³⁺ oxidation by BMO under both aerobic and anaerobic conditions consisted of cerianite with crystal sizes of 5.00–7.23 Å. Such nano-sized CeO₂ (CeO_2,BMO) showed faster auto-catalytic Ce³⁺ oxidation than that on well-crystalized cerianite under aerobic conditions, where the normalized pseudo-first order rate constants for auto-catalytic Ce³⁺ oxidation on CeO_2,BMO was two orders of magnitude higher. Consequently, we concluded that Ce³⁺ contact with BMOs sequesters Ce³⁺ through two oxidation paths: primary Ce³⁺ oxidation by BMOs produces nano-sized crystalline cerianite, and subsequent auto-catalytic Ce³⁺ oxidation efficiently occurs using dissolved oxygen as the oxidizing agent. Pretreatment of newly formed BMOs with La³⁺ solution resulted in decreased rate constants for primary Ce³⁺ oxidation by BMO due to site blocking by La³⁺ sorption. The results presented herein increase our understanding of the role of BMO in oxidative Ce³⁺ sequestration process(es) through enzymatic and abiotic paths in natural environments and provide supporting evidence for the potential application of BMOs towards the recovery of Ce³⁺ from contaminated waters.     

Luminescence excitation spectra of Mn2+ in synthetic forsterite

Detailed ligand-field spectra of Mn²⁺ in both microcrystalline and single-crystal synthetic forsterite are obtained using the technique of luminescence excitation spectroscopy. It is shown that Mn²⁺ has an almost exclusive preference for one particular cation site which is most probably the M2 site. Low temperature measurements reveal a no-phonon (purely electronic) transition at 16,260 cm^?1 (615 nm) which is the energy of the lowest split component of the ⁴ T ₁(G) state above the ground state. Phonon replicas of this transition are evident showing that a particular phonon mode (180 cm^?1) is dominantly involved. An analysis of the polarized spectra of Mn²⁺ in single-crystal forsterite shows the choice of C _2v (C ₂, σ _d ) pseudosymmetry for the M2 site yields the best agreement with the polarization dependence of the transitions between the ligand-field states of the Mn²⁺ ion in this site.     

The magnetic structure of the langbeinite K2Mn2 (SO4)3

K₂Mn₂(SO₄)₃ orders magnetically at T_N= 1.75 K. One of the orthorhombic cell edges of the low temperature Langbeinite structure becomes doubled in the magnetically ordered state. The antiferromagnetic spin structure found is characterized by weak or vanishing molecular fields due to nearest neighbours. There are no indications of magnetic order down to 1.45 K in the isomorphic compound K₂Co₂ (SO₄)₃.     

Crystal chemistry of selenates with mineral-like structures: V. Crystal structures of (H3O)2[(UO2)(SeO4)2(H2O)](H2O)2 and (H3O)2[(UO2)(SeO4)2(H2O)](H2O), new compounds with rhomboclase and goldichite topology

The crystal structures of two new compounds (H₃O)₂[(UO₂)(SeO₄)₂(H₂O)](H₂O)₂ (1, orthorhombic, Pnma, a = 14.0328(18), b = 11.6412(13), c = 8.2146(13) Å, V = 134.9(3) ų) and (H₃O)₂[(UO₂)(SeO₄)₂(H₂O)](H₂O) (2, monoclinic, P2₁/c, a = 7.8670(12), b = 7.5357(7), c = 21.386(3) Å, β = 101.484(12)°, V = 1242.5(3) ų) have been solved by direct methods and refined to R ₁ = 0.076 and 0.080, respectively. The structures of both compounds contain sheet complexes [(UO₂)(SeO₄)₂]^2? formed by cornershared [(UO₂)O₄(H₂O)] bipyramids and SeO₄ tetrahedrons. The sheets are parallel to the (100) plane in structure 1 and to (?102) in structure 2. The [(UO₂)(SeO₄)₂(H₂O)]^2? layers are linked by hydrogen bonds via interlayer groups H₂O and H₃O⁺. The sheet topologies in structures 1 and 2 are different and correspond to the topologies of octahedral and tetrahedral complexes in rhomboclase (H₂O₂)⁺[Fe(SO₄)₂(H₂O)₂] and goldichite K[Fe(SO₄)₂(H₂O)₂](H₂O)₂, respectively.     

A pre-edge analysis of Mn K-edge XANES spectra to help determine the speciation of manganese in minerals and glasses

High-resolution manganese K-edge X-ray absorption near edge structure spectra were collected on a set of 40 Mn-bearing minerals. The pre-edge feature information (position, area) was investigated to extract as much as possible quantitative valence and symmetry information for manganese in various “test” and “unknown” minerals and glasses. The samples present a range of manganese symmetry environments (tetrahedral, square planar, octahedral, and cubic) and valences (II to VII). The extraction of the pre-edge information is based on a previous multiple scattering and multiplet calculations for model compounds. Using the method described in this study, a robust estimation of the manganese valence could be obtained from the pre-edge region at 5% accuracy level. This method applied to 20 “test” compounds (such as hausmannite and rancieite) and to 15 “unknown” compounds (such as axinite and birnessite) provides a quantitative estimate of the average valence of manganese in complex minerals and silicate glasses. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Electronic absorption spectra of Cr3+ ions in natural clinopyroxenes

The polarized (E‖a′, E‖b and E‖c) electronic absorption spectra of five natural chromium-containing clinopyroxenes with compositions close to chromdiopside, omphacite, ureyite-jadeite (12.8% Cr₂O₃), jadeite, and spodumene (hiddenite) were studied. The polarization dependence of the intensities of the Cr³⁺ bands in the clinopyroxene spectra cannot be explained by the selection rules for the point groups C ₂ or C _2v but can be accounted for satisfactorily with the help of the higher order pseudosymmetry model, i.e. with selection rules for the point symmetry group C _3v. The trigonal axis of the pseudosymmetry crystal field forms an angle of 20.5° with the crystallographic direction c in the (010) plane. D _q increases from diopside (1542 cm^?1) through omphacite (1552 cm^?1), jadeite (1574 cm^?1) to spodumene (1592 cm^?1). The parameter B which is a measure of covalency for Cr³⁺-O bonds at M1 sites in clinopyroxene depends on the Cr³⁺ concentration and the cations at M2 sites.     

The crystal structure of ilinskite, NaCu5O2(SeO3)2Cl3, and review of mixed-ligand CuOmCln coordination geometries in minerals and inorganic compounds

The crystal structure of ilinskite, NaCu₅O₂(SeO₃)₂Cl₃, a rare copper selenite chloride from volcanic fumaroles of the Great fissure Tolbachik eruption (Kamchatka peninsula, Russia), has been solved by direct methods and refined to R ₁?=?0.044 on the basis of 2720 unique observed reflections. The mineral is orthorhombic, Pnma, a?=?17.769(7), b?=?6.448(3), c?=?10.522(4) Å, V?=?1205.6(8) ų, Z?=?4. The The CuO_mCl_n coordination polyhedra share edges to form tetramers that have 'additional' O1 and O2 atoms as centers. The O1Cu₄ and O2Cu₄ tetrahedra share common Cu atoms to form [O₂Cu₅]⁶⁺ sheets. The SeO₃ groups and Cl atoms are adjacent to the [O₂Cu₅]⁶⁺ sheets to form complex layers parallel to (100). The Na⁺ cations are located in between the layers. A review of mixed-ligand CuO_mCl_n coordination polyhedra in minerals and inorganic compounds is given. There are in total 26 stereochemically different mixed-ligand Cu-O-Cl coordinations.     

Polarized single crystal absorption spectroscopy of the Pnam-P2 1/a transition of Ilvaite Ca(Fe2+, Fe3+)Fe2+Si2O8(OH) as measured between 300 K and 450 K

The temperature dependence of the absorption spectra of ilvaite, Ca(Fe²⁺,Fe³⁺)Fe²⁺Si₂O₈(OH), shows strongly one dimensional transport behaviour with no singularity at the Pnam-P2₁/a phase transition point near 335 K. Polarized single crystal transmission measurements were carried out between 300 K and 450 K in a frequency range between 600 and 23 000 cm⁻¹. No Drude —absorption at low energies was found at any temperature. A macroscopic, thermodynamic model based on Landau-Ginzburg theory is given which accounts for the observed macroscopic properties of the structural phase transition and its coupling with the Fe²⁺-Fe³⁺ ordering. This ordering scheme is discussed on an atomistic level and compared with the behaviour of magnetite and trans-(CH) _x .     

Synthetic Mn3+-kyanite and viridine, (Al2-xMn x 3+ ) SiO5, in the system Al2O3-MnO-MnO2-SiO2

Experiments on the join Al₂SiO₅-“Mn₂SiO₅” of the system Al₂O₃-SiO₂-MnO-MnO₂ in the pressure/temperature range 10–20 kb/900–1050° C with gem quality andalusite, Mn₂O₃, and high purity SiO₂ as starting materials and using /_O2-buffer techniques to preserve the Mn³⁺ oxidation state had following results: At 20 kb/1000°C orange-yellow kyanite mixed crystals are formed. The kyanite solid solubility is limited at about (Al_1.88Mn _0.12 ³⁺ )SiO₅ and, thus, equals approximately that on the join Al₂SiO₅-“Fe₂SiO₅” (Langer and Frentrup, 1973) indicating that there is no Jahn-Teller stabilisation of Mn³⁺ in the kyanite matrix. 5 mole % substitution causes the kyanite lattice constants a _o, b _o, c _o, and V _o to increase by 0.015, 0.009, 0.014 Å, and 1.6 ų, resp., while α, β, γ, remain unchanged. Between 10 and 18 kb/900°C, Mn³⁺-substituted, strongly pleochroitic (emeraldgreen-yellow) andalusite_ss (viridine) was obtained. At 15 kb/900°C, the viridine compositional range is about (Al_1.86Mn _0.14 ³⁺ )SiO₅-(Al_1.56Mn _0,44 ³⁺ )SiO₅. Thus, Al→Mn³⁺ substitutional degrees are appreciably higher in andalusite than in kyanite, proving a strong Jahn-Teller effect of Mn³⁺ in the andalusite structure, which stabilises this structure type at the expense of kyanite and sillimanite and, thus, enlarges its PT-stability range extremely. 17 mole % substitution cause the andalusite constants a _o, b _o, c _o, and V _o to increase by 0.118, 0.029, 0.047 Å and 9.4 ų, resp. At “Mn₂SiO₅”-contents smaller than about 7 mole %, viridine coexists with Mn-poor kyanite. At “Mn₂SiO₅”-concentrations higher than the maximum kyanite or viridine miscibility, braunite (tetragonal, ideal formula Mn²⁺Mn³⁺[O₈/Si0₄]), pyrolusite and SiO₂ were found to coexist with the Mn³⁺-saturated ky _ss or and _ss, respectively. In both cases, braunites were Al-substituted (about 1 Al for 1 Mn³⁺). Pure synthetic braunites had the lattice constants a _o 9.425, c _o, 18.700 Å, V _o 1661.1 ų (ideal compn.) and a _o 9.374, c _o 18.593 ų, V _o 1633.6 ų (1 Al for 1 Mn³⁺). Stable coexistence of the Mn²⁺-bearing phase braunite with the Mn⁴⁺-bearing phase pyrolusite was proved by runs in the limiting system MnO-MnO₂-SiO₂.     

Evidence for absorption by exchange-coupled Fe2+-Fe3+ pairs in the near infra-red spectra of minerals

Reduction of Fe³⁺ to Fe²⁺ by heating in hydrogen reduces the absorbance of the bands at 9,000 and 13,800 cm^?1 in the E ⊥ c spectrum of tourmaline, and the 9,000 and 11,000 cm^?1 bands in the E ∥ (001) spectrum of biotite. This behaviour is consistent with the presumed d-d origin of these bands (which seems well established) only if they gain much of their intensity from exchange-coupling with neighbouring Fe³⁺ ions. Intensification of spin-forbidden bands in sapphire by Fe³⁺-Fe³⁺ exchange-coupling was recognized by Ferguson and Fielding (1971, 1972), but exchange-coupling has not previously been thought to intensify spin-allowed d-d bands. Spin-allowed exchange-coupled bands resulting from Fe²⁺-Fe³⁺ pairs have features in common with both normal single ion d-d bands, which they resemble in energy, width and pressure dependence, and Fe²⁺+Fe³⁺→Fe³⁺+Fe²⁺ charge transfer bands, which they resemble in temperature-, heat treatment-, composition-, and polarization-dependence. Distinction between normal d-d, charge transfer, and pair d-d absorptions is thus complicated, and criteria for assigning these bands are discussed. Spin-allowed exchange-coupled pair bands should be sought in the spectra of transition metal clusters (trimers and polymers as well as pairs may be involved) whenever geometry favours their origin. It is possible that the bands near 10,000 and 11,500 cm^?1 in blue sapphire, and at about 5,000 cm^?1 in titanian garnets are of this type, but many other examples are likely to occur. Exchange-coupling may involve ions other than Fe³⁺ (e.g., Mn²⁺, also d ⁵), although Fe²⁺-Fe²⁺ coupling is unlikely to be important at laboratory temperatures.     

X-ray absorption fine structure study of As(V) and Se(IV) sorption complexes on hydrous Mn oxides

X-ray absorption fine structure (XAFS) spectroscopic analysis at the As, Se, and Mn K-edges was used to study arsenate [As(V)O₄³⁻] and selenite [Se(IV)O₃²⁻] sorption complexes on the synthetic hydrous manganese oxides (HMOs) vernadite (δ-MnO₂) and K-birnessite (nominal composition: K₄Mn₁₄O₂₇ · 9H₂O). No significant changes were observed in sorption complex structure as a function of sorbent, pH (5 to 8), surface coverage (0.04 to 0.73 μmol/m²), or reaction time (5 to 22 h) in the arsenate or selenite systems. In the arsenate/HMO system, extended XAFS parameters indicate an average second-neighbor As(V) coordination of 2.0 ± 0.4 Mn at an average distance of 3.16 ± 0.01 Å, which is consistent with formation of As(V)O₄ sorption complexes sharing corners with two adjacent Mn(IV)O₆ surface species (i.e., bidentate, binuclear). In the selenite/HMO system, selenite surface complexes are surrounded by two shells of Mn atoms, which could represent two different adsorption complexes or a precipitate. The first shell consists of 1.6 ± 0.4 Mn at 3.07 ± 0.01 Å, which is consistent with the selenite anion forming bidentate (mononuclear) edge-sharing complexes with Mn(II)O₆ or Mn(III)O₆ octahedra. The second shell consists of 1.4 ± 0.4 Mn at 3.49 ± 0.03 Å, consistent with selenite forming monodentate, corner-sharing complexes with Mn(II)O₆ or Mn(III)O₆ octahedra. Pauling bond valence analysis that uses the extended XAFS-derived bond lengths for As(V)-O, Se(IV)-O, and Mn-O bonds indicates that the proposed surface complexes of selenite and arsenate on HMOs should be stable. Although a nearly identical Se(IV) coordination environment is found in a crystalline Mn(II)-Se(IV) precipitate (which has a structure similar to that of MnSeO₃ · H₂O), there are significant differences in the X-ray absorption near-edge structure and extended XAFS spectra of this precipitate and the selenite/HMO sorption samples. These differences coupled with transmission electron microscopy results suggest that if a precipitate is present it lacks long-range order characteristic of crystalline MnSeO₃ · H₂O.