Correlation of 29Si nuclear magnetic resonance chemical shifts in silicates with orbital energy differences obtained from X-ray spectra

Trends in ²⁹Si nuclear magnetic resonance (NMR) chemical shifts in silicates are shown to arise primarily from changes in the paramagnetic contribution to the shift, σ^para. Using the average excitation energy approximation, changes in σ^para may be correlated with changes in the separation of high energy occupied and low energy unoccupied molecular orbitals. The required molecular orbital (MO) energy differences may be obtained from x-ray emission and absorption spectra. Shifts in σ to high field as silicate polymerization increases are associated with increased occupied to unoccupied MO energy differences. Substitution of Al for Si (and consequent introduction of electropositive cations, such as Na) reduces the occupied to unoccupied orbital energy gap causing shifts of σ to low field. The observed correlation indicates that ²⁹Si NMR is capable of giving information on silicate electronic structure.     

蓝晶石矿中氟钠镁铝硅铁钛钾钙元素的X射线荧光光谱分析

蓝晶石的分析通常采用碱熔体系,重量、容量、比色、原子吸收光谱法、离子选择电极法等多种分析手段进行单独测定,对含有刚玉、金红石矿物的难熔性蓝晶石样品,这种分析方法常因熔矿不完全而导致测定结果偏低,而且分析手续冗长,操作复杂,不能满足地质测试的需要。本文采用玻璃熔融法制样,建立了X射线荧光光谱法同时测定蓝晶石矿中F、Na、Mg、Al、Si、Fe、Ti等主量元素的分析方法。以不同矿种的标准物质和自制含多种矿物组分的蓝晶石管理样拟合校准曲线,对玻璃熔融法的稀释比、熔矿温度及对F元素的影响因素等测定条件进行优化,确定选择样品与四硼酸锂-偏硼酸锂混合熔剂的稀释比为1:10,在1050℃温度下实际样品熔矿完全,各元素的分析结果与化学分析法的测定值基本吻合。方法检出限小于0.05%,方法精密度(RSD, n=7)小于4.5%。本法操作简单,重现性好,准确可靠,解决了难熔样品的熔矿问题,同时也很好地解决了传统方法费时、耗材、不能同时测定多元素的问题。     

Vibrational spectra,lattice dynamics and chemical constitution of oxides

The relationships among lattice dynamics of oxides and their chemical constitution are discussed in terms of the concept of dynamic crystal chemistry. A new approach to the design of a dynamic model of an ionic-covalent crystal combining molecular force constants with explicit treatment of long-range electrostatic forces in a lattice is outlined and its applications exemplified. The conditions of introduction of scaled quantum chemical force constants of a suitable molecular system into the secular equation for lattice vibrations are determined rigorously. The importance of unified treatment of stability conditions and various dynamic properties of crystals including phonon spectra, macroscopic elastic and piezoelectric constants etc., is emphasized. Several results of ab initio quantum chemical SCF computations by the gradient method for molecular species containing silicon-oxygen bonds are presented and are used to explain some empirical regularities in silicate structures.     

On the theory of Al,Si ordering in albite

Calculations of the equilibrium distribution of Al, Si in the albite framework based on quasi-chemical theories of order, disorder transformations (Yang 1945; Yang and Li 1947; Li 1949) were made for a two-dimensional framework model. The ordering is caused by the energy of Al, Si interchange between sites of different crystal-chemical types and the energy of nearest neighbour interaction. By taking into account the decrease in the energy of interchange between sites with increasing disordering and with increasing temperature, and by examining different relationships for site-to-site interchange energy and the nearest neighbour interaction, it is possible to understand the basic characteristics of the transformation from low (essentially ordered) to high (essentially disordered) albite as revealed by experiment. These characteristics are: (1) abrupt variation of the equilibrium degree of order within a narrow temperature range and possible first order phase transformation for the transition from low-albite to high-albite, (2) hysteresis of the synthetic high albite transformation path and of the low albite hydrothermal “annealing” path, (3) presence of a temperature range where high albite is stable and has a continually changing equilibrium degree of order.     

Computational study of tetrahedral Al–Si and octahedral Al–Mg ordering in phengite

 As part of a wider study of the nature and origins of cation order–disorder in micas, a variety of computational techniques have been used to investigate the nature of tetrahedral and octahedral ordering in phengite, K₂ ^[6](Al₃Mg)^[4](Si₇Al)O₂₀(OH)₄. Values of the atomic exchange interaction parameters J _n used to model the energies of order–disorder were calculated. Both tetrahedral Al–Si and octahedral Al–Mg ordering were studied and hence three types of interaction parameter were necessary: for T–T, O–O and T–O interactions (where T denotes tetrahedral sites and O denotes octahedral sites). Values for the T–T and O–O interactions were taken from results on other systems, whilst we calculated new values for the T–O interactions. We have demonstrated that modelling the octahedral and tetrahedral sheets alone and independently produces different results from modelling a whole T–O–T layer, hence justifying the inclusion of the T–O interactions. Simulations of a whole T–O–T layer of phengite indicated the presence of short-range order, but no long-range order was observed. Received: 8 August 2002 / Accepted: 14 February 2003 Acknowledgements The authors are grateful to EPSRC (EJP) and the Royal Society (CIS) for financial support. Monte Carlo simulations were performed on the Mineral Physics Group's Beowulf cluster and the University of Cambridge's High Performance Computing Facility.     

敞口酸溶-电感耦合等离子体发射光谱法测定多金属矿中的铝锰钾钠钙镁硫

以铜、铅、锌为主,富含多种有色金属的矿床,其所含矿物种类较多,通过矿石主量成分的分析,可对矿床评价与利用起到重要作用.多金属矿石的主量元素检测多采用传统化学方法进行各项指标的测定,需使用多种方法,实验过程复杂,且人为误差影响较大.本文建立了一种敞口酸溶-电感耦合等离子体发射光谱法测定多金属矿中的铝锰钾钠钙镁硫的方法.以氢氟酸-硝酸-高氯酸敞口酸溶样品,使用电感耦合等离子体发射光谱仪进行同时测定,标准曲线线性良好,相关系数均大于0.9990%,方法检出限为0.0003%~0.0041%,测定结果相对标准偏差(RSD)为0.82%~7.93%,方法准确、可靠,满足地质样品的分析需求,且测试过程简单,相较传统分析方法更加省时高效.     

Calculation of the equation of state and elastic moduli of MgO using molecular orbital theory

The success of molecular orbital theory in calculating crystal properties such as bond lengths and atomic force constants has been well documented in the literature. Calculations can be extended to crystals under simulated compression and strain to determine elastic moduli and their pressure derivatives. Comparison of the molecular orbital results with both experimental values and results obtained by calculations such as the potential induced breathing model provides insight into the nature of chemical bonding in MgO. In this study, several molecular clusters were investigated as possible models for MgO; the cluster Mg₄O₄H₂₄ was chosen as the best model. Molecular energies were calculated with respect to bond length for both compression and expansion based on clusters that had been optimized for minimum energy. The resulting energy-volume curve was fitted to a recently derived equation of state (Brown, in preparation) to derive the values of K ₀ and dK₀/dP and the individual elastic moduli and their pressure derivatives were calculated by applying strain to the molecular cluster at both zero and elevated pressures. Agreement between theory and experiment varies between parameters, but the overall trend is encouraging. Since the molecular orbital model includes only short range interactions, its ability to approximating model the elastic moduli of MgO suggests a strong contribution to the elastic energy from short range interactions.     

High temperature single crystal X-ray diffraction studies of the ortho-proto phase transition in enstatite,Mg2Si2O6 at 1360 K

A high temperature single-crystal X-ray diffraction study of enstatite, Mg₂Si₂O₆ was undertaken at 296, 900, 1200, 1360 and 1400 K. During the X-ray data collection at 1360 K (T₀), orthoenstatite (Pbca) transformed to protoenstatite (Pbcn). The unit cell parameters measured at T₀ are a=18.456(4), b=8.960(2) and c=5.270(1) Å for ortho and a=9.306(1), b=8.886(1) and c=5.360(1) Å for proto. The discontinuous increase in c and decrease in b due to the ortho to proto transformation are associated with the drastic unkinking of the silicate chains, whereas the abrupt increase in a results from the large expansion of the M2 — O distances along a coupled with the increase in the out-of-plane tilting of the silicate tetrahedra. Stacking faults form in ortho prior to the phase transition, as well as in proto between 1360 and 1400 K. With increasing temperature, the silicate B chain in ortho straightens faster than the A chain as the configurations of the SiA and SiB tetrahedra tend to become similar. At T₀, the A and B chains with the O3-O3-O3 angles (O3 being the bridging oxygen atom) of 163.0° and 149.5° in ortho, respectively, attain an identical angle of 168.4° in proto. The configuration of the silicate chain in proto resembles that of the A chain in ortho. Rigid-body thermal vibration analysis suggests that between 1200 and 1400 K the largest, the second largest and the smallest thermal librational motions of the [SiO₄] tetrahedra in both ortho and proto are approximately around a, c and b, respectively. Below 1200 K, the largest thermal librational amplitudes of the SiA and SiB tetrahedra in ortho are quite different, but become nearly equivalent at T₀. In contrast to the results reported for all iron-bearing orthopyroxenes at high temperature, switching of the O3B atoms coordinated with the M2 cation occurs during the ortho to proto transformation, but not in ortho below T₀. The ortho-proto transition does not affect the configuration of the M1 octahedron significantly, but results in a decrease of the mean M2 — O bond distance by 0.043 Å and a highly distorted M2 octahedron in proto.     

Pressure effect on the electronic structure of iron in (Mg,Fe)(Si,Al)O3 perovskite: a combined synchrotron Mössbauer and X-ray emission spectroscopy study up to 100 GPa

We investigated the valence state and spin state of iron in an Al-bearing ferromagnesian silicate perovskite sample with the composition (Mg_0.88Fe_0.09)(Si_0.94Al_0.10)O₃ between 1 bar and 100 GPa and at 300 K, using diamond cells and synchrotron Mössbauer spectroscopy techniques. At pressures below 12 GPa, our Mössbauer spectra can be sufficiently fitted by a “two-doublet” model, which assumes one ferrous Fe²⁺-like site and one ferric Fe³⁺-like site with distinct hyperfine parameters. The simplest interpretation that is consistent with both the Mössbauer data and previous X-ray emission data on the same sample is that the Fe²⁺-like site is high-spin Fe²⁺, and the Fe³⁺-like site is high-spin Fe³⁺. At 12 GPa and higher pressures, a “three-doublet” model is necessary and sufficient to fit the Mössbauer spectra. This model assumes two Fe²⁺-like sites and one Fe³⁺-like site distinguished by their hyperfine parameters. Between 12 and 20 GPa, the fraction of the Fe³⁺-like site, Fe³⁺/∑Fe, changes abruptly from about 50 to 70%, possibly due to a spin crossover in six-coordinate Fe²⁺. At pressures above 20 GPa, the fractions of all three sites remain unchanged to the highest pressure, indicating a fixed valence state of iron within this pressure range. From 20 to 100 GPa, the isomer shift between the Fe³⁺-like and Fe²⁺-like sites increases slightly, while the values and widths of the quadruple splitting of all three sites remain essentially constant. In conjunction with the previous X-ray emission data, the Mössbauer data suggest that Fe²⁺ alone, or concurrently with Fe³⁺, undergoes pressure-induced spin crossover between 20 and 100 GPa.     

Analysis of the Na,Mg, Al,and Si abundances in the atmospheres of red giants of different spectral subgroups

We analyze the Na, Mg, Al, and Si abundances in the atmospheres of more than 40 stars, includingred giants of different spectral subgroups (normal red giants, mild and classical barium stars) and several supergiants. All these elements exhibit abundance excesses, with the overabundance increasing with the star’s luminosity. The dependence of the overabundances for each of these elements on the luminosity (or log g) is the same for all the spectral subgroups, testifying to a common origin: they are all products of hydrogen burning in the NeNa and MgAl cycles that have been dredged up from the stellar interiors to the outer atmospheric layers by convection that gradually develops during the star’s evolution from the main sequence to the red-giant stage. The sodium abundances derived for several stars are lower than for other stars with similar atmospheric parameters. The ages and kinematic characteristics of these two groups of stars suggest that they probably belong to different stellar generations.     

等离子体发射光谱法同时测定大洋富钴结壳中硅锰铁钙镁铝钛

大洋富钴结壳试样经ＨＣｌ＋ＨＮＯ３＋ＨＦ溶解,在０．２ｍｏｌ／ＬＨＦ和０．１３ｍｏｌ／ＬＨ３ＢＯ３介质中,用感耦等离子发射光谱法同时测定Ｓｉ、Ｍｎ、Ｆｅ、Ｃａ、Ｍｇ、Ａｌ、Ｔｉ元素的含量。方法经国家标准物质ＧＢＷ０７２４９大洋多金属结核验证,其结果与标准值相符,精密度ＲＳＤ〈３．２％（ｎ＝６）;已应用于太平洋富钴结壳中多元素分析。     

微波消解法测定生物燃料样品中铝钙铬钾镁锰磷锌

生物燃料来源于可再生的有机材料。绝大多数的生物质材料是植物产品或废物,如粮食、油料作物、木头、木板、树皮或碎屑。这些生物质材料由于可以作为替代能源而变得越来越重要,所以人们迫切需要一个能准确测定生物燃料中的主次量元素的分析方法。生物燃料中Al、K、Na、Ca、Mg、Fe和P等主要灰分形成元素,与生物质热工艺转化过程中的灰融、沉积形成和腐蚀等问题有关。而As、Cd、Co、Cu、Cr、Hg、Mo、Mn、Ni、Pb、V、Zn等次量元素由于大气排放问题也十分重要。虽然现在也有一些方法来分析固体燃料中的主量和次量元素;但是测量的精度仍不理想。其中的一个关键问题是这些生物质燃料中的灰分形成元素具有非常复杂     

Ab initio calculation of H, O, Al and Si NMR parameters, vibrational frequencies and bonding energetics in hydrous silica and Na-aluminosilicate glasses

Ab initio, molecular orbital (MO) calculations were performed on model systems of SiO₂, NaAlSi₃O₈ (albite), H₂O-SiO₂ and H₂O-NaAlSi₃O₈ glasses. Model nuclear magnetic resonance (NMR) isotropic chemical shifts (δ_iso) for ¹H, ¹⁷O, ²⁷Al and ²⁹Si are consistent with experimental data for the SiO₂, NaAlSi₃O₈, H₂O-SiO₂ systems where structural interpretations of the NMR peak assignments are accepted. For H₂O-NaSi₃AlO₈ glass, controversy has surrounded the interpretation of NMR and infrared (IR) spectra. Calculated δ_iso¹H, δ_iso¹⁷O, δ_iso²⁷Al and δ_iso²⁹Si are consistent with the interpretation of Kohn et al. (1992) that Si-(OH)-Al linkages are responsible for the observed peaks in hydrous Na-aluminosilicate glasses. In addition, a theoretical vibrational frequency associated with the Kohn et al. (1992) model agrees well with the observed shoulder near 900 cm⁻¹ in the IR and Raman spectra of hydrous albite glasses. MO calculations suggest that breaking this Si-(OH)-Al linkage requires ∼+56 to +82 kJ/mol which is comparable to the activation energies for viscous flow in hydrous aluminosilicate melts.     

Adsorption of zinc on colloidal (hydr)oxides of Si,Al and Fe in the presence of a fulvic acid

The adsorption of Zn (total concentration 10⁻⁶ M) to colloidal quartz, hydrargillite and goethite (50, 300 and 70 mg/l, respectively) was studied by a batch technique at a constant ionic strength (0.01 M) but with variation of pH (3–10) and fulvic acid (FA) concentration (0, 2 or 20 mg/l). The adsorption had similar pH-dependence in all systems in the absence of FA giving a pH₅₀ (pH of 50% adsorption) of 7.6 under these conditions. The presence of the FA reduced the overall adsorption to quartz (pH₅₀ of 7.9 at 2 mg FA/I and 9.3 at 20 mg/1) and shifted the adsorption curves downwards (pH₅₀ of 6.8) in the hydrargillite and goethite systems at 2 mg FA/l. At 20 mg FA/l, the adsorption in the two latter systems was increased at pH <6.5 and reduced at pH >6.5. The results reflect the affinity of the surfaces for FA as well as the formation of Zn–FA complexes (in solution and on solid surfaces).     

Investigation of acidic dissolution of mixed clays between pH 1.0 and −3.0 using Si and Al X-ray absorption near edge structure

Although widely investigated in relation to acid mine drainage systems at pH > 1.0, we know little about the impact of sulfuric acid (H₂SO₄) on the geochemistry and mineralogy of clays at pH < 1.0 (including negative pH values). Thus, laboratory batch experiments were conducted on three mixed clay samples with different mass ratios of phyllosilicates (smectite, illite, and kaolinite) to investigate the impact of H₂SO₄ from pH 1.0 to −3.0 for exposure periods of 14, 90, 180, and 365 days. Si and Al K- and L_2,3-edge X-ray absorption near edge structure (XANES) spectroscopy were employed on these samples to determine the chemical and structural changes that occur during acidic dissolution of phyllosilicates that cannot be distinguished using X-ray diffraction analyses. A series of silicate, phyllosilicate, and Al-bearing standard compounds were also studied to provide an explanation for the observed changes in the clay samples. The Si XANES results indicated the preferential dissolution of the phyllosilicates (pH ? 1.0, t ? 14 d), the persistence of quartz even at pH ? −3.0 and t ? 365 d, and the formation of an amorphous silica-like phase that was confined to the surface layer of the altered clay samples at pH ? 0.0 and t ? 90 d). Al XANES results demonstrated dissolution of Al-octahedral layers (pH ? 1.0, t ? 14 d), the persistence of four-fold relative to six-fold coordinated Al, and the precipitation of an Al-SO₄-rich phase (pH ? −1.0, t ? 90 d). An existing conceptual model of phyllosilicate dissolution under extremely acidic conditions was modified to include the results of this study.     

Kinetic rate laws derived from order parameter theory IV: Kinetics of Al,Si disordering in Na-feldspars

The kinetic rate laws of Al-Si disordering under dry conditions (T = 1353K, 1253 K, 1223 K, 1183 K) and in the presence of water (p = 1 kbar, T = 1023 K, 1073 K, 1103 K) were studied both experimentally and theoretically. A gradual change of the degree of order was found under dry conditions. For intermediate degrees of order broad distributions of the order parameter Q _od occur. The variations of Q _od are correlated with structural modulations as observed in the transmission electron microscope. The time evolution of the mean value of Q _od can be well described by the rate law: $$\frac{{dQ_{od} }}{{dt}} = - \frac{\gamma }{{RT}}\exp \sum\limits_{i = 1}^n {X_i^2 } \left[ {\frac{{ - (G_a^0 + \varepsilon (\Delta Q_{od} )^2 )}}{{RT}}} \right]\frac{{dG}}{{dQ_{od} }}$$ with the excess Gibbs energy G and G _a ⁰ = 433.8 kJ/mol, ?= -27.4 kJ/mol, γ = 1.687 · 10¹⁴ h ^?1. Under wet conditions, two processes were found which occur simultaneously. Firstly, some material renucleated with the equilibrium degree of order. Secondly, the bulk of the material transformed following the same rate law as under dry conditions but with the reduced activation energy G _a ⁰ = 332.0 kJ/mol and ? = -43.0 kJ/ mol, γ = 1.047 · 10¹³ h^?1. The applicability of the kinetic theory is discussed and some ideas for the analysis of geological observations are evolved.     

电感耦合等离子体发射光谱法同时测定长石矿物中钾钠钙镁铝钛铁

样品用HCl-HNO₃-HClO₄-HF分解,电感耦合等离子体发射光谱法同时测定长石矿物中的K、Na、Ca、Mg、Al、Ti、Fe。对元素的分析谱线、溶解用酸的量等工作条件进行了优化,选择了各元素的最佳分析谱线及背景校正模式,探讨HF的用量,试验加内标和不加内标两种测定方法对测定结果的影响。结果表明,对于样品中含量较高的Al,选用次灵敏线396.152 nm,用内标法补偿非光谱干扰,以长石中含量极低的Au作为内标元素(测定谱线为242.795 nm)可以获得满意的结果,方法检出限为0.45~3.56 μg/g。经国家一级标准物质GBW 03134、GBW 3116验证,测定值的相对误差(RE)为-1.32%~10.0%,ICP-AES法与其他测定方法的测定结果无显著性差异,方法精密度(RSD, n=10)为0.55%~7.2%,能够满足长石矿物中相关组分的准确测定。     

微波消解-电感耦合等离子体发射光谱法同时测定白云石中铁铝钙镁钾钠硫

采用微波消解白云石样品，电感耦合等离子体发射光谱法测定其中铁、铝、钙、镁、钾、钠、硫的氧化物，对微波消解条件和等离子体发射光谱仪的工作条件进行了选择，克服了常规化学分析方法步骤繁琐、耗时长、工作量大的不足。利用所建立的方法快速分析了白云石中铁、铝、钙、镁、钾、钠、硫的含量，结果与标准值或化学法相符，10次测定的相对标准偏差（RSD）7种元素均小于1％，可以满足生产和科研的要求。