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1.
Two new protoamphibole-type amphiboles with space group type Pnmn, have been found in nature: protoferro-anthophyllite (Fe0.80Mn0.20)2 (Fe0.98Mg0.02)5 (Si4O11)2(OH)2, and protomangano-ferro-anthophyllite, (Mn0.70Fe0.30)2 (Fe0.82Mg0.18)5 (Si4O11)2(OH)2. Protoferro-anthophyllite (PFA) occurs in pegmatites at both Gifu Prefecture, Japan and at Cheyenne Mountain, El Paso County, Colorado, USA. Protomangano-ferro-anthophyllite, (PMFA) occurs in pegmatites at Fukushima Prefecture and in a Mn mine at Tochigi Prefecture, Japan. Structure determinations of the two amphiboles show that both are isostructural with the synthetic fluorian-amphibole, protoamphibole (= protofluorian-lithian-anthophyllite). A calculation of the procrystal electron density distributions, the bond paths and the bond critical point properties of PFA, PMFA, grunerite and protoamphibole indicates that the M4 cation in these amphiboles is 4-coordinated. A calculation of the electron density distributions at the Becke3LYP/6-311G(2d,p) level for model silicate tetrahedra for these amphiboles and anthophyllite reveals that the value of the electron density at the bond critical points, ρ(r c ), for the SiO(nbr) bonds is larger, on average (0.93 e/Å3), than that for the SiO(br) bonds (0.90 e/Å3). The observed SiO bond lengths decrease linearly with increasing ρ(r c ) while the magnitudes of the curvatures of ρ(r c ) both perpendicular and parallel to the bonds and the Laplacian of ρ(r c ) each increases. These trends are associated with an increase in the electronegativity of the Si cation, a possible increase in the covalent character of the SiO bond and a tendency for SiO(nbr) bonds to be involved in wider OSiO angles than SiO(br) bonds. It is possible, if not likely, that protoanthophyllite has often been misidentified as anthophyllite.  相似文献   

2.
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) Å3) 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) Å3) 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.  相似文献   

3.
In a group of minerals of reasonable complexity in which the structure topology is related but not identical, the general relation between structure topology and chemical composition is not known. This problem is of major significance. The structural hierarchy and stereochemistry are described for 27 titanium disilicate minerals that contain the TS (titanium-silicate) block, a central trioctahedral (O) sheet and two adjacent (H) sheets of [5]- and [6]-coordinated polyhedra and (Si2O7) groups and related delindeite. The TS block is characterized by a planar cell based on translation vectors, t 1 and t 2 , with t 1 ～ 5.5 and t 2 ～ 7 Å and t 1 ∧ t 2 close to 90°. The general formula of the TS block is A 2 P B 2 P M 2 H M 4 O (Si 2 O 7 ) 2 X 4 + n, where M 2 H and M 4 O = cations of the H and O sheets; MH = Ti (= Ti + Nb), Zr, Mn2+, Ca; MO = Ti, Zr, Mn2+, Ca, Na; A P and B P are cations at the peripheral (P) sites = Na, Ca, Ba; X = anions = O, OH, F; n = 0, 2, 4; the core part of the TS block is shown in bold and is invariant. Cations in each sheet of the TS block form a close-packed layer and the three layers are cubic close packed.There are three topologically distinct TS blocks, depending on the type of linkage of two H sheets and the central O sheet. The H sheets of one TS block attach to the O sheet in the same manner. All structures consist of a TS block and an I (intermediate) block that comprises atoms between two TS blocks. Usually, the I block consists of alkali and alkaline-earth cations, (H2O) groups and oxyanions (PO4)3?, (SO4)2? and (CO3)2?. These structures naturally fall into four groups, based on differences in topology and stereochemistry of the TS block. In Group I, Ti = 1 apfu Ti occurs in the O sheet, and (Si2O7) groups link to a Na polyhedron of the O sheet (linkage 1). In Group II, Ti = 2 apfu, Ti occurs in the H sheet, and (Si2O7) groups link to two M 2+ octahedra of the O sheet adjacent along t 2 (linkage 2). In Group III, Ti = 3 apfu, Ti occurs in the O and H sheets, and (Si2O7) groups link to the Ti octahedron of the O sheet (linkage 1). In Group IV, Ti = 4 apfu (the maximum possible content of Ti in the TS block), Ti occurs in the O and H sheets, and (Si2O7) groups link to two Ti octahedra of the O sheet adjacent along t 1 (linkage 3). The stability of the TS block is due to the ability of Ti (Nb) to have an extremely wide range in Ti (Nb)-anion bond lengths, 1.68–2.30 Å, which allows the chemical composition of the TS block to vary widely. In crystal structures so far known, only one type of TS block occurs in a structure. The TS block propagates close-packing of cations onto the I block. The general structural principles and the relation between structure topology and chemical composition are described for the TS-block minerals. These principles allow prediction of structural arrangements and possible chemical compositions, and testing whether or not all aspects of the structure and chemical formula of a mineral are correct. Here, I show how these principles work, and review recent results that show the effectiveness of these principles as a predictive technique.  相似文献   

4.
In the lattice energy expression of forsterite, based on a Born-Mayer (electrostatic+repulsive+dispersive) potential, the oxygen charge z o, the hardness parameter ρ and the repulsive radii r Mg and r Si appear as unknown parameters. These were determined by calculating the first and second partial derivatives of the energy with respect to the cell edges, and equalizing them to quantities related to the crystal elastic constants; the overdetermined system of equations was solved numerically, minimizing the root-mean-square deviation. To test the results obtained, the SiO 4 4? ion was assumed to move in the unit-cell, and the least-energy configuration was sought and compared with the experimental one. By combining the two methods, the optimum set of parameters was: z o=?1.34, ρ=0.27 Å, r Mg=0.72 Å, r Si=0.64 Å. The values ?8565.12 and ?8927.28 kJ mol?1 were obtained, respectively, for the lattice energy E Land for its ionic component E L 0 ,which accounts for interactions between Mg2+ and SiO 4 4? ions only. The charge distribution calculated on the SiO 4 4? ion was discussed and compared with other results. Using appropriate thermochemical cycles, the formation enthalpy and the binding energy of SiO 4 4? were estimated to be: ΔH f(SiO 4 4? )=2117.6 and E(SiO 4 4? )=708.6 kJ mol?1, respectively.  相似文献   

5.
The power law regression equation, <R(M–O)> = 1.46(<ρ(r c)>/r)?0.19, relating the average experimental bond lengths, <R(M–O)>, to the average accumulation of the electron density at the bond critical point, <ρ(r c)>, between bonded pairs of metal and oxygen atoms (r is the row number of the M atom), determined at ambient conditions for oxide crystals, is similar to the regression equation R(M–O) = 1.41(ρ(r c)/r)?0.21 determined for three perovskite crystals at pressures as high as 80 GPa. The pair are also comparable with the equation <R(M–O)> = 1.43(<s>/r)?0.21 determined for oxide crystals at ambient conditions and <R(M–O)> = 1.39(<s>/r)?0.22 determined for geometry-optimized hydroxyacid molecules that relate the geometry-optimized bond lengths to the average Pauling bond strength, <s>, for the M–O bonded interactions. On the basis of the correspondence between the equations relating <ρ(r c)> and <s> with bond length, it seems plausible that the Pauling bond strength might serve a rough estimate of the accumulation of the electron density between M–O bonded pairs of atoms. Similar expressions, relating bond length and bond strength hold for fluoride, nitride and sulfide molecules and crystals. The similarity of the expressions for the crystals and molecules is compelling evidence that molecular and crystalline M–O bonded interactions are intrinsically related. The value of <ρ(r c)> = r[(1.41)/<R(M–O)>]4.76 determined for the average bond length for a given coordination polyhedron closely matches the Pauling’s electrostatic bond strength reaching each the coordinating anions of the coordinated polyhedron. Despite the relative simplicity of the expression, it appears to be more general in its application in that it holds for the bulk of the M–O bonded pairs of atoms of the periodic table.  相似文献   

6.
For ABO 4 type ternary oxides, high pressure phase transformations known up to the present are reviewed, and an attempt is made to explain and predict crystal structures of their high pressure phases. When ABO 4 type compounds are plotted based on the two variables, k=r A /r B and t=(r A +r B )/2r O, where r A , r B , and r O are the ionic radii of A and B cations and divalent oxygen, they can be classified into the major structure types. It is found empirically that a compound basically transforms to the structure type isostructural with a compound lying in a classified area with the same k and larger t values in the diagram.  相似文献   

7.
The crystal structures of synthetic K-dravite [XKYMg 3 Z Al 6 T Si6O18(BO3) 3 V (OH) 3 W (OH)], dravite [XNaYMg 3 Z Al 6 T Si6O18(BO3) 3 V (OH) 3 W (OH)], oxy-uvite [XCaYMg 3 Z Al 6 T Si6O18(BO3) 3 V (OH) 3 W O], and magnesio-foitite [X?Y(Mg2Al)ZAl 6 T Si6O18(BO3) 3 V (OH) 3 W (OH)] are investigated by polarized Raman spectroscopy, single-crystal structure refinement (SREF), and powder X-ray diffraction. The use of compositionally simple tourmalines characterized by electron microprobe analysis facilitates the determination of site occupancy in the SREF and band assignment in the Raman spectra. The synthesized K-dravite, oxy-uvite, and magnesio-foitite have significant Mg–Al disorder between their octahedral sites indicated by their respective average 〈Y–O〉 and 〈Z–O〉 bond lengths. The Y- and Z-site compositions of oxy-uvite (YMg1.52Al1.48(10) and ZAl4.90Mg1.10(15)) and magnesio-foitite (YAl1.62Mg1.38(18) and ZAl4.92Mg1.08(24)) are refined from the electron densities at each site. The Mg–Al ratio of the Y and Z sites is also determined from the relative integrated peak intensities of the Raman bands in the O–H stretching vibrational range (3250–3850 cm?1), producing values in good agreement with the SREF data. The unit cell volume of tourmaline increases from magnesio-foitite (1558.4(3) Å3) to dravite (1569.5(4)–1571.7(3) Å3) to oxy-uvite (1572.4(2) Å3) to K-dravite (1588.1(2) Å3), mainly due to lengthening of the crystallographic c-axis. The increase in the size of the X-site coordination polyhedron from dravite (Na) to K-dravite (K) is accommodated locally in the crystal structure, resulting in the shortening of the neighboring O1–H1 bond. In oxy-uvite, Ca2+ is locally associated with a deprotonated W (O1) site, whereas vacant X sites are neighbored by protonated W (O1) sites. Increasing the size of the X-site-occupying ion does not detectably affect bonding between the other sites; however, the higher charge of Ca and the deprotonated W (O1) site in oxy-uvite are correlated to changes in the lattice vibration Raman spectrum (100–1200 cm?1), particularly for bands assigned to the T 6O18 ring. The Raman spectrum of magnesio-foitite shows significant deviations from those of K-dravite, dravite, and oxy-uvite in both the lattice and O–H stretching vibrational ranges (100–1200 and 3250–3850 cm?1, respectively). The vacant X site is correlated with long- and short-range changes in the crystal structure, i.e., deformation of the T 6O18 ring and lengthening of the O1–H1 and O3–H3 bonds. However, X-site vacancies in K-dravite, dravite, and oxy-uvite result only in the lengthening of the neighboring O1–H1 bond and do not result in identifiable changes in the lattice-bonding environment.  相似文献   

8.
Thaumasite, Ca3Si(OH)6(CO3)(SO4)12H2O, occurs as a low-temperature secondary alteration phase in mafic igneous and metamorphic rocks, and is recognized as a product and indicator of sulfate attack in Portland cement. It is also the only mineral known to contain silicon in six-coordination with hydroxyl (OH)? that is stable at ambient P–T conditions. Thermal expansion of the various components of this unusual structure has been determined from single-crystal X-ray structure refinements of natural thaumasite at 130 and 298 K. No phase transitions were observed over this temperature range. Cell parameters at room temperature are: a= 11.0538(6) Å, c=10.4111(8) Å and V=1101.67(10) Å3, and were measured at intervals of about 50 K between 130 and 298 K, resulting in mean axial and volumetric coefficients of thermal expansion (×10?5K?1); α a =1.7(1), α c =2.1(2), and α V =5.6(2). Although the unit cell and VIIICaO8 polyhedra show significant positive thermal expansion over this temperature range, the silicate octahedron, sulfate tetrahedron, and carbonate group show zero or negative thermal expansion, with α V (VISiO6) = ?0.6 ± 1.1, α V (IVSO4)=?5.8 ± 1.4, and α R (C–O)= 0.0 ± 1.8 (×10?5 K?1). Most of the thermal expansion is accommodated by lengthening of the R(O...O) hydrogen bond distances by on average 5σ, although the hydrogen bonds involving hydroxyl sites on VISi expand twice as much as those on molecular water, causing the [Ca3Si(OH)6(H2O)12]4+ columns to expand in diameter more than they move apart over this temperature range. The average Si–OH bond length of the six-coordinated Si atom 〈R(VISi–OH)〉 in thaumasite is 1.783(1) Å, being about 0.02 Å (?20σ) shorter than VISi–OH in the dense hydrous magnesium silicate, phase D, MgSi2H2O6.  相似文献   

9.
The dissolution of H2O and CO2 in structurally dense, nominally anhydrous and non-carbonate oxide matrices such as MgO and CaO is reviewed. H2O and CO2 are treated as gaseous oxide components which enter into solid solution with the refractory oxide hosts. They form anion complexes associated with cation vacancy sites. Evidence is presented that OH? pairs which derive from the dissolution of H2O are subject to a charge transfer (CT) conversion into peroxy moieties and molecular hydrogen, O 2 2? ... H2. Because the O 2 2? moiety is small (O?-O? distance ≈ 1.5 Å) high pressure probably favors the CT conversion. Mass spectroscopic studies show that molecular H2 may be lost from the solid which retains excess oxygen in the form of O 2 2? , leading to the release of atomic O. The dissociation of O 2 2? moieties into a vacancy-bound O? state and an unbound O? state can be followed by measuring the internal redox reactions involving transition metal impurities, the transient paramagnetism of the O? and their effect on the d.c. conductivity. Evidence is presented that CO2 molecules dissolve dissociatively in the structurally dense oxide matrix, as if they were first to dissociate into CO+O and then to form separate solute moieties CO 2 2? and O 2 2? , both associated with cation vacancy sites. In the CO 2 2? moiety (C-O? distance 1.2–1.3 Å, OCO angle ≈ 130°) the C atom probably sits off center. The transition of the C atom into interstitial sites is accompanied by dissociation of the CO 2 2? moiety into CO? and O?. This transition can be followed by infrared spectroscopy, using OH? as local probes. Further support derives from magnetic susceptibility, thermal expansion, low frequency dielectric loss and low temperature deformation measurements. The recently observed emission of O and Mg atoms besides a variety of molecules such as CO, CO2, CH4, HCN and other hydrocarbons during impact fracture of MgO single crystals is presented and discussed in the light of the other experimental data.  相似文献   

10.
We relate a single-crystal FTIR (Fourier transform infrared) and neutron diffraction study of two natural cancrinites. The structural refinements show that the oxygen site of the H2O molecule lies off the triad axis. The water molecule is almost symmetric and slightly tilted from the (0001) plane. It is involved in bifurcated hydrogen bridges, with Ow···O donor–acceptor distances >2.7 Å. The FTIR spectra show two main absorptions. The first at 3,602 cm?1 is polarized for E ⊥ c and is assigned to the ν3 mode. The second, at 3,531 cm?1, is also polarized for E ⊥ c and is assigned to ν1 mode. A weak component at 4,108 cm?1 could possibly indicate the presence of additional OH groups in the structure of cancrinite. Several overlapping bands in the 1,300–1,500 cm?1 range are strongly polarized for E ⊥ c, and are assigned to the vibrations of the CO3 group.  相似文献   

11.
NMR shieldings (σ) and electric field gradients (eq) are calculated using ab initio methods at the O and T nuclei (where T=P, Si) in two different types of molecules-TH3 dimers, i.e. H3SiOSiH3 and H3POPH 3 2+ , and TO4 trimeric rings, i.e., Si3O 9 6- and P3O 9 3- , which serve as models for assessing the effects of polymerization, bond length and bond angle variation on the NMR properties of polymerized silicates and phosphates. In agreement with earlier ab initio studies on H3SiOSiH3 we confirm that σ(29Si), σ(31P), σ(17O) and eq(17O) all decrease as θ(SiOSi) decreases in the range from 180° to 100°. However, correction for artifacts due to distant core electrons leads to a considerably reduced value for the anisotropy in σ O, bringing it into better agreement with estimated experimental values. The qualitative change in σ(29Si) with θ(SiOSi) can be understood on the basis of changes in the energies of the highest energy occupied MO's and consequent variations in their contributions to the paramagnetic part of the shielding. For H3POPH 3 2+ we calculate a larger value of eqO than for the analog Si compound but the same type of variation of σ(17O) with θ(TOT). The change in σ(31P) with θ(POP) is, however, calculated to be much smaller than in the Si case and a maximum is predicted for intermediate angles. For the trimeric rings we obtain energy optimized geometries in good agreement with x-ray structural data, with T-O terminal distances systematically shorter than the T-O bridging distances. Calculated σ(T) anisotropies are also in good agreement with experiment and can be simply related to the calculated structure. After correction for distant core effects we obtain a change in σ(31P) between PO 4 3- and P3O 9 3- in reasonable agreement with experiment.  相似文献   

12.
The nicotinamide (pyridine-3-carboxamide, nia) complexes of silver(I), [Ag(nia)2(NO3)]·H2O (1), [Ag(nia)2(NO3)] (2), and {K[Ag(nia)2](NO3)2} n (3), were prepared and characterised by IR spectroscopy and TG/DTA thermal methods. The solid state structures of 2 and 3 were determined by single-crystal X-ray diffraction analysis. In both complexes two nicotinamide ligands are coordinated to silver(I) through the nitrogen atom of the pyridine ring in a near-linear fashion. In 2, additional coordination by two oxygen atoms of one nitrate group leads to the distorted tetrahedral coordination environment of silver(I). In 3, nitrate ions bridge potassium cations giving rise to a 2D coordination network which is further stabilised by cross-bridging of each two potassium atoms in [1 0 0] direction by complex cations, [Ag(nia)2]+. Despite different aggregation of 2 and 3 in the solid state, both complexes demonstrate quite similar thermal stability. The amide self-complementary hydrogen bonds appear to be the main driving force for establishing the crystal structures of both 2 and 3.  相似文献   

13.
A computer model for cubic sodalite structures, general formula M 8(T 12O24)X 2 where M, X and T are the cavity cation and anion and framework cation respectively, has been devised. It has been used to determine the effect of changing cavity cation and anion radii on the cell edge, tilt angle of the tetrahedra and T-O-T angle for the following sodalite frameworks: (Al6Si6O24)6?, (Be6Si6O24)12?, (Al12O24)12?, and (B12O24)12?. After fixing the T-O distance(s), the cavity cation-framework oxygen distance and taking a value of 1.4 Å for the radius of oxygen the model was used to calculate atomic coordinates and interatomic distances and angles for selected aluminosilicate-sodalites. The structure calculated for Na8(Al6Si6O24)Cl2 agrees closely with that determined for natural sodalite (Löns and Schulz, 1967). The model is also applied to the estimation of the effective radii of the tetrahedrally-coordinated cavity anions which can be accommodated in natural and synthetic sodalites: OH? 1.48–1.51, Cl? 1.78, Br? 1.93, I? 2.14–2.17, SO 4 2? 2.37–2.57, MoO 4 2? 2.70 and WO 4 2? 2.79 Å.  相似文献   

14.
Ab initio STO-3G molecular orbital theory has been used to calculate energy-optimized Si-O bond lengths and angles for molecular orthosilicic and pyrosilicic acids. The resulting bond length for orthosilicic acid and the nonbridging bonds for pyrosilicic acid compare well with Si-OH bonds observed for a number of hydrated silicate minerals. Minimum energy Si-O bond lengths to the bridging oxygen of the pyrosilicic molecule show a close correspondence with bridging bond length data observed for the silica polymorphs and for gas phase and molecular crystal siloxanes when plotted against the SiOSi angle. In addition, the calculations show that the mean Si-O bond length of a silicate tetrahedron increases slightly as the SiOSi angle narrows. The close correspondence between the Si-O bond length and angle variations calculated for pyrosilicic acid and those observed for the silica polymorphs and siloxanes substantiates the suggestion that local bonding forces in solids are not very different from those in molecules and clusters consisting of the same atoms with the same coordination numbers. An extended basis calculation for H4SiO4 implies that there are about 0.6 electrons in the 3d-orbitals on Si. An analysis of bond overlap populations obtained from STO-3G* calculations for H6Si2O7 indicates that Si-O bond length and SiOSi angle correlations may be ascribed to changes in the hybridization state of the bridging oxygen and (d – p) π-bonding involving all five of the 3d AO's of Si and the lone-pair AO's of the oxygen. Theoretical density difference maps calculated for H6Si2O7 show a build-up of charge density between Si and O, with the peak-height charge densities of the nonbridging bonds exceeding those of the bridging bonds by about 0.05 e Å?3. In addition, atomic charges (+1.3 and ?0.65) calculated for Si and O in a SiO2 moiety of the low quartz structure conform reasonably well with the electroneutrality postulate and with experimental charges obtained from monopole and radial refinements of diffraction data recorded for low quartz and coesite.  相似文献   

15.
The thermal evolution of 10-Å phase Mg3Si4O10(OH)2·H2O, a phyllosilicate which may have an important role in the storage/release of water in subducting slabs, was studied by X-ray single-crystal diffraction in the temperature range 116–293 K. The lattice parameters were measured at several intervals both on cooling and heating. The structural model was refined with intensity data collected at 116 K and compared to the model refined at room temperature. As expected for a layer silicate on cooling in this temperature range, the a and b lattice parameters undergo a small linear decrease, α a  = 1.7(4) 10?6 K?1 and α b  = 1.9(4) 10?6 K?1, where α is the linear thermal expansion coefficient. The greater variation is along the c axis and can be modeled with the second order polynomial c T  = c 293(1 + 6.7(4)10?5 K?1ΔT + 9.5(2.5)10?8 K?2(ΔT)2) where ΔT = T ? 293 K; the monoclinic angle β slightly increased. The cell volume thermal expansion can be modeled with the polynomial V T  = V 293 (1 + 8.0 10?5 K?1 ΔT + 1.4 10?7 K?2 (ΔT)2) where ΔT = T ? 293 is in K and V in Å3. These variations were similar to those expected for a pressure increase, indicating that T and P effects are approximately inverse. The least-squares refinement with intensity data measured at 116 K shows that the volume of the SiO4 tetrahedra does not change significantly, whereas the volume of the Mg octahedra slightly decreases. To adjust for the increased misfit between the tetrahedral and octahedral sheets, the tetrahedral rotation angle α changes from 0.58° to 1.38°, increasing the ditrigonalization of the silicate sheet. This deformation has implications on the H-bonds between the water molecule and the basal oxygen atoms. Furthermore, the highly anisotropic thermal ellipsoid of the H2O oxygen indicates positional disorder, similar to the disorder observed at room temperature. The low-temperature results support the hypothesis that the disorder is static. It can be modeled with a splitting of the interlayer oxygen site with a statistical distribution of the H2O molecules into two positions, 0.6 Å apart. The resulting shortest Obas–OW distances are 2.97 Å, with a significant shortening with respect to the value at room temperature. The low-temperature behavior of the H-bond system is consistent with that hypothesized at high pressure on the basis of the Raman spectra evolution with P.  相似文献   

16.
The mixed valence iron silicate ilvaite, CaFe 2 2+ Fe3+Si2O8(OH), displays electron delocalization associated with Fe2+→Fe3+ charge transfer as observed by Mössbauer spectroscopy. Previous studies report the observation of an ‘electron hopping phenomenon’ with resolution of discrete valence states below 320 K. Mössbauer spectra of a suite of naturally occurring ilvaites were recorded over a temperature range, 80 K to 575 K. Five quadrupole doublets were resolved by computer fitting and assigned to Fe2+(A), Fe2+(B), Fe3+(A), and Fe2+(A)→Fe3+(A)‖c and ⊥c. Contrary to prior work, doublets associated with electron delocalization are resolved at 80 K and preclude the use of a Verwey-type order-disorder model. We propose a thermal activation model and discuss its criteria from molecular orbital and mineralogical viewpoints.  相似文献   

17.
A new mineral aklimaite, Ca4[Si2O5(OH)2](OH)4 · 5H2O, has been found near Mount Lakargi, Upper Chegem caldera, Kabardino-Balkaria, the Northern Caucasus, Russia, in the skarnified limestone xenolith in ignimbrite. This hydrothermal mineral occurs in a cavity of altered larnite skarn and is associated with larnite, calcium humite-group members, hydrogarnets, bultfonteinite, afwillite, and ettringite. Aklimaite forms transparent, colorless (or occasionally with pinkish tint) columnar or lath-shaped crystals up 3 × 0.1 × 0.01 mm in size, flattened on {001} and elongated along {010}; they are combined in spherulites. The luster is vitreous; the cleavage parallel to the {001} is perfect. D calc = 2.274 g/cm3. The Mohs’ hardness is 3–4. Aklimaite is optically biaxial, negative, 2V meas > 70°, 2V calc = 78°, α = 1.548(2), β = 1.551(3), γ = 1.553(2). The IR and Raman spectra are given. The chemical composition (wt %, electron microprobe) is as follows: 0.06 Na2O, 0.02 K2O, 45.39 CaO, 0.01 MnO, 0.02 FeO, 24.23 SiO2, 0.04 SO3, 3.22 F, 27.40 H2O(calc.), ?1.36 -O=F2; the total is 99.03. The empirical formula calculated on the basis of 2Si apfu with O + OH + F = 16 is as follows: (Ca4.02Na0.01)Σ4.03[Si2.00O5.07(OH)1.93][(OH)3.16F0.84] Σ4.00 · 5H2O. The mineral is monoclinic, space group C2/m, a = 16.907(5), b = 3.6528(8), c = 13.068(4) Å, β = 117.25(4)·, V= 717.5(4) Å3, Z = 2. Aklimaite is representative of the new structural type, the sorosilicate with disilicate groups [Si2O5(OH)2]. The strongest reflections in the X-ray powder patterns [d, Å (hkl)] are: 11.64(100)(001), 2.948(32)(310, 203), 3.073(20) ( $\bar 404$ , $\bar 311$ ), 2.320(12)(005, 510), 2.901 (11)(004), 8.30(10) $\left( {\bar 201} \right)$ . The type specimen is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.  相似文献   

18.
Variations in the 17O nuclear quadrupole coupling constant, NQCC, and the 17O NMR shielding constant, σO, are evaluated for bridging oxygens in H3T-O-T′H3 linkages (with T, T′=Al, Si, P), and for nonbridging O in SiH3O?, SiH3OH and SiH3OMg+ and the 19F NMR shielding constant, σF, is evaluated for bridging F in H3SiFSiH 3 ? using Hartree-Fock methods with large, flexible Gaussian basis sets. Trends in 17O NQCC as a function of T and T′ identity agree with experiment but the value for the Si-O-Al case is underestimated, indicative of neglected contributions from charge compensating cations. For H3SiOSiH3 the decrease in NQCC over the range from 180° to 140° is substantial but somewhat slower than the variation of -cosSi and σO decrease as Si of the nonbridging oxygen of SiH3O? compared to the bridging oxygen of H3SiOSiH3 but the calculated σSi in H3SiOAlH 3 ? is too large and σO too small, indicative of important contributions from counter ions. By contrast, σO for PH3OAlH3 compared to SiH3OSiH3 is consistent with experiment. In H3SiFSiH 3 + (a model for bridging F in amorphous Si:H:F) the value of σSi is smaller and the NQCC at F is considerably larger than for H3SiF, suggesting distinctive 29Si and 19F NMR spectra for this species.  相似文献   

19.
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.  相似文献   

20.
Single-crystal study of the structure (R = 0.0268) was performed for garyansellite from Rapid Creek, Yukon, Canada. The mineral is orthorhombic, Pbna, a = 9.44738(18), b = 9.85976(19), c = 8.14154(18) Å, V = 758.38(3) Å3, Z = 4. An idealized formula of garyansellite is Mg2Fe3+(PO4)2(OH) · 2H2O. Structurally the mineral is close to other members of the phosphoferrite–reddingite group. The structure contains layers of chains of M(2)O4(OH)(H2O) octahedra which share edges to form dimers and connected by common edges with isolated from each other M(1)O4(H2O)2 octahedra. The neighboring chains are connected to the layer through the common vertices of M(2) octahedra and octaahedral layers are linked through PO4 tetrahedra.  相似文献   

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