Measurement of the effects of temperature and partial pressure of oxygen on the oxidation states of europium in silicate glasses     

R.V Morris  L.A Haskin  G.M Biggar  M.J O&#x;hara 《Geochimica et cosmochimica acta》1974,38(9):1447-1459

The techniques of electron paramagnetic resonance (EPR) were used to measure the concentration ratio of Eu²⁺ to Eu³⁺ in quenched CaMgSi₂O₆, Ca₃Si₃O₁₂, and CaAl₂Si₂O₈ liquids as functions of partial pressure of oxygen and temperature. The redox equilibrium of the Eu ions was described by the reaction 4Eu³⁺ + 20^2? = 4Eu²⁺ + O₂. The reduction of Eu³⁺ to Eu²⁺ was endothermic, and for CaMgSi₂O₆ and Ca₃Al₂Si₃O₁₂ liquids the mean value of ΔH⁰ and the standard deviation from that mean were 25 ± 7 kcal/mole.The magnitude of the Eu anomaly in the distribution coefficients is discussed in terms of the compositions of the solid and liquid phases.  相似文献   

Polymerization of silicate and aluminate tetrahedra in glasses,melts and aqueous solutions—II. The network modifying effects of Mg²⁺ , K⁺, Na⁺, Li⁺, H⁺, OH⁻, F⁻, Cl⁻, H₂O,CO₂ and H₃O⁺ on silicate polymers     

B.H.W.S. de Jong  Gordon E. Brown 《Geochimica et cosmochimica acta》1980,44(11):1627-1642

The effect of the group IA and VIIA ions, as well as Mg²⁺, and the molecules H₂O, CO₂, H₃O⁺ and OH^? on the energy of the Si-O bond in a H₆Si₂O₇ cluster has been calculated using semiempirical molecular orbital calculations (CNDO/2). Three types of elementary processes, i.e. substitution, addition, and polymerization reactions have been used to interpret data on the dynamic viscosity, surface tension and surface charge, hydrolytic weakening, diffusivity, conductivity, freezing point depression, and degree of polymerization of silicates in melts, glasses, and aqueous solutions. As a test of our calculational procedure, observed X-ray emission spectra of binary alkali silicate glasses were compared with calculated electronic spectra. The well known bondlength variations between the bridging bond [Si-O(br)] and the non-bridging bond [Si-O(nbr)] in alkali silicates are shown to be due to the propagation of oscillating bond-energy patterns through the silica framework. A kinetic interpretation of some results of our calculations is given in terms of the Bell-Evans-Polanyi reaction principle.  相似文献   

An electrochemical study of Ni²⁺, Co²⁺, and Zn²⁺ ions in melts of composition CaMgSi₂O₆     

Krystyna W. Semkow  Ronald A. Rizzo  Larry A. Haskin  David J. Lindstrom 《Geochimica et cosmochimica acta》1982,46(10):1879-1889

Cyclic voltammetry has been done for Ni²⁺, Co²⁺, and Zn²⁺ in melts of diopside composition in the temperature range 1425 to 1575°C. Voltammetric curves for all three ions excellently match theoretical curves for uncomplicated, reversible charge transfer at the Pt electrode. This implies that the neutral metal atoms remain dissolved in the melt. The reference electrode is a form of oxygen electrode. Relative to that reference assigned a reduction potential of 0.00 volt, the values of standard reduction potential for the ions are $\text{E}{}^1\text{(}\text{Ni}{}^{\mathrm{2+}}\text{Ni}{}^0\text{,}\text{diopside}\text{, 1500°C) = ?0.32 ± .01}\text{V}$, $\text{E}{}^1\text{(}\text{Co}{}^{\mathrm{2+}}\text{Co}{}^0\text{,}\text{diopside}\text{, 1500°C) = ?0.45 ± .02}\text{V}$, and $\text{E}{}^1\text{(}\text{Zn}{}^{\mathrm{2+}}\text{Zn}{}^0\text{,}\text{diopside}\text{, 1500°C) = ?0.53 ± .01}\text{V}$. The electrode reactions are rapid, with first order rate constants of the order of 10^?2 cm/sec. Diffusion coefficients were found to be 2.6 × 10^?6 cm²/sec for Ni²⁺, 3.4 × 10^?6 cm²/sec for Co²⁺, and 3.8 × 10^?6 cm²/sec for Zn²⁺ at 1500°C. The value of $\text{E1}$ ($\text{Ni}{}^{\mathrm{2+}}\text{Ni}{}^0$, diopside) is a linear function of temperature over the range studied, with values of ?0.35 V at 1425°C and ?0.29 V at 1575°C. At constant temperature the value of $\text{E}{}^1\text{(}\text{Ni}{}^{\mathrm{2+}}\text{Ni}{}^0\text{, 1525°C}$) was not observed to vary with composition over the range CaO · MgO · 2SiO₂ to CaO·MgO·3SiO₂ or from 1.67 CaO·0.33MgO·2SiO₂ to 0.5 CaO·1.5MgO·2SiO₂. The value for the diffusion coefficient for Ni²⁺ decreased by an order of magnitude at 1525°C over the compositional range CaO · MgO · 1.25SiO₂ to CaO · MgO · 3SiO₂. This is consistent with a mechanism by which Ni²⁺ ions diffuse by moving from one octahedral coordination site to another in the melt, with the same Ni²⁺ species discharging at the cathode regardless of the SiO₂ concentration in the melt.  相似文献   

The incorporation of Mg²⁺ and Sr²⁺ into calcite overgrowths: influences of growth rate and solution composition     

Alfonso Mucci  John W Morse 《Geochimica et cosmochimica acta》1983,47(2):217-233

The concentrations of Mg²⁺ and Sr²⁺ incorporated within calcite overgrowths precipitated from seawater and related solutions, determined at 25°C, were independent of the precipitation rate over approximately an order of magnitude. The saturation states used to produce this range of precipitation rates varied from 3 to 17 depending on the composition of the solution.The amount of Mg²⁺ incorporated in the overgrowths was not directly proportional to $\text{Mg}{}^{\mathrm{2+}}\text{Ca}{}^{\mathrm{2+}}$ in solution over the entire range (1–20) of ratios studied. Below a ratio of 7.5, the overgrowth was enriched in MgCO₃ relative to what is predicted by the constant distribution coefficient measured above a ratio of 7.5. This increased MgCO₃ correlates with the relative enrichment of adsorbed Mg²⁺. Above a ratio of 7.5 the concentration of MgCO₃ in the calcite overgrowths followed a classical thermodynamic behavior characterized by a constant distribution coefficient of 0.0123 (±0.008 std dev).The concentration of SrCO₃ incorporated in the overgrowths was linearly related to the MgCO₃ content of the overgrowths, and is attributed to increased solubility of SrCO₃ in calcite due to the incorporation of the smaller Mg²⁺ ions.The kinetic data indicate that the growth mechanism involves the adsorption of the cations on the surface of the calcite prior to dehydration and final incorporation. It is suggested that dehydration of cations at the surface is the rate controlling step.  相似文献   

Partitioning of Ni²⁺, Co²⁺, Fe²⁺, Mn²⁺ and Mg²⁺ between olivine and silicate melts: compositional dependence of partition coefficient   总被引：1，自引：0，他引：1  

Eiichi Takahashi 《Geochimica et cosmochimica acta》1978,42(12):1829-1844

Partitioning of Ni²⁺, Co²⁺, Fe²⁺, Mn²⁺ and Mg²⁺ between olivine and silicate melts has been determined near the join (Mg_0.5·-Fe_0.5)₂SiO₄-K₂O 4SiO₂ and for seven different basaltic compositions. The experiments were made at 1 atm total pressure, 1500-1100°C, and under moderate to reducing oxygen fugacities. The concentration factor, defined as K^MO = (MO)^ol/(MO)^liq (molar ratio), increases markedly for all the cations studied as the olivine component of the liquid decreases. Much of the increase in K^MO is considered as due to the compositional effect of the coexisting liquid: the temperature effect on K^MO is probably opposite to the compositional effect (K^MO decreases as temperature decreases).The partition coefficient K^MO-MgO = (MO/MgO)^ol/(MO/MgO)^liq for the reaction, M_ol²⁺ + Mg_liq²⁺ = M_liq²⁺ + Mg_ol²⁺. is relatively constant over a wide range of SiO₂ content of the liquid, except in the case of Ni²⁺. The partition coefficients have similar ranges both in synthetic and natural rock systems: K^NiO-MgO = 1.8–3.0, K^CoO-MgO = 0.6–0.8, K^FeO-MgO = 0.27–0.38, and K^MnO-MgO = 0.23–0.32. There is a systematic variation in the partition coefficient K^MO-MgO with the composition of liquid; K^MO-MgO increases with increasing SiO₂ content of melt. The partition coefficients for Co²⁺, Fe²⁺ and Mn²⁺ are useful to test the equilibration of olivine with magma of a wide compositional range.  相似文献   

Temperature-dependent Fe2+–Mn2+ order–disorder behaviour in amphiboles     

J. J. Reece  S. A. T. Redfern  M. D. Welch  C. M. B. Henderson  C. A. McCammon 《Physics and Chemistry of Minerals》2002,29(8):562-570

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 Ca_0.1Mn_1.9 Mg_1.25Fe²⁺ _3.56Fe³⁺ _0.38Si_7.81O₂₂(OH)₂ and Ca_0.24Mn_1.57 Mg_2.27 Fe²⁺ _2.76Fe³⁺ _0.32Si_7.84O₂₂(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   

Distribution of cations at two tetrahedral sites in Ca2MgSi2O7-Ca2Fe3+AlSiO7 series synthetic melilite and its relation to incommensurate structure     

Maki Hamada  Masahide Akasaka 《Physics and Chemistry of Minerals》2013,40(3):259-270

Synthetic melilites on the join Ca₂MgSi₂O₇ (åkermanite: Ak)-Ca₂Fe³⁺AlSiO₇ (ferrialuminium gehlenite: FAGeh) were studied using X-ray powder diffraction and ⁵⁷Fe Mössbauer spectroscopic methods to determine the distribution of Fe³⁺ between two different tetrahedral sites (T1 and T2), and the relationship between ionic substitution and incommensurate (IC) structure. Melilites were synthesized from starting materials with compositions of Ak₁₀₀, Ak₈₀FAGeh₂₀, Ak₇₀FAGeh₃₀ and Ak₅₀FAGeh₅₀ by sintering at 1,170–1,350 °C and 1 atm. The average chemical compositions and end-member components, Ak, FAGeh and Geh (Ca₂Al₂SiO₇), of the synthetic melilites were Ca_2.015Mg_1.023Si_1.981O₇ (Ak₁₀₀), Ca_2.017Mg_0.788Fe _0.187 ³⁺ Al_0.221Si_1.791O₇ (Ak₇₈FAGeh₁₉Geh₃), Ca_1.995Mg_0.695Fe _0.258 ³⁺ Al_0.318Si_1.723O₇ (Ak₆₉FAGeh₂₅Geh₆) and Ca_1.982Mg_0.495Fe _0.449 ³⁺ Al_0.519Si_1.535O₇ (Ak₄₉FAGeh₄₄Geh₇), respectively. Rietveld refinements using X-ray powder diffraction data measured using CuK _α -radiation at room temperature converged successfully with goodness-of-fits of 1.15–1.26. The refined Fe occupancies at the T1 and T2 sites and the Mg and Si contents determined by electron microprobe analysis gave the site populations of [0.788Mg + 0.082Fe³⁺ + 0.130Al]_T1[0.104Fe³⁺ + 0.104Al + 1.792Si]_T2 for Ak₇₈FAGeh₁₉Geh₃, [0.695Mg + 0.127Fe³⁺ + 0.178Al]_T1[0.132Fe³⁺ + 0.144Al + 1.724Si]_T2 for Ak₆₉FAGeh₂₅Geh₆ and [0.495Mg + 0.202Fe³⁺ + 0.303Al]_T1[0.248Fe³⁺ + 0.216Al + 1.536Si]_T2 for Ak₄₉FAGeh₄₄Geh₇ (apfu: atoms per formula unit), respectively. The results indicate that Fe³⁺ is distributed at both the T1 and the T2 sites. The mean T1–O distance decreases with the substitution of Fe³⁺ + Al³⁺ for Mg²⁺ at the T1 site, whereas the mean T2–O distance increases with substitution of Fe³⁺ + Al³⁺ for Si⁴⁺ at the T2 site, causing decrease in the a dimension and increase in the c dimension. However, in spite of the successful Rietveld refinements for the X-ray powder diffraction data measured using CuK _α-radiation at room temperature, each Bragg reflection measured using CuK _α1-radiation at room temperature showed weak shoulders, which were not observed in those measured at 200 °C. The Mössbauer spectra of the melilites measured at room temperature consist of two doublets assigned to Fe³⁺ at the T1 site and two or three doublets to Fe³⁺ at the T2 site, implying the existence of multiple T1 and T2 sites with different site distortions. These facts can be interpreted in terms of the IC structure in all synthetic melilites at room temperature, respectively. The results of Mössbauer analysis indicate that the IC structure in melilite is caused by not only known multiple T1 site, but also multiple T2 site at room temperature.  相似文献   

Luminescent properties of natural barite: Evidence for its genesis     

N. N. Boroznovskaya  L. A. Zyryanova  I. V. Pekov 《Doklady Earth Sciences》2016,471(1):1171-1173

The study of radiation of intrinsic and impurity excitations in natural barite showed that the patterns of BaSO₄ luminescence were mostly controlled by the presence of the [SO₄] anion complex. Several types of self-radiation were registered including those at the expense of the presence of O^2– ions of the axial and nonaxial configurations of the anionic group (emission bands within the wavelength ranges of 209–213 and 330–350 nm, respectively). Exitons located near the impurity and intrinsic defects largely participate in emission. Impurity defects participating in the luminescent centers of barite from the Ore Altai include Pb²⁺, Gd³⁺, Eu²⁺, Eu³⁺, Cu⁺, and Ag⁺ (under X-ray excitation). Variations in the spectral composition of barite indicate the different conditions of its formation.  相似文献   

多硅白云母中Fe²⁺的高压有序效应          下载免费PDF全文

应育浦  宋仁奎 《地质科学》1995,30(4):355-362

21个多硅白云母来自4个低温高压变质带。用电子探针、X射线粉末衍射及穆斯堡尔谱测定其化学成分、b0值及Fe²⁺占位。建立Fe²⁺(M1)/Fe²⁺(M2)对b0值相关图,发现Fe²⁺在八面体晶位有序化并解释其有序机理。  相似文献   

EPR measurement of the effect of glass composition on the oxidation states of europium     

R.V Morris  L.A Haskin 《Geochimica et cosmochimica acta》1974,38(9):1435-1445

The techniques of electron paramagnetic resonance (EPR) were used to measure the concentration ratio of Eu²⁺ to Eu³⁺ in quenched silicate liquids as a function of their compositions. The compositional end members were CaAl₂Si₂O₈ and either MSiO₃ or M₂Si0₄, M = Mg, (Ca_0.5, Mg_0.5), and Ca. All of the liquids were quenched from 1650 ± 25°C, 10^?6.9±0.6 atm of oxygen, and 10^?6.1±0.6 atm total pressure. For a particular choice of M, the ratio of Eu²⁺ to Eu³⁺ increased as much as a factor of 24 with increasing atomic ratio (Al + Si)/(O); for a constant value of (Al + Si)/(O), the ratio of Eu²⁺ to Eu³⁺ increased in the order Mg > (Ca_0.5,Mg_0.5) >Ca. In order to interpret the compositional dependence of the redox equilibrium of Eu in a systematic manner, the concept of a solvent coefficient was introduced.  相似文献   

Molecular dynamics simulations of pressure and temperature effects on MgSiO3 and Mg2SiO4 melts and glasses     

J. D. Kubicki  A. C. Lasaga 《Physics and Chemistry of Minerals》1991,17(8):661-673

Ab-initio interionic potentials for Mg²⁺, Si⁴⁺, and O^2– have been used in molecular dynamics (MD) simulations to investigate diffusivity changes, pressure-induced structural transitions, and temperature effects on polymerization in MgSiO₃ and Mg₂SiO₄ melts and glasses. The potential gives reasonable agreement with the 0.1 MPa radial distribution function of MgSiO₃ glass. Maxima in the diffusion coefficients of Si⁴⁺ and O^2– occur as pressure is increased on the MgSiO₃ melt. The controlling structural mechanism for this behavior is the Q¹ species of SiO₄ tetrahedra. Mg²⁺ diffusion coefficients decrease monotonically with pressure in both melt compositions. Increasing Mg²⁺ coordination number and population of 3- and 4-membered SiO₄ rings with pressure combine to hinder translation of the Mg²⁺ ions. The dominant changes in structure with pressure are a decrease in the intertetrahedral (Si-O--Si) angle up to approximately 4 g/cm³ and coordination changes of the ions above this density. Temperature effects on viscosity in these simulated melts are indirectly studied by analyzing polymerization changes with temperature. Polymerization and coordination numbers increase with decreasing temperature and a small quench rate effect is observed. Fair agreement is found between the MD simulations and experimental equation of state for Mg₂SiO₄, but the equation of state predictions for MgSiO₃ melts are much less accurate. The zero pressure volume, V ₀, is significantly higher and K ₀ is lower in the simulations than empirical values. The inadequacies reflect error in using the ionic approximation for polymerized systems and a need to collect more data for a variety of molecular configurations in the development of ab-initio potentials.  相似文献   

Partition of Sr,Ba, Ca,Y, Eu²⁺, Eu³⁺, and other REE between plagioclase feldspar and magmatic liquid: an experimental study   总被引：1，自引：0，他引：1  

Michael J Drake  Daniel F Weill 《Geochimica et cosmochimica acta》1975,39(5):689-712

Plagioclase feldspar/magmatic liquid partition coefficients for Sr, Ba, Ca, Y, Eu²⁺, Eu³⁺ and other REE have been determined experimentally at 1 atm total pressure in the temperature range 1150–1400°C. Natural and synthetic melts representative of basaltic and andesitic bulk compositions were used, crystallizing plagioclase feldspar in the composition range An₃₅–An₈₅. Partition coefficients for Sr are greater than unity at all geologically reasonable temperatures, and for Ba are less than unity above approximately 1060°C. Both are strongly dependent upon temperature. Partition coefficients for the trivalent REE are relatively insensitive to temperature. At fixed temperature they decrease monotonically from La to Lu. The partition of Eu is a strong function of oxygen fugacity. Under extreme reducing conditions D_Eu approaches the value of D_Sr.  相似文献   

Mayenite-supergroup minerals from burned dump of the Chelyabinsk Coal Basin     

《Russian Geology and Geophysics》2015,56(11):1603-1621

Three minerals of the mayenite supergroup have been found in fluorellestadite-bearing metacarbonate rock (former fragment of petrified wood of ankeritic composition) from the dump at the Baturinskaya-Vostochnaya-1-2 mine. These are eltyubyuite Ca₁₂Fe1°Si₄O3₂Cl6, its fluorine analog Ca₁₂Fe₁₀³⁺Si₄O₃₀F₁₀, and chlormayenite-wadalite Ca₁₂(Al,Fe)₁₄O₃₂Cl₂-Ca₁₂(Al,Fe)₁₀Si₄O₃₂Cl₂. The first two phases occur in the reaction mantle around hematite, magnesioferrite, and Ca-ferrite aggregates (“calciohexaferrite” CaFe₁₂O₁₉, “grandiferrite” CaFe₄O₇, and “dorrite phase” Ca₂(Fe₅^3 +Mn0^0.5Mg_0.5)(Si_0.5Fe_5.5³⁺)O₂₀) and, rarely, as individuals in grained aggregates of fluorellestadite-cuspidine (± lar- nite ± rusinovite Ca₁₀(Si₂O₇)₃Cl₂). Assemblages of zoned chlormayenite-wadalite crystals are found in grained aggregates of fluorellestadite- cuspidine, which lack Ca-ferrite. Also, harmunite CaFe₂O₄, chlorellestadite, fluorapatite, anhydrite, rondorfite CasMg(SiO₄)₄Cl₂, fluorine analog of rondorfite CasMg(SiO₄)₄F₂, “Mg-cuspidine” Ca_3.5(Mg,Fe)_0.5(Si₂O₇)F₂, fluorite, barioferrite BaFe₁₂O₁9, zhangpeishanite BaFCl, and other rare phases are identified in this rock. Data on the chemical composition and Raman spectroscopy of the mayenite-supergroup minerals are given. The genesis of metacarbonate rock is considered in detail: “oxidizing calcination” of Ca-Fe-carbonates with the formation of hematite and lime; reaction between hematite and lime with the formation of different Ca-ferrites; formation of larnite as a result of reaction between SiO₂ and lime or CaCO₃; and reactionary impact of hot Cl-F-S-bearing gases on early assemblages. Eltyubyuite and its fluorine analog crystallized at the stages of gas impact. It is presumed that the maximum temperature during the formation of rock reached 1200–1230 °C. © 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.  相似文献   

Characterization of leached layers on olivine and pyroxenes using high-resolution XPS and density functional calculations     

V.P. Zakaznova-Herzog  H.W. Nesbitt  J.S. Tse 《Geochimica et cosmochimica acta》2008,72(1):69-86

High-resolution core level and valence band (VB) X-ray photoelectron spectra (XPS) of olivine [(Mg_0.87Fe_0.13)₂SiO₄], bronzite [(Mg_0.8Fe_0.2)₂Si₂O₆] and diopside [Ca(Mg_0.8Fe_0.2)Si₂O₆] 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., Ca²⁺ and Mg²⁺) 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 (Ca²⁺ and Mg²⁺) via electrophilic attack of H⁺ on the M-O-Si moiety to produce the H₂Mg(M1)SiO_4(surf) complex at olivine surfaces, and two complexes, H₂Mg(M1)Si₂O_6(surf) and H₄Si₂O_6(surf) at diopside and enstatite surfaces. The olivine and pyroxene surface complexes H₂Mg(M1)SiO_4(surf) and H₂Mg(M1)Si₂O_6(surf) have been proposed previously, but the second pyroxene surface complex H₄Si₂O_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 H₂Mg(M1)SiO_4(surf); while the second attacks the M1-O-Si moiety ultimately producing H₄SiO₄ which is released to solution. For pyroxenes, the first electrophilic reaction produces H₂Mg(M1)Si₂O_6(surf), while the second produces.H₄Si₂O_6(surf). These two reactions are followed by a nucleophilic attack of H₂O (or H₃O⁺) on Si of H₄Si₂O_6(surf). This reaction is responsible for rupture of the brigding oxygen bond of the Si-O-Si moiety and release of H₄SiO₄ 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 Ca²⁺ and Mg²⁺ 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 H₂Mg(M1)SiO₄, as indicated by high quality density functional calculations on an olivine where Mg²⁺ 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   

A multi-analytical study of the crystal structure of unusual Ti–Zr–Cr-rich Andradite from the Maronia skarn, Rhodope massif, western Thrace, Greece     

A. Katerinopoulou  A. Katerinopoulos  P. Voudouris  A. Bieniok  M. Musso  G. Amthauer 《Mineralogy and Petrology》2009,95(1-2):113-124

Unusual Ti–Cr–Zr-rich garnet crystals from high-temperature melilitic skarn of the Maronia area, western Thrace, Greece, were investigated by electron-microprobe analysis, powder and single-crystal X-ray diffraction, IR, Raman and Mössbauer spectroscopy. Chemical data showed that the garnets contain up to 8 wt.% TiO₂, 8 wt.% Cr₂O₃ and 4 wt.% ZrO₂, representing a solid solution of andradite (Ca₃Fe³⁺ ₂Si₃O₁₂ ≈46 mol%), uvarovite (Ca₃Cr₂Si₃O₁₂ ≈23 mol%), grossular (Ca₃Al₂Si₃O₁₂ ≈10 mol%), schorlomite (Ca₃Ti₂[Si,(Fe³⁺,Al³⁺)₂]O₁₂ ≈15 mol%), and kimzeyite (Ca₃Zr₂[Si,Al₂]₃O₁₂ ≈6 mol%). The Mössbauer analysis showed that the total Fe is ferric, preferentially located at the octahedral site and to a smaller extent at the tetrahedral site. Single-crystal XRD analysis, Raman and IR spectroscopy verified substitution of Si mainly by Al³⁺, Fe³⁺ and Ti⁴⁺. Cr³⁺ and Zr⁴⁺ are found at the octahedral site along with Fe³⁺, Al³⁺ and Ti⁴⁺. The measured H₂O content is 0.20 wt.%. The analytical data suggest that the structural formula of the Maronia garnet can be given as: (Ca_2.99Mg_0.03)_Σ=3.02(Fe³⁺ _0.67Cr_0.54Al_0.33Ti_0.29Zr_0.15)_Σ=1.98(Si_2.42Ti_0.24Fe_0.18Al_0.14)_Σ=2.98O₁₂OH_0.11. Ti-rich garnets are not common and their crystal chemistry is still under investigation. The present work presents new evidence that will enable the elucidation of the structural chemistry of Ti- and Cr-rich garnets.  相似文献   

Thermochemistry of glasses along joins of pyroxene stoichiometry in the system Ca₂Si₂O₆-Mg₂Si₂O₆-Al₄O₆     

Richard L. Hervig  Derry Scott  Alexandra Navrotsky 《Geochimica et cosmochimica acta》1985,49(7):1497-1501

Enthalpies of solution in 2PbO · B₂O₃ at 974 K have been measured for glasses along the joins Ca₂Si₂O₆ (Wo)-Mg₂Si₂O₆ (En) and Mg₂Si₂O₆-MgAl₂SiO₆ (MgTs). Heats of mixing are symmetric and negative for Wo-En with W_H = ?31.0 ± 3.6 kJ mol^?. Negative heats of mixing were also found for the En-MgTs glasses (W_H = ?33.4 ± 3.7 kJ mol^?).Enthalpies of vitrification of pyroxenes and pyroxenoids generally increase with decreasing alumina content and with decreasing basicity of the divalent cation.Heats of mixing along several glassy joins show systematic trends. When only non-tetrahedral cations mix (outside the aluminosilicate framework), small exothermic heats of mixing are seen. When both nontetrahedral and framework cations mix (on separate sublattices, presumably), the enthalpies of mixing are substantially more negative. Maximum enthalpy stabilization near compositions with Al/Si ≈ 1 is suggested.  相似文献   

The distribution of Fe3+ and Ga3+ between two tetrahedral sites in melilites,Ca2(Mg,Fe3+, Ga,Si)3O7     

Masahide Akasaka  Haruo Ohashi  Isamu Shinno 《Physics and Chemistry of Minerals》1986,13(3):152-155

The distribution of Fe³⁺ and Ga³⁺ between the two tetrahedral sites in three synthetic melilites has been studied by using ⁵⁷Fe Mössbauer spectroscopy. In the melilite, (Ca₂Ga₂SiO₇)₅₀ (Ca₂Fe³⁺GaSiO₇)₅₀ (mol %), the distribution of Fe³⁺ and Ga³⁺ in T1 and T2 sites is apparently random, which can be explained in terms of the electrostatic valence rule. However in the melilites, (Ca₂MgSi₂O₇)₅₂ (Ca₂Fe³⁺GaSiO₇)₄₂ (Ca₂Ga₂SiO₇)₆ and (Ca₂MgSi₂O₇)₆₂ (Ca₂Fe³⁺GaSiO₇)₃₆ (Ca₂Ga₂SiO₇)₂ (mol %), Fe³⁺ shows preference for the more ionic T1 site and Ga³⁺ for the more covalent T2 site. If the electronegativity of Ga³⁺ is assumed to be larger than that of Fe³⁺, the mode of distribution of Fe³⁺ and Ga³⁺ can be explained in terms of our previous hypothesis that a large electronegativity induces a stronger preference for the more covalent T2 site.  相似文献   

Electric conductivity of Fe2SiO4–Fe3O4 spinel solid solutions     

T. Yamanaka  H. Shimazu  K. Ota 《Physics and Chemistry of Minerals》2001,28(2):110-118

 The spinel solid solution was found to exist in the whole range between Fe₃O₄ and γ-Fe₂SiO₄ at over 10 GPa. The resistivity of Fe_{3− x} Si _x O₄ (0.0<x<0.288) was measured in the temperature range of 80∼300 K by the AC impedance method. Electron hopping between Fe³⁺ and Fe²⁺ in the octahedral site of iron-rich phases gives a large electric conductivity at room temperature. The activation energy of the electron hopping becomes larger with increasing γ-Fe₂SiO₄ component. A nonlinear change in electric conductivity is not simply caused by the statistical probability of Fe³⁺–Fe²⁺ electron hopping with increasing the total Si content. This is probably because a large number of Si⁴⁺ ions occupies the octahedral site and the adjacent Fe²⁺ keeping the local electric neutrality around Si⁴⁺ makes a cluster, which generates a local deformation by Si substitution. The temperature dependence of the conductivity of solid solutions indicates the Verwey transition temperature, which decreases from 124(±2) K at x=0 (Fe₃O₄) to 102(±5) K at x=0.288, and the electric conductivity gap at the transition temperature decreases with Si⁴⁺ substitution. Received: 15 March 2000 / Accepted: 4 September 2000  相似文献   

Crystal chemistry of a Mg-vesuvianite and implications of phase equilibria in the system CaO-MgO-Al2O3-SiO2-H2O-CO21     

J. W. VALLEY  D. R. PEACOR  J. R. BOWMAN  E. J. ESSENE  M. J. ALLARD 《Journal of Metamorphic Geology》1985,3(2):137-153

Abstract Chemical analysis (including H₂, F₂, FeO, Fe₂O₃) of a Mg-vesuvianite from Georgetown, Calif., USA, yields a formula, Ca_18.92Mg_1.88Fe³⁺_0.40Al_10.97Si_17.81- O_69.0.1(OH)_8.84F_0.14, in good agreement on a cation basis with the analysis reported by Pabst (1936). X-ray and electron diffraction reveal sharp reflections violating the space group P4/nnc as consistent with domains having space groups P4/n and P4nc. Refinement of the average crystal structure in space group P4/nnc is consistent with occupancy of the A site with Al, of the half-occupied B site by 0.8 Mg and 0.2 Fe, of the half-occupied C site by Ca, of the Ca (1,2,3) sites by Ca, and the OH and O(10) sites by OH and O. We infer an idealized formula for Mg-vesuvianite to be Ca₁₉Mg(MgAl₇)Al₄Si₁₈O₆₉(OH)₉, which is related to Fe^3+-vesuvianite by the substitutions Mg + OH = Fe³⁺+ O in the B and O(10) sites and Fe³⁺= Al in the AlFe site. Thermodynamic calculations using this formula for Mg-vesuvianite are consistent with the phase equilibria of Hochella, Liou, Keskinen & Kim (1982) but inconsistent with those of Olesch (1978). Further work is needed in determining the composition and entropy of synthetic vs natural vesuvianite before quantitative phase equilibria can be dependably generated. A qualitative analysis of reactions in the system CaO-MgO-Al₂O₃-SiO₂-H₂O-CO₂ shows that assemblages with Mg-vesuvianite are stable to high T in the absence of quartz and require water-rich conditions (XH₂O > 0.8). In the presence of wollastonite, Mg-vesuvianite requires very water-rich conditions (XH₂O > 0.97).  相似文献   

Thermochemical study of glasses in the system NaAlSi₃O₈-KAlSi₃O₈-Si₄O₈ and the join Na_1.6Al_1.6Si_2.4O₈-K_1.6Al_1.6Si_2.4O₈     

Richard L. Hervig  Alexandra Navrotsky 《Geochimica et cosmochimica acta》1984,48(3):513-522

Enthalpies of solution in 2PbO · B₂O₃ at 981 K have been measured for glasses in the system albite-orthoclase-silica and along the join Na_1.6Al_1.6Si_2.4O₈-K_1.6Al_1.6Si_2.4O₈. The join KAlSi₃O₈-Si₄O₈ shows zero heat of mixing similar to that found previously for NaAlSi₃O₈-Si₄O₈ glasses. Albite-orthoclase glasses show negative heats of mixing symmetric about Ab₅₀Or₅₀ (W_n = ? 2.4 ± 0.8 kcal). Negative heats of (Na, K) mixing are also found at $\text{Si}\text{(Si + Al)}\text{= 0.6.}$ Ternary excess enthalpies of mixing in the glassy system Ab-Or-4Q are positive but rarely exceed 1 kcal mol^?1.Using earlier studies of the thermodynamic properties of the crystals, the present calorimetric data and the “two-lattice” entropy model, the albite-orthoclase phase diagram is calculated in good agreement with experimental data. Attempts to calculate albite-silica and orthoclase-silica phase diagrams reveal complexities probably related to significant (but unknown) mutual solid solubility between cristobalite and alkali feldspar and to the very small heat and entropy of fusion of SiO₂.  相似文献