A CNDO/2 molecular orbital study of the silica polymorphs quartz,cristobalite, and coesite

CNDO/2 MO calculations on H₁₂Si₅O₁₆ clusters modeling silicate tetrahedral linkage in the silica polymorphs show total energy minima at bent SiOSi angles and a correlation between the Si-O bond lengths, d(Si-O), used in the calculation and the minimum energy value of the SiOSi angle. Calculations on hydrogen saturated Si₅O₁₆ clusters isolated from the structures of low quartz, low cristobalite and coesite which were adjusted by DLS methods so that all d(Si-O) equal 1.61 Å and all _L OSiO equal 109.47° yield Mulliken bond overlap populations, n(Si-O), and Si-O two-center energies, E(Si-O), which correlate with observed bond lengths; shorter bonds involve larger n(Si-O) values and more negative E(Si-O) values.

     

A first-principles study of helium diffusion in quartz and coesite under high pressure up to 12GPa

Helium diffusion in mantle minerals is crucial for understanding mantle structure and the dynamic processes of Earth's degassing.In this paper,we report helium incorporation and the mechanism of its diffusion in perfect crystals of quartz and coesite.The diffusion pathways,activation energies(Ea),and frequency factors of helium under ambient and high pressure conditions were calculated using Density Functional Theory(DFT)and the climbing image nudged elastic band(CI-NEB)method.The calculated diffusive coefficients of He in the quartz in different orientations are:D[100]=1.24×10^?6exp.(?26.83 kJ/mol/RT)m²/s D[010]=1.11×10^?6exp.(?31.60 kJ/mol/RT)m²/s.and in the coesite:D[100]=3.00×10^?7exp.(?33.79 kJ/mol/RT)m²/s D[001]=2.21×10^?6exp.(?18.33 kJ/mol/RT)m²/s.The calculated results indicate that diffusivity of helium is anisotropic in both quartz and coesite and that the degree of anisotropy is much more pronounced in coesite.Helium diffusion behavior in coesite under high pressures was investigated.The activation energies increased with pressure:Ea[100]increased from 33.79 kJ/mol to 58.36 kJ/mol,and Ea[001]increased from 18.33 kJ/mol to 48.87 kJ/mol as pressure increased from0 GPa to 12 GPa.Our calculations showed that helium is not be quantitatively retained in silica at typical surface temperatures on Earth,which is consistent with the findings from previous studies.These results have implications for discussion of the Earth's mantle evolution and for recognition thermal histories of ultra-high pressure(UHP)metamorphic terranes.     

Simple covalent potential models of tetrahedral SiO2: Applications to α-quartz and coesite at pressure

We present a covalent potential model of tetrahedrally coordinated SiO₂. The interactions include covalent effects in the form of a Si-O bond-stretching potential, O-Si-O and Si-O-Si angle-bending potentials, and oxygen-oxygen repulsion. Calculated equations of state of α-quartz and coesite agree well with experiment (calculated densities within 1 percent of experiment up to 6 GPa). The calculated α-quartz-coesite transition pressure agrees with the experimental value of ≈2 GPa. Furthermore, the compression mechanisms predicted by the model (i.e. pressure induced changes in Si-O bond lengths and O-Si-O and Si-O-Si angles) are accurate.     

SiO bonded interactions in coesite: a comparison of crystalline, molecular and experimental electron density distributions

Bond critical point properties of electron density distributions calculated for representative Si₅O₁₆ moieties of the structure of coesite are compared with those observed and calculated for the bulk crystal. The values calculated for the moieties agree with those observed to within ∼5%, on average, whereas those calculated for the crystal agree to within ∼10%. As the SiOSi angles increase and the SiO bonds shorten, there is a progressive build-up in the calculated electron density along the bonds. This is accompanied by an increase in both the curvatures of the electron density, both perpendicular and parallel to each bond, and the Laplacian of the electron density distribution at the bond critical points. The cross sections of the bonds at the critical points become more circular as the angle approaches 180o. Also, the bonded radius of the oxide anion decreases about twice as much as that of the Si cation as the SiO bond length decreases and the fraction of s-character of the bond is indicated to increase. A knowledge of electron density distributions is central to our understanding of the forces that govern the structure, properties, solid state reactions, surface reactions and phase transformations of minerals. The software (CRYSTAL95 and TOPOND) used in this study to calculate the bond critical properties of the electron density and Laplacian distributions is bound to promote a deeper understanding of crystal chemistry and properties. Received: 23 February 1998 / Revised, accepted: 16 July 1998     

鄂东南付家山脉石英矿杂质特征及其用作高纯石英原料的潜力

作为战略性矿产资源之一,高纯石英已广泛应用于集成电路、半导体芯片、太阳能等高新技术产业中,但是能够生产高纯石英的原料矿床极为稀缺,我国尤为紧缺高纯石英原料矿。鄂东南地区是湖北省脉石英矿床的主要分布区。本文针对鄂东南付家山脉石英矿床,通过光学显微镜、扫描电子显微镜观察了脉石英的脉石矿物类型和包裹体特征,采用电感耦合等离子发射光谱法(ICP-OES)对原矿进行了微量元素分析,旨在获得付家山脉石英矿床的杂质元素特征,进而评价矿床用作高纯石英原料的潜力。结果表明,付家山脉石英矿石SiO₂含量大于99.95%,杂质元素主要为Al、K、Fe、Ti、Ca等,脉石矿物主要有白云母、钾长石、铁氧化物等,流体包裹体较为发育。杂质元素分析结果表明,付家山脉石英原矿质量达到低端高纯石英标准,经传统工艺提纯后,可能具有生产中高端高纯石英的潜力。     

Thermodynamic properties of quartz,cristobalite and amorphous SiO2: drop calorimetry measurements between 1000 and 1800 K and a review from 0 to 2000 K

We report relative enthalpy measurements on quartz, cristobalite and amorphous SiO₂ between 1000 and 1800 K. We have observed a glass transition around 1480 K for amorphous SiO₂. From our results and available C_p, relative enthalpy, and enthalpy of solution data we have derived a consistent set of thermodynamic data for these phases. Our calculated enthalpies of fusion are 8.9 ± 1.0 kJ mole^?1 for cristobalite at 1999 K and 9.4 ± 1.0 kJ mole^?1 at 1700 K for quartz.     

A study of factors affecting on the zeta potential of kaolinite and quartz powder

The purpose of this study was to determine the effects of pH, ion type (salt and metal cations), ionic strength, cation valence, hydrated ionic radius, and solid concentration on the zeta potential of kaolinite and quartz powder in the presence of NaCl, KCl, CaCl₂, CuCl₂, BaCl₂, and AlCl₃ solutions. The kaolinite and quartz powder have no isoelectric point (iep) within the entire pH range (3 < pH < 11). In the presence of hydrolysable metal ions, kaolinite and quartz powder have two ieps. As the cationic valence increases, the zeta potential of kaolinite and quartz powder becomes less negative. Monovalent cation, K⁺, yields more negative zeta potential values than the divalent cation Ba²⁺. As concentration of solid increases, the zeta potential of the minerals becomes more positive under acidic conditions; however, under alkaline conditions as solid concentration increases the zeta potential becomes more negative. Hydrated ionic radius also affects the zeta potential; the larger the ion, the thicker the layer and the more negative zeta potential for both kaolinite and quartz powder.     

Ab initio calculation of interatomic force constants in H6Si2O7 and the bulk modulus of α quartz and α cristobalite

Ab initio force constants calculated for Si-O stretch and Si...Si non-bonded interactions in H₆Si₂O₇ are found comparable with experimental values derived from the lattice dynamics of α quartz. The bulk moduli of α quartz and α cristobalite are calculated using the molecular Si...Si force constant and assuming rigid regular SiO₄ tetrahedra. In the case (α quartz) where data are available the calculation agrees well with experiment.     

Effects of substrate structure and composition on the structure, dynamics, and energetics of water at mineral surfaces: A molecular dynamics modeling study

Molecular dynamics computer simulations of the molecular structure, diffusive dynamics and hydration energetics of water adsorbed on (0 0 1) surfaces of brucite Mg(OH)₂, gibbsite Al(OH)₃, hydrotalcite Mg₂Al(OH)₆Cl · 2H₂O, muscovite KAl₂(Si₃Al)O₁₀(OH)₂, and talc Mg₃Si₄O₁₀(OH)₂ provide new insight into the relationships between the substrate structure and composition and the molecular-scale structure and properties of the interfacial water. For the three hydroxide phases studied here, the differences in the structural charge on the octahedral sheet, cation occupancies and distributions, and the orientations of OH groups all affect the surface water structure. The density profiles of water molecules perpendicular to the surface are very similar, due to the prevalent importance of H-bonding between the surface and the water and to their similar layered crystal structures. However, the predominant orientations of the surface water molecules and the detailed two-dimensional near-surface structure are quite different. The atomic density profiles and other structural characteristics of water at the two sheet silicate surfaces are very different, because the talc (0 0 1) surface is hydrophobic whereas the muscovite (0 0 1) surface is hydrophilic. At the hydrophilic and electrostatically neutral brucite and gibbsite (0 0 1) surfaces, both donating and accepting H-bonds from the H₂O molecules are important for the development of a continuous hydrogen bonding network across the interfacial region. For the hydrophilic but charged hydrotalcite and muscovite (0 0 1) surfaces, only accepting or donating H-bonds from the water molecules contribute to the formation of the H-bonding network at the negatively and positively charged interfaces, respectively. For the hydrophobic talc (0 0 1) surface, H-bonds between water molecules and the surface sites are very weak, and the H-bonds among H₂O molecules dominate the interfacial H-bonding network. For all the systems studied, the orientation of the interfacial water molecules in the first few layers is influenced by both the substrate surface charge and the ability by the surfaces to facilitate H-bond formation. The first layer of water molecules at all surfaces is well ordered in the xy plane (parallel to the surface) and the atomic density distributions reflect the substrate crystal structure. The enhanced ordering of water molecules at the interfaces indicates reduced orientational and translational entropy. In thin films, water molecules are more mobile parallel to the surface than perpendicular to it due to spatial constraints. At neutral, hydrophilic substrates, single-monolayer surface coverage stabilizes the adsorbed water molecules and results in a minimum of the surface hydration energy. In contrast, at the charged and hydrophilic muscovite surface, the hydration energy increases monotonically with increasing water coverage over the range of coverages studied. At the neutral and hydrophobic talc surface, the adsorption of H₂O is unfavorable at all surface coverages, and the hydration energy decreases monotonically with increasing coverage.     

A three-dimensional model and its potential application to spring protection

Withdrawal of groundwater can dry up springs. It is, however, often difficult to determine the exact causes for the depletion of spring flow by the traditional methods. A 3-D geological model is presented in this paper to explore the groundwater flow conditions in the aquifers that feed springs. Because of the various geological data in spring areas, a date integration technique is adopted to ensure the accuracy of the model. This model is implemented in the spring protection program in Jinan City, China, and the reasons for the drying up of the springs in the city are studied. The proposed model would be particularly helpful in the spring protection, and the appropriate application of the model would result in economic and social benefits of the city.     

Micro-deformation of quartz under the dynamic action and its applications

Laboratory-controlled shock experiments on samples from two types of granite were carried out by use of a series of high explosive lenses differing in peak pressures. Microdeformation features (e.g., Micro-fractures, shock lamellae and changes in refractive index (No) in ordinary light) were observed under the petrographic microscope for quartz selected from the shocked granite samples. According to our observations, we have related the microdeformation of quartz with peak pressures. Similar studies of the micro-deformation of quartz was also performed from the granite medium in which underground nuclear explosion had been carried out. By comparing the deformation features of quartz subjected to nuclear-explosion-induced shock with those mentioned above, we have deduced the stress distribution in the free-field in the underground-nuclear-explosion medium. Based upon the above results, together with some other data, damaged zones in the free-field in respect to this underground nuclear explosion can be distinguished in terms of micro-deformation features.     

岩体结构三维无缝建模与四面体优化剖分

复杂地质特征给数值模拟的建模工作带来很大困难,针对该问题,提出并实现了岩体结构三维无缝建模方法。将直接法拓展到四面体网格剖分中,实现了基于岩体结构无缝模型的四面体优化剖分方法,从而为数值模拟提供既符合地质实际,又有较高网格质量的计算模型。利用该方法建立了某拱坝坝肩岩体三维地质模型,并进行四面体优化剖分。     

Numerical modeling of a landslide-generated tsunami following a potential explosion of the Montserrat volcano

The explosion of the Montserrat volcano (Caribbean Sea) could trigger a major landslide and lead to the generation of a tsunami in the Caribbean Sea. In the worst case scenario, the volume of material reaching the sea has been estimated at 80 millions of cubic meters. The sliding of this mass and the generation of the associated tsunami have been simulated numerically, assuming that the debris behave like a heavy fluid flowing into the sea. The numerical model solves the 3D Navier-Stokes equations for a mixture composed of rocks and water. The generated water waves is then propagated around the coast of Montserrat by means of a shallow water model. The numerical results show that the water heights above sea level are higher than 5 meters within a radius of 5 km of the source.     

Dislocation structure of the deformation lamellae in synthetic quartz; a study by electron and optical microscopy

Single crystals of experimentally deformed synthetic quartz showing optical deformation lamellae were examined by transmission electron microscopy. Dislocations are distributed fairly uniformly throughout the crystal. However, parallel to the trace of the deformation lamellae, which may be irrational, there are walls of tangled dislocations whose characteristics suggest that they are directly associated with the lamellae. The nature and formation of the optical image is discussed in detail.     

Rigid unit modes in the molecular dynamics simulation of quartz and the incommensurate phase transition

A molecular dynamics simulation of quartz at different temperatures both in the a and in the phase has been conducted. The - phase transition could be observed. A phonon analysis of the -phase confirms and rounds out in a quantitative way the origin of the incommensurate (ic) modulated phase. In particular it traces the optic soft mode at becoming (to a good approximation) a so-called rigid unit mode (RUM) at q0, and elucidates its coupling to the transverse acoustic mode which precipitates the incommensurate transition. This success underpins and illuminates the concept of RUMs and their role in structural phase transitions.     

石英砂表面锰胶膜的合成及其影响因素研究

以石英砂为载体在其颗粒表面合成锰胶膜运用X射线衍射(XRD)和扫描电镜(SEM)等手段对胶膜中矿物的结构进行表征,探讨体系中不同锰摩尔比R(Mn~(2+)、Mn~(7+)摩尔比分别为1:1、1:2、1:3、1:4)、合成温度(30、50、70、90、110℃)、搅拌时间(48、72、96 h)和石英砂添加量(5、10、20、30、40 g)等因素对锰胶膜合成的影响。研究结果表明,随着R的减小,锰胶膜中的锰钾矿向水钠锰矿转化且矿物的结晶度增大,锰含量减小,晶体颗粒逐渐增大;随着温度的升高,胶膜中矿物的结晶度增大,锰含量下降,但矿物类型没有变化;搅拌不会影响胶膜的矿物类型,但搅拌时间越长胶膜中锰矿物含量越高;随石英砂添加量的增大胶膜中锰矿物类型不变,但其结晶度减弱,锰含量先降低但在石英砂添加量为10 g时开始升高。     

A scale of dissolution for quartz and its implications for diagenetic processes in sandstones

During diagenesis quartz grains undergo selective dissolution, controlled in location by the surface energy characteristics of the individual grains. Experimental etching in HF of isolated quartz grains reproduces comparable textures to those of natural occurrences. Some experimental results illustrate the specific effects of surface textures on their initial dissolution rates, so demonstrating the control surface energy variation has over dissolution. A hierarchy of grain surface characteristics, according to surface energies, provides a useful guide to the relative rates of dissolution during decomposition.     

石英中铝、钙和镁的纳米离子探针原位分析标准物质研究

石英胶结物是砂岩中重要的胶结物类型。用纳米离子探针(CAMECA Nano SIMS 50L)对石英胶结物中Al、Ca和Mg等重要微量元素进行原位分析,可为成岩作用研究提供重要基础依据。然而,由于缺乏相应的标准物质,一直不能对石英胶结物中Al、Ca和Mg进行研究。针对不均一的石英颗粒样品,文中采用细磨、混合、高温熔融(2000℃,6个小时)与快速冷却等措施,制作均一化的石英玻璃。经纳米离子探针验证,该石英玻璃中Al、Ca和Mg在亚微米尺度上是均一的,已能满足成岩作用研究的需要。随后,用电感耦合等离子体发射光谱和激光剥蚀电感耦合等离子体质谱测定了石英玻璃中Al、Ca和Mg的含量。相对灵敏度因子分析显示,这些测试结果是协调、合理的。该均一化的石英玻璃可作为亚微米尺度Al、Ca和Mg原位分析的标准物质。