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1.
2.
The Origin of Sillimanite in Glen Clova, Angus 总被引：2，自引：3，他引：2
At the sillimanite isograd in Glen Clova, sillimanite appearsto have formed within biotite, rather than in kyanite. Biotiteis thought to have been a nucleating agent, the trigonally arrangedoxygen octahedra and tetrahedra in the alternate mica layersacting as nuclei for the growth of the octahedral Al–Oand the tetrahedral (Al, Si)–O chains that constitutethe sillimanite structure. Nucleation seems to have been dominantlyepitaxial; no permanent breakdown of biotite was involved, andit is suggested that Al and Si for sillimanite growth was mainlyderived from the solution of unstable kyanite. 相似文献
3.
Silicon K-edge x-ray absorption near-edge structure (XANES) spectra of a selection of silicate and aluminosilicate minerals have been measured using synchrotron radiation (SR). The spectra are qualitatively interpreted based on MO calculation of the tetrahedral SiO 4 4? cluster. The Si K-edge generally shifts to higher energy with increased polymerization of silicates by about 1.3 eV, but with considerable overlap for silicates of different polymerization types. The substitution of Al for Si shifts the Si K-edge to lower energy. The chemical shift of Si K-edge is also sensitive to cations in more distant atom shells; for example, the Si K-edge shifts to lower energy with the substitution of Al for Mg in octahedral sites. The shifts of the Si K-edge show weak correlation with average Si-O bond distance (dSi-O), Si-O bond valence (sSi-O) and distortion of SiO4 tetrahedra, due to the crystal structure complexity of silicate minerals and multiple factors effecting the x-ray absorption processes. 相似文献
4.
E. Salje 《Physics and Chemistry of Minerals》1987,14(2):181-188
Landau theory of the \(P\bar 1 - I\bar 1\) phase transition in Ca-rich plagioclases reveals the sensitivity of the phase transition behaviour to a) Al, Si disorder, b) structural replacement of Ca by Na, and c) inhomogeneities of lattice strains. The following effects are predicted:
- A tricritical phase transition exists in fully ordered anorthite. Al, Si disorder and Na, Ca exchange lead to second order phase transitions.
- The transition temperatures depend sensitively on the degree of Al, Si disorder and the chemical composition of the Ca-rich plagioclases. Increasing Na-content decreases the transition temperatures.
- The thermal evolution of c and d reflections depends on the homogeneity of the crystal and do not necessarily reflect the temperature evolution of the macroscopic lattice strain. A simple quadratic dependence of the X-ray scattering intensity on the order parameter exists only for fully ordered, homogeneous anorthite.
5.
D. Zherebetskyy S. Lebernegg G. Amthauer M. Grodzicki 《Physics and Chemistry of Minerals》2012,39(5):351-361
The magnetic structure of almandine has been investigated by electronic structure calculations in the local spin density approximation
in order to arrive at a more detailed understanding of the magnetic structure and the exchange pathways. The calculations
are based on experimentally determined geometrical data of the crystal structure at 100 K. The calculated quadrupole splittings,
spin-allowed d–d transitions and magnetic moment for iron atoms are in reasonable agreement with the respective experimental values obtained
by M?ssbauer and absorption spectroscopy, and magnetization measurements demonstrating the reliability of the calculations.
The spin structure is derived from the calculated coupling constants for the possible exchange pathways. The competing superexchange
pathways exist via oxygen bridges between directly neighboured iron ions and via edges of silicon tetrahedra and aluminium
octahedra connecting more distant iron dodecahedra. Careful consideration revealed that almandine structure contains two identical
interpenetrative sublattices of Fe dodecahedra connected via Al octahedra and Si tetrahedra. The calculations provide the
information about ferromagnetic interaction between the iron spins within each sublattice, whereas the coupling between two
magnetic sublattices is weakly antiferromagnetic via Al octahedra and Si tetrahedra. This antiferromagnetic interaction of
two identical magnetic sublattices is in good agreement with the experiments and explains the M?ssbauer spectra of almandine
below the Néel temperature. Since almandine belongs to most abundant crystallized silicates that are main constituents of
the earth and main components of cosmic dust, these results have scientific importance of studying the universe. 相似文献
6.
The ordering of Al and Si in gehlenite is considered using computer simulation. The enthalpy of ordering ΔH per 2Al+2Si atoms is found to be 0.52 eV. It is dominated by the nearest neighbour interaction, but the analysis is carried out to fifth neighbours. The nearest neighbour interaction differs significantly from that for other materials. The structure does not have a connected network of ordering sites, which mainly accounts for the unobservably low transition temperature for Al/Si ordering. Two alternatives are given for the likely ordering pattern. 相似文献
7.
Eric Dowty 《Physics and Chemistry of Minerals》1987,14(2):122-138
Normal coordinate calculations, producing synthetic infrared and Raman spectra, were carried out on melilites, pyroxenes, silica polymorphs and feldspars. Atomic motions are complex in the high-frequency Raman modes of melilites and aluminous pyroxenes. The symmetric T-Onb stretching vibrations of Si and Al tetrahedra with different numbers of bridging oxygens are separate from each other, but may combine individually with oscillation of bridging oxygens between Si and Al tetrahedra. The latter type of vibration tends to dominate as Al/Si increases. The frequencies of these vibrational components and the degree of such intermixing depend on T-O force constants, which vary greatly depending on local bonding configurations; individual bands in the high-frequency Raman cannot in general be assigned to single structural entities or fixed combinations thereof. Calculations confirm that in some Al-Si glasses such as jadeite and spodumene, i.e. those in which all Al can be tetrahedral without non-bridging oxygens, Al-O-Al linkages or linkage of more than two tetrahedra by a single oxygen, aluminum is predominantly in tetrahedral coordination. Other Al-Si glasses which are richer in aluminum or which have non-bridging oxygens may contain Al tetrahedral triclusters, non-tetrahedral Al, or both. On the basis of distinctive 450–750 cm?1 infrared bands, both silica and feldspar glasses resemble tridymite and related stuffed derivatives, not other crystalline silica polymorphs or feldspars. Either these glasses have a structure like that of tridymite on a local scale, or the disorder of the glasses causes drastic modification to the vibrations in question. 相似文献
8.
The origin of sector trilling in cordierite is due to the hexagonal-orthorhombic Al, Si ordering transformation which under non-equilibrium conditions proceeds via a short-range ordered modulated structure. The growth of these distortion waves associated with progressive ordering produces a strain field which is minimized by a cyclic distribution of symmetrically equivalent modulations.Sector and complexly trilled cordierites in metamorphic rocks grew as the hexagonal polymorph with a considerable degree of Al, Si disorder. The enthalpy and entropy of disorder are evaluated from recent experimental work. The implication is that, in metamorphic rocks, substantial overstepping of stable equilibrium phase boundaries is required to nucleate hexagonal cordierite. Moreover, its composition coexisting with other phases will also be significantly different from that of the stable ordered form. 相似文献
9.
A methodology based on the Hartree–Fock theory is used to study pure MgSiO3 crystals as well as Al doping in perovskite and ilmenite modifications of this mineral. Atomic displacements in the neighbourhood
of the defect are obtained for cases when the Al substitutes for either Mg or Si host atoms. The atomic relaxation is due
to the changes produced upon the chemical bonding within the defective region and in some occasions obeys the Coulomb electrostatic
interaction. Band structure properties are briefly revised for the pure and doped minerals. The occurrence of Al-bound hole
polaron is predicted in the ilmenite mineral. The results of output are compared to the available reports on the same subject
in both experimental and theoretical fields of the investigation. 相似文献
10.
Synthetic (SiGe)-pyroxenoids are often observed to have superperiods. Whether or not these superperiods grow in connection
with Si-Ge ordering is a fundamental question. The size difference between Ge and Si tetrahedra leads to CdGeO3 having the pyroxmangite structure, whereas CdSiO3 has the wollastonite structure. Consequently, considerable strain is expected for a pyroxenoid with a disordered Ge-Si distribution.
A 29Si MAS-NMR study of a Cd-pyroxmangite of nominal composition Cd7[Ge6Si]O7 points to considerable Si clustering, probably as GeSiSiSiGe triples corresponding to the wollastonite-like units of the
siebener chain. It is proposed that such ordering relieves strain within the tetrahedral chain. Residual strain is also relieved
by insertion of an extra pair of GeO4 tetrahedra into the pyroxene-like component of the siebener-chain units, leading to single neuner-chain units which terminate
the superperiods. This growth pattern is cyclic. The presence of several types of superperiod may reflect similar energetics
for different Ge-Si-ordering patterns within the siebener chains. Ordering of Si-rich unit cells and of unit cells having
no Si is proposed as the reason for the occurrence of the superperiods in Cd-pyroxmangite.
Received: 25 August 1999 / Accepted: 19 May 2000 相似文献
11.
Order/disorder phase transition in cordierite and its possible relationship to the development of symplectite reaction textures in granulites 总被引：1，自引：0，他引：1
V. L. Vinograd L. L. Perchuk T. V. Gerya A. Putnis B. Winkler J. D. Gale 《Petrology》2007,15(5):427-440
Based on a consistent set of empirical interatomic potentials, static structure energy calculations of various Al/Si configurations in the supercell of Mg-cordierite and Monte Carlo simulations the phase transition between the orthorhombic and hexagonal modifications of cordierite (Crd) is predicted at 1623 K. The temperature dependences of the enthalpy, entropy, and free energy of the Al/Si disorder were calculated using the method of thermodynamic integration. The simulations suggest that the commonly observed crystallization of cordierite in the disordered hexagonal form could be related to a tendency of Al to occupy T1 site, which is driven by local charge balance. The increase in the Al fraction in the T1 site over the ratio of 2/3(T1): 1/3(T2), that characterizes the ordered state, precludes formation of the domains of the orthorhombic phase. This intrinsic tendency to the crystallization of the metastable hexagonal phase could have significantly postponed the formation of the association of orthorhombic cordierite and orthopyroxene over the association of quartz and garnet in metapelites subjected to granulite facies metamorphism. The textures of local metasomatic replacement (the formation of Crd + Opx Or Spr + Crd symplectites between the grains of garnet and quartz) indicate the thermodynamic instability of the association of Qtz + Grt at the moment of the metasomatic reaction. This instability could have been caused by the difficulty of equilibrium nucleation of orthorhombic cordierite. 相似文献
12.
S. B. Liu A. Pines M. Brandriss J. F. Stebbins 《Physics and Chemistry of Minerals》1987,15(2):155-162
The nuclear magnetic relaxation of 23Na and 29Si in albite glass and liquid has been studied from 800 K to 1400 K. The dominant spin-lattice relaxation mechanism for 23Na is found to be nuclear quadrupole interaction arising from the Na+ diffusion. The activation energy of the Na diffusion is found to be 71±3 kJ/mol, in close agreement with the results on electrical conductivity and on Na self-diffusion from radio-tracer experiments. The correlation time of the Na motion is estimated to be about 8.5×10?11 s near the melting point (～1390 K). Both nuclear dipole-dipole interaction and chemical shift anisotropy interaction are large enough to contribute to the 29Si relaxation. However, calculations based on a simplified model which employ single correlation time and exponential correlation function, with interactions typical of crystalline silicates, cannot completely account for the experimental data. NMR relaxation data also reveal that the Si motion is correlated to the Na motion and that the Si is relatively immobile. Several possible motions of SiO4 tetrahedra that can cause 29Si relaxation are suggested. The motion responsible for 29Si relaxation differs from that which is responsible for viscosity: the apparent activation energy for the former is much lower. Measurements of spin-spin relaxation times and linewidths are also presented and the significance of their temperature dependence is discussed. 相似文献
13.
E. E. Senderov 《Physics and Chemistry of Minerals》1980,6(4):251-268
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. 相似文献
14.
Using a combination of dielectric spectroscopy and atomistic computer simulation techniques, the dynamical behaviour of the
loosely bound (Na+ and K+) channel ions in nepheline has been investigated. The low-frequency dielectric properties of a natural Bancroft nepheline
have been studied from room temperature to 1100 K. At each temperature, the dielectric constant, conductivity and dielectric
loss were determined over a range of frequencies from 100 Hz to 10 MHz. At high temperatures a distinct Debye-type relaxation
in the dielectric loss spectrum was observed; the activation energy for this process was determined to be 1.38 ± 0.02 eV.
Atomistic simulation techniques were used to elucidate the mechanism and energetics of cation migration. A mechanism involving
the hopping of Na+ ions between oval sites and partially occupied hexagonal (K+) sites, via a bottleneck consisting of a distorted sixfold ring of (Al,Si)O4 tetrahedra, was found to give a calculated energy barrier in very good agreement with the experimentally determined activation
energy. These results confirm the nature of the process responsible for the observed dielectric behaviour. Overall, this study
demonstrates the intrinsic, microscopic control of cation diffusion processes in rock-forming minerals. Identifying specific
energy barriers and preferred diffusion pathways is fundamental to the prediction of diffusion energetics.
Received: 8 May 2000 / Accepted: 21 July 2000 相似文献
15.
B. Mysen 《Contributions to Mineralogy and Petrology》1997,127(1-2):104-118
The anionic structure of aluminosilicate melts of intermediate degree of polymerization (NBO/T = 0.5) and with along the composition join (LS4-LA4) has been examined in-situ to ˜1480 °C, and compared with recent data for melts along the analog composition join
and with less polymerized melts along the join and O_5. With , the anionic equilibrium, (1) , adequately describes the structure. With , a second expression, (2) , is required because an additional structural unit, Q1, is stabilized in the melts. The enthalpy, , of reaction (1) increases from − 36 ±4 kJ/mol in the absence of aluminum to 34± 5 kJ/mol at and 64 ± 4 kJ/mol at Al/(Al + Si) = 0.45. Similar trends are reported for other alkali aluminosilicate melts. Least-squares
fitting of abundance of structural units as a function of temperature and bulk composition has been conducted. The unit abundance
is dominantly a function of temperature, Al/(Al +Si), and bulk melt polymerization. Configurational entropy and heat capacity
of mixing of melts above their glass transition temperatures have been calculated with the aid of the least-squares fitted
equations. The values of these parameters indicate that as the ionization potential of the metal cations increases, configurational
heat capacity of alkali aluminosilicate melts becomes temperature dependent. As a result, transport properties (viscosity,
diffusivity, and conductivity) of such melts will not show Arrhenian behavior even in the high-temperature range. Further,
discontinuous changes in entropy and heat capacity of mixing results from temperature-induced changes in types of structural
units in the melts. Such discontinuous changes would also be reflected in discontinuous changes of temperature-dependent transport
properties.
Received: 26 September 1996 / Accepted: 18 October 1996 相似文献
16.
Atomistic computer simulation techniques have been employed to model mechanisms of hydrogen incorporation in the clinopyroxenes diopside and jadeite. Calculation of solution reaction energies for the pure phases indicates that hydrogen is most easily incorporated via the formation of [VSi(OH)4]
x
hydrogarnet type defects. When components of the two phases are mixed, then solution energies can become exothermic. The substitution of Al for Si in diopside and of Mg or Ca for Al in jadeite, provides favourable routes for hydrogen incorporation, with exothermic values of solution energy. Thus the amount of water present in these minerals in the Earth’s upper mantle will vary with composition. Simulation of IR frequencies associated with O–H stretching at specific defect clusters has also been carried out. An analysis of hydrogen–oxygen bond lengths gives good agreement, although comparison of experimental and calculated IR frequencies are problematic. This is partly due to the complexity of experimental spectra, but may also be due in part to deficiencies in the ability of the model to accurately describe the O–H stretching frequency. 相似文献
17.
Entropies of Al-Si in layer silicates have been calculated using a series of CVM approximations for the honeycomb lattice.
The parameters of the models have been constrained by 29Si NMR data. The results of low order approximations such as “pair” and “star” have been rejected because of their low accuracy
at high Al/(Al+Si) ratios. Reasonably accurate results have been achieved with the help of the “hexagon” and “star-hexagon”
approximations.
Received: 31 October 1997 / Revised, accepted: 17 April 1998 相似文献
18.
George B. Morgan Antonio Acosta-Vigil David London 《Contributions to Mineralogy and Petrology》2008,155(2):257-269
This study examines the systematics and rate of alkali transport in haplogranite diffusion couples in which a chemical potential
gradient in Al is established between near water-saturated metaluminous and peraluminous liquids that differ only in their
initial content of normative corundum. At 800°C, measurable chemical diffusion of alkalis occurs throughout the entire length
(∼1 cm) of the diffusion couples in 2–6 h, indicating long range diffusive communication through melt. Alkali transport results
in homogenization of initially different Na/Al and ASI [=mol. Al2O3/(CaO + Na2O + K2O)] throughout the couples within ∼24 h, whereas initially homogenous K* evolves to become uniformly different between metaluminous
and peraluminous ends. Calculated effective binary diffusion coefficients for alkalis in experiments that do not significantly
violate the requirement of a semi-infinite chemical reservoir (0- to 2-h duration at 800°C) are similar to those observed
in previous studies: in the range of (1–8) × 10−12 m2/s. Such a magnitude of diffusivity, however, is inadequate to account for the observed changes of alkali concentrations and
molecular ratios throughout the couples in 2- to 6-h experiments. The latter changes are consistent with diffusivities estimated
via the x
2 = Dt approximation, which yields effective values around 10−9 m2/s. These observations suggest that Fick’s law alone does not adequately describe the diffusive transport of alkalis in granitic
liquids. In addition to simple ionic diffusion associated with local gradients in concentration or chemical potential of the
diffusing component described by Fick’s second law (local diffusion), alkali transport through melt involves system-wide diffusion (field diffusion) driven by chemical potential gradients that also include components with which the alkalis couple or complex (e.g., Al).
Field diffusion involves the coordinated migration of essentially all alkali cations, resembling a positive ionic current
that drives the system to a metastable state having a minimum energy configuration with respect to alkali distribution. The
net result is effective transport rates perhaps three orders of magnitude faster than simple local alkali diffusion, and at least seven to eight orders of magnitude faster than the diffusive equilibration of Al and Si. 相似文献
19.
Pascal Richet Richard A. Robie Jacques Rogez Bruce S. Hemingway Philippe Courtial Christophe Téqui 《Physics and Chemistry of Minerals》1990,17(5):385-394
The thermodynamic properties of carnegieite and NaAlSiO4 glass and liquid have been investigated through C p determinations from 10 to 1800 K and solution-calorimetry measurements. The relative entropies S 298-S0 of carnegieite and NaAlSiO4 glass are 118.7 and 124.8 J/mol K, respectively. The low-high carnegieite transition has been observed at 966 K with an enthalpy of transition of 8.1±0.3 kJ/mol, and the enthalpy of fusion of carnegieite at the congruent melting point of 1799 K is 21.7±3 kJ/mol. These results are consistent with the reported temperature of the nepheline-carnegieite transition and available thermodynamic data for nepheline. The entropy of quenched NaAlSiO4 glass at 0 K is 9.7±2 J/mol K and indicates considerable ordering among AlO4 and SiO4 tetrahedra. In the liquid state, progressive, temperature-induced Si, Al disordering could account for the high configurational heat capacity. Finally, the differences between the entropies and heat capacities of nepheline and carnegieite do not seem to conform to current polyhedral modeling of these properties 相似文献
20.
W R Taylor 《Journal of Earth System Science》1990,99(1):99-117
Infrared (IR) and Raman spectroscopic methods are important complementary techniques in structural studies of aluminosilicate glasses. Both techniques are sensitive to small-scale (<15 Å) structural features that amount to units of several SiO4 tetrahedra. Application of IR spectroscopy has, however, been limited by the more complex nature of the IR spectrum compared with the Raman spectrum, particularly at higher frequencies (1200–800 cm?1) where strong antisymmetric Si-O and Si-O-Si absorptions predominate in the former. At lower frequencies, IR spectra contain bands that have substantial contributions from ‘cage-like’ motions of cations in their oxygen co-ordination polyhedra. In aluminosilicates these bands can provide information on the structural environment of Al that is not obtainable directly from Raman studies. A middle frequency envelope centred near 700 cm?1 is indicative of network-substituted AlO4 polyhedra in glasses with Al/(Al+Si)>0·25 and a band at 520–620cm?1 is shown to be associated with AlO6 polyhedra in both crystals and glasses. The IR spectra of melilite and melilite-analogue glasses and crystals show various degrees of band localization that correlate with the extent of Al, Si tetrahedral site ordering. An important conclusion is that differences in Al, Si ordering may lead to very different vibrational spectra in crystals and glasses of otherwise gross chemical similarity. 相似文献