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1.
Accurate interatomic potentials have been employed to compute the phonon density of states of αquartz, stishovite and coesite polymorphs of silica. The temperature variation of several thermodynamic properties is calculated by using the phonon density of states to describe the vibrational entropy contribution to the free energy. Results for these polymorphs are in surprisingly good agreement with available experimental data. Moreover, the microscopic origin of quantitative differences in the heat capacity behavior of low and high density polymorphs is established.  相似文献   

2.
Micro- and non-crystalline opals, chalcedony and flint show diffuse small angle neutron scattering (SANS). Precious opals give rise to two additional intensity maxima at very small scattering angles which are due to Bragg reflections from the closest packed non-crystalline silica spheres. A small angle texture diagram reveals that the closest packing is faulty. Synthetic non-crystalline opals yield much less intense small angle scattering due to lower contrast between silica spheres and interstitial cement or particles; in this case intensity maxima were not observed. The outer part of the scattering curves of opal-CT and microcrystalline quartz deviates from Porod's law. The specific surface of natural non-crystalline opals ranges from 0.006 to 0.018 nm–1. In microcrystalline opals, the specific surface is about 10 times larger than in non-crystalline opals.  相似文献   

3.
《International Geology Review》2012,54(11):1750-1754
Examination of a large number of specimens of α-quartz, α-cristobalite, chalcedonies, opals, quartzines, and agates shows that the so-called amorphous forms of silica do not represent any particular specificities, but are simply amorphous mixtures, more often than not, for example, α-quartz with tridymite, on the α-quartz base (chalcedonies, opals). In chalcedonies the tridymitic constituent is somewhat larger than in opals. Another large group of opals is but an altered α-cristobalite, high in molecular water. The α-quartz opals characteristically contain far less water. — Authors.  相似文献   

4.
The electron localization function, ELF, generated for a number of geometry-optimized earth materials, provides a graphical representation of the spatial localization of the probability electron density distribution as embodied in domains ascribed to localized bond and lone pair electrons. The lone pair domains, displayed by the silica polymorphs quartz, coesite and cristobalite, are typically banana-shaped and oriented perpendicular to the plane of the SiOSi angle at ~0.60 Å from the O atom on the reflex side of the angle. With decreasing angle, the domains increase in magnitude, indicating an increase in the nucleophilic character of the O atom, rendering it more susceptible to potential electrophilic attack. The Laplacian isosurface maps of the experimental and theoretical electron density distribution for coesite substantiates the increase in the size of the domain with decreasing angle. Bond pair domains are displayed along each of the SiO bond vectors as discrete concave hemispherically-shaped domains at ~0.70 Å from the O atom. For more closed-shell ionic bonded interactions, the bond and lone pair domains are often coalesced, resulting in concave hemispherical toroidal-shaped domains with local maxima centered along the bond vectors. As the shared covalent character of the bonded interactions increases, the bond and lone pair domains are better developed as discrete domains. ELF isosurface maps generated for the earth materials tremolite, diopside, talc and dickite display banana-shaped lone pair domains associated with the bridging O atoms of SiOSi angles and concave hemispherical toroidal bond pair domains associated with the nonbridging ones. The lone pair domains in dickite and talc provide a basis for understanding the bonded interactions between the adjacent neutral layers. Maps were also generated for beryl, cordierite, quartz, low albite, forsterite, wadeite, åkermanite, pectolite, periclase, hurlbutite, thortveitite and vanthoffite. Strategies are reviewed for finding potential H docking sites in the silica polymorphs and related materials. As observed in an earlier study, the ELF is capable of generating bond and lone pair domains that are similar in number and arrangement to those provided by Laplacian and deformation electron density distributions. The formation of the bond and lone pair domains in the silica polymorphs and the progressive decrease in the SiO length as the value of the electron density at the bond critical point increases indicates that the SiO bonded interaction has a substantial component of covalent character.  相似文献   

5.
The occurrence of water in natural opaline silicas has been studied by differential thermal analysis, thermogravimetric analysis, infra‐red analysis and nuclear magnetic resonance. The results show that in the “crystalline” opals studied, some 90 per cent or more of the total water is physically adsorbed whereas in “amorphous” opals, at least 20 per cent but perhaps much more of the total water is held as hydroxyl groups chemically bonded to the silica surface. The rate of water loss on heating is also different, being chiefly controlled by the pore structure in “crystalline” opals but to a significant extent by the surface structure in “amorphous”.  相似文献   

6.
Periodic Hartree-Fock STO-3G calculations have been performed on several tetracoordinated silica polymorphs: low and high quartz, low and idealized high cristobalite and prototype tridymite. The optimized structural parameters are in overall good agreement with experimental data. In the particular case of -quartz, the SiO4 tetrahedra are found to be irregular. The optimized values of the two different SiO bond lengths are respectively 1.608 Å and 1.613 Å. The potential energy versus tilt angle curves suggest a picture of the high temperature phases in terms of delocalized oxygen atoms which is consistent with a disordered structure. Finally, the bonding in silica polymorphs is discussed from electron density maps and Mulliken population analysis.  相似文献   

7.
The main parameters of infrared spectra have been investigated for marine siliceous skeletons and for silica polymorphs. From infrared (IR)data, the mean values for the Si-O bond length (1.62 Å) and for the Si-O-Si angle (about 142°6) are inferred for biogenic silica. A model is proposed for the molecular structure of biogenic, amorphous silica, which consists of a discontinuous, three-dimensional framework of short chains of [SiO4] tetrahedra, bounded with apical hydroxyls. According to the relationship between the structure of the silica polymorphs and their infrared spectra, it is shown that a quantitative analysis of biogenic silica seems to be possible in quartz-rich sediments. Some geological implications of these data are emphasized, including the processes of siliceous skeleton sedimentation and diagenetic evolution.  相似文献   

8.
The means and timing of the formation of Australian sediment-hosted precious opal remain a subject of continuing debate. In this study, the question of which water formed the opal is addressed by examination of rare earth element data for opals and host rocks. The available data, mainly for Lightning Ridge, NSW, suggest a positive Eu anomaly, relative to the neighbouring Sm and Dy, occurs in opals whereas no such anomaly was found for the weathered Cretaceous sediments hosting the opal. Such anomalies may be inherited from the source rock with a similar positive Eu anomaly or generated in situ by severe reduction. There is no indication of major reduction processes during the opal formation that could have led to such a Eu anomaly so this is likely inherited from a source rock. As the opal host rocks did not show this anomaly, the source rocks must be external to the opal fields. Calcite cements within rocks hosting the aquifers of the Eromanga and Surat basins of the Great Artesian Basin have been reported to have a positive Eu anomaly, which strongly suggests that opal was formed by upwelling Great Artesian Basin artesian waters. This work has also highlighted variations in trace-element concentrations in opals, which indicate significant variation in the source water composition during opal formation or different water sources were involved. Either of these is indicative of the source for the opal with its trace elements derived from external sources. These conclusions have significant implications to considerations of how opal formed, and hence, for the exploration for other deposits and to the chemistry and timing that led to the formation of opal.  相似文献   

9.
The Gladstone-Dale law (specific refraction) and the Drude law (molecular refraction) for silica polymorphs, at “sodium light” (λ D =0.5893 μm), are derived from simple atomic properties of SiO2 complex (atomic weight, first ionization potential). The considerations are based on the Lorentz electron theory of solids. The characteristic frequency (or eigenfrequency) v 0 of elementary electron oscillators (in energy units, hv) is identified with the band gap E G of a solid; on the other hand, this E G -gap is identified with the single ionization potential \(\tilde U\) of non-free atoms. For \(\tilde U\) =E G =10.2 eV (energy gap of quartz, see Nitsan and Shankland 1976b) the Gladstone-Dale law, or specific refraction, is (n?1)/ρ=0.208 cm3/g, where n and ρ are the refractive index and the density of medium, respectively. According to empirical data, the average value of the specific refraction of pure SiO2 polymorphs (except stishovite-high density phase of silica) is (〈n〉?1)/ρ=0.207±0.001 (〈n〉 denotes the mean refractive index of crystal). For stishovite the Drude law (n 2?1)/ρ=0.542 cm3/g is valid under an assumption that the first ionization potential \(\tilde U\) =E G ≈9 eV; this result is good agreement with the empirical value (〈n〉2?1)/ρ=0.536 cm3/g.  相似文献   

10.
直至目前,我国相关欧泊的矿藏及其相应的矿物学特征鲜见报道,近期在浙江丽水缙云地区发现的蓝色类欧泊石引起了宝玉石科研工作者的广泛关注,但此矿物的相关研究至今尚未见论述。本文应用X射线荧光光谱(XRF)、场发射扫描电镜(FE-SEM)结合X射线粉晶衍射(XRD)与傅里叶变换红外光谱(FTIR)等多种综合检测技术首次对该类蓝色原矿的矿物学及光学变彩性质进行研究。结果表明:XRF分析显示蓝色原矿的主体化学组成为SiO2(质量含量95%),仅含极少量的Fe2O3与Al2O3等杂质。同时,蓝色原矿及其相应的热处理后样品局部区域具有明显的变彩特征,主体的化学组成与其光学特征与天然欧泊的组成及变彩效应类同。XRD分析显示原矿粉体的无机相的晶型为方石英相,衍射峰形锐化,该晶体的结构特征明显相异于天然欧泊即微晶类的蛋白石(Opal-CT)、Opal-C与非晶蛋白石类(Opal-A)。此外,样品的红外光谱与天然欧泊间存在一定的频率位移。鉴于此,初步将该类原矿归属为类欧泊,因蓝色原矿所在地丽水缙云地区属于中生代陆相火山岩的地质,因此进一步将该原矿命名为一类火山岩类的类欧泊石,而有关此类欧泊是否可以归属为广义上的欧泊有待再作商榷。同时基于对蓝色原矿有无变彩效应区域的FE-SEM测试结论,本文初步推测该类矿物的光学变彩特征与原矿表面的“叠瓦状”结构形貌有关。  相似文献   

11.
An interatomic interaction potential for silica polymorphs is derived based on the SCD model (cfr. Tijskens et al. 1994). This interaction potential incorporates all classical electrostatic interactions arising from the spherical part of the spatial extent of the atoms including many body interactions. The potential is derived from Hartree-Fock energies and electron densities for a set 72 [SiO4]4-- and [Si2O7]6--clusters with variable configuration. The long range impact of the surroundings on these clusters in the infinite system has been successfully mimicked by embedding the clusters in a finite three-dimensional array of point charges. This three-dimensional array of point charges is optimized as to reproduce the average site potential and its gradient occurring in II–IV-coordinated silica polymorphs at the central atoms of the clusters. The resulting interaction potential consists of two functions of the configurational coordinates, ?, describing spherical “atomic” electron densities, σa(x, ?) for A=Si, O. All classical electrostatic interactions are derived from these densities. A Born-Mayer type correction term ΔE qm(?) models the quantum mechanical interactions and the electrostatic interactions arising from the non-spherosymmetrical component of the electron density. The new interaction potential model shows a slightly improved reproduction of the potential surface with respect to the classical Born-Mayer ionic model and demonstrates the importance of many body interactions as charge transfer and expansion/contraction of the atomic electron densities in these systems. Also the dependence of the quantum mechanical correction term ΔE qm(?) on the Si-O-Si-bond angle proves covalent effects to be larger than suggested by the classical Born-Mayer ionic model thereby clarifying the controversy in literature on the importance of covalent effects in silica polymorphs and polymerised silicates in general.  相似文献   

12.
The 1D τ-p inversion algorithm is widely employed to generate starting models with most computer programs that implement refraction tomography. However, this algorithm emphasizes the vertical resolution of many layers, and as a result, it frequently fails to detect even large lateral variations in seismic velocities, such as the decreases that are indicative of shear zones. This study presents a case that demonstrates the failure of the 1D τ-p inversion algorithm to define or even detect a major shear zone that is 50 m or ten stations wide. Furthermore, the majority of refraction tomography programs parameterize the seismic velocities within each layer with vertical velocity gradients. By contrast, the 2D generalized reciprocal method (GRM) inversion algorithms emphasize the lateral resolution of individual layers. This study demonstrates the successful detection and definition of the 50-m wide shear zone with the GRM inversion algorithms. The existence of the shear zone is corroborated by a 2D analysis of the head wave amplitudes and by numerous closely spaced orthogonal seismic profiles carried out as part of a later 3D refraction investigation. Furthermore, a 1D analysis of the head wave amplitudes indicates that a reversal in the seismic velocities, rather than vertical velocity gradients, occurs in the weathered layers. While all seismic refraction operations should aim to provide as accurate depth estimates as is practical, the major conclusion reached in this study is that refraction Inversion algorithms that emphasize the lateral resolution of individual layers generate more useful results for geotechnical and environmental applications. The advantages of the Improved lateral resolution are obtained with 2D profiles in which the structural features can be recognized from the magnitudes of the variations in the seismic velocities. Furthermore, the spatial patterns obtained with 3D investigations facilitate the recognition of structural features that do not display any intrinsic variation or "signature" in seismic velocities.  相似文献   

13.
Abstract

Black opal (opal-AG) owes its dark coloration to a fine-grained pigment commonly inferred to be mainly carbon, yet chemical compositions for black opals suggest there may be additional components. Here we search for such components in pigment concentrates prepared by dissolving black opal nodules (nobbies) from Lightning Ridge (NSW) in hydrofluoric acid, using electron microscopy (scanning electron microscopy, transmission electron microscopy), X-ray diffraction and laser-ablation ICP-MS. The results demonstrate the presence of sulfides—predominantly pyrite and chalcopyrite, with minor galena and Ti-oxide phases, as additional components of the pigment. ATR-FTIR analysis indicates the presence of C=O and C–H groups, consistent with an organic origin. Transmission electron microscopy images of pigment show variously deformed, originally spherical ～100?nm particles rich in sulfide and carbon, which are interpreted as thin coatings of pigment on now dissolved opaline silica spheres. Laser-ablation ICP-MS analysis identifies remnant silica in pigment concentrates, which may be interpreted as opaline silica surviving HF treatment protected as inclusions in sulfides. When examined within the context of petrographic observations from more than 1000 opal nodules (nobbies) at Lightning Ridge, these new results suggest that pigment carbon and sulfides in the nodules formed microbially under initially anoxic groundwater conditions, within pre-existing cavities concurrently being filled with silica sol ultimately derived from chemical weathering of feldspar-rich volcaniclastic sediment. Intensely black pigment layers observed at the floor of many nodules indicate settling of dark, high-density (sulfide–Ti-oxide-rich) pigment within cavities, with the implication that sulfate-reducing bacterial (SRB) activity commences early during the silica sol-gel ripening process. Microbial activity may persist until after the cavity has completely filled with the silica sol, as illustrated by abundant black opals with uniformly distributed pigment. Pigment formed at this stage may no longer be able to settle out within the ripening and increasingly viscous silica gel, thus forming pigmentation throughout the opal cavity. The existence of ‘amber’, pigment-poor opal with intensely black basal pigment layers is interpreted as signalling a lack of sulfate to sustain further SRB activity, or a change to more oxidising conditions, possibly related to interaction with surface waters within a downward-penetrating weathering front. A change in redox conditions would shut off activity of SRB and thus sulfide pigment production and allow development of aerobic microbial activity as described by others.  相似文献   

14.
Ab-Initio quantum mechanical calculations on molecular clusters are used to obtain potential surfaces for the SiO bond in silicates. These potential surfaces form the basis for extracting the key parameters in various commonly employed potential functions. Applications to the usual ionic model demonstates a close relation between the ab-initio derived ionic potential and those empirically dervied. The ionic model is then used to predict structures and elastic properties of orthosilicates and of the silica polymorphs. The deficiencies in the ionic model lead to the application of the quantum results to covalent models. These latter models are then used in theoretical calculations of the properties of silica polymorphs.  相似文献   

15.
近期国内珠宝市场上出现一种产自巴西的绿欧泊,该类绿欧泊表现出的宝石矿物学特征与传统的欧泊有着明显差异。采用常规宝石学测试手段,并结合电子探针、X射线粉末衍射、扫描电子显微镜、红外吸收光谱、紫外吸收光谱等测试分析方法,就其化学成分、物相、微结构、谱学特征及颜色成因等问题展开初步研究。结果表明,巴西绿欧泊的主要矿物组成为蛋白石,含不等量的α-方石英和α-鳞石英等次要矿物,整体呈不规则胶柬状结构,局部微波纹理发育;该类欧泊主要由直径约10～30nm的SiO2球粒聚集而成;由H2O和M—OH倍频振动致近红外吸收谱带位于7000cm-1处,由H2O合频振动致近红外吸收谱带分别位于5200,5612cm叫处,由M—OH与[SiO4]合频振动致近红外吸收谱带位于4600-4100cm-1范围内。笔者还对巴西绿欧泊的呈色机理一并给予了探讨。  相似文献   

16.
Raman spectra of the two high-pressure polymorphs of SiO2 (coesite and stishovite) were investigated in the temperature range 105–875 K at atmospheric pressure. Coesite remained intact after the highest temperature run, but stishovite became amorphous at temperatures above about 842～872 K. Most Raman modes exhibit a negative frequency shift with temperature for these polymorphs, but positive trends were also observed for some modes. Except for some weak modes, nonlinear temperature variation were established for these polymorphs within the experimental uncertainty and temperature range spanned. The slopes of the variation (δvi/δT)P for these polymorphs were compared with the published values. When compared with quartz and stishovite, the four-membered rings of SiO4-tetrahedra in coesite exhibit very little change with both temperature and pressure. It is also suggested that temperature and pressure should have opposite effects on the Raman shift of each vibrational mode.  相似文献   

17.
This paper presents the results of the δ18O study of the precious opals from Primor’e (Raduzhnoe deposit), Australia, and Ethiopia and the modern opals from the hydrotherms of the Mendeleev Volcano (Kunashir Island, Kuril Islands). It is established that the oxygen isotope ratio in opals may serve as a criterion for the estimation of their formation temperature. The low-temperature sedimentary opals are relatively enriched in the heavy oxygen isotope independently of the sedimentary or volcanic host rocks. Examples are the Australian and Slovakian opals of the A-type. The hydrothermal opals are enriched in the light oxygen isotope, which depends on the precipitation temperature. The higher the temperature, the lighter the oxygen isotope ratio of the precipitating opal is and the closer it is to that of the hydrothermal fluid.  相似文献   

18.
Five different refraction formulas were applied to SiO2 polymorphs in order to determine the most suitable refractive index-density relation. 13 SiO2 polymorphs with topological different tetrahedral frameworks are used in this study including eight new low density SiO2 polymorphs — so called “guest free porosils”. These SiO2 polymorphs cover a density range from 1.76 to 2.92 g/cm3. The mean refractive indices (ovn) of the porosils have been determined by the immersion method, the densities (ρ) were calculated from the unit cell parameters. Assuming the polarizability (α) of all SiO2 polymorphs to be constant the general refractivity formula $$\{ 2\overline {11} 0\} \langle 0001\rangle $$ turned out to be the most suitable for SiO2 polymorphs. Regression analysis yields an electronic overlap parameter b=1.2(1).  相似文献   

19.
It is suggested that an explanation of the observed metastable formation of cristobalite and tridymite in the stability field of quartz lies in the topology of these silica polymorphs and the resulting energies at the time of formation.  相似文献   

20.
首次提出了用于解决塔里木盆地地震资料数据处理中浅部地震速度分析的折射层析成象方法。该方法可以通过求解任意变速介质中的二维程函方程、射线追踪和ＳＩＲＴ迭代解法实现。通过对塔里木盆地地震资料的层析反演取得了较好的结果,揭示了浅部地震速度具有较大的横向变化。这一变化对建立地下速度模型及反射构造成象有较大影响,钻井资料证实了这一结果。这项研究的成功对研究塔里木盆地地震波速度的变化规律及进一步提高该区整体地震资料处理与解释的质量具有重要的实际意义。  相似文献   

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