共查询到20条相似文献，搜索用时 15 毫秒
1.
I. L. Jones Volker Heine Maurice Leslie Geoffrey D. Price 《Physics and Chemistry of Minerals》1990,17(3):238-245
The technique of calculating lattice dissociation energies using static, minimum lattice energy, ionic models has been extended to allow for multiple occupancy of the ionic sites. A particular lattice site can have a fraction x of an ionic species A and a fraction y of an ionic species B, where the position of each can be relaxed separately along with the unit cell dimensions until an equilibrium is reached. Various degrees of long and short range order can be modelled. This technique has been applied to the mineral sillimanite, Al2SiO5, to calculate the effect on the lattice energy of (Al, Si) ordering over the tetrahedral sites. It is found using this method that (Al, Si) ordering with space group Pbmn stabilizes the material by 29.25 kcal/mol (Aliv-O-Aliv), with respect to the completely disordered material. 相似文献
2.
W. E. Cameron 《Physics and Chemistry of Minerals》1977,1(3):265-272
Complete solid solution between sillimanite and mullite is shown to occur when Fe3+, in the virtual absence of Ti, replaces AlVI. All members of the series adopt the sillimanite (Al, Si)IV ordering scheme in preference to the modulated structure typical of synthetic and titaniferous natural mullites. 相似文献
3.
A microscopic model is introduced to discuss the modulated structure of mullite. The oxygen vacancies of this aluminosilicate are known to play a central role. In particular, a single vacancy strongly orders its surrounding Al/Si tetrahedral sites. It is shown in this work that if two oxygen vacancies approach too closely to one another, their Al/Si dressing overlap. This situation results in repulsive interaction. The field of interaction between the vacancies is estimated with the use of an atomistic computer simulation. We use a Bragg-Williams type of theory to dicuss the ordering pattern of the vacancies. Due to frustration between the two dominant repulsive interactions, our model predicts a modulated phase transition in agreement with observation. 相似文献
4.
U. C. Bertram Volker Heine I. L. Jones Geoffrey D. Price 《Physics and Chemistry of Minerals》1990,17(4):326-333
Computer simulation is used to investigate the effect of Al/Si disordering over the tetrahedral sites on the lattice energy and the lattice constants of the mineral sillimanite Al2SiO5. A methodology for an atomistic assessment of the energy of the reaction 2(Si-O-Al)→(Si-O-Si)+(Al-O-Al) and its various contributions is established. This ordering energy is 0.97 eV for nearest neighbour sites in the ab-plane and 0.56 eV for those separated in the c-direction. The large difference is due to a greater constraint on the atomic relaxation in the ab-plane and shows the structural dependence of the ordering energy. Its magnitude appears to be determined by a complicated balance between Coulomb and short-range repulsive energy involving strain over many bonds, both in the ordered and disordered structures. There is also a significant interaction between second neighbour sites whereas the contribution of more distant neighbours is negligible. The lattice energies of most of the 154 configurations studied show a linear behaviour as a function of short-range order, specified by the number of Al-Al pairs. The ordering temperature Tc, estimated on the basis of a statistical mechanical model of disordering, and the calculated ordering energies are in semi-quantitative agreement with experimental values. 相似文献
5.
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
6.
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
7.
E. J. Palin M. T. Dove S. A. T. Redfern A. Bosenick C. I. Sainz-Diaz M. C. Warren 《Physics and Chemistry of Minerals》2001,28(8):534-544
The nature of Al–Si ordering across the tetrahedral sites in muscovite, K2Al4(Si6Al2O20)(OH)4, was investigated using various computational techniques. Values of the atomic exchange interaction parameters J
l
were obtained. From these parameters, a two-dimensional Al–Si ordering scheme was deduced. The transition temperature T
c for this two-dimensional ordering is 1900 K. There are several possible ordering schemes in three dimensions, based on different
stacking sequences of ordered sheets of tetrahedral sites. Monte Carlo simulations of both two-dimensional and three-dimensional
ordering were performed, but in the three-dimensional simulation only the two-dimensional ordering is seen, implying that
three-dimensional ordering is too slow to be attained during the timescale of the simulation. The effect of the three-dimensional
interactions is to raise the two-dimensional ordering temperature to 2140 K. From the three-dimensional Monte Carlo simulation,
the frequency of occurrence of 4Si0Al, 3Si1Al, 2Si2Al and 1Si3Al clusters was determined, which match those inferred by 29Si MAS–NMR measurements reasonably well. In fact, the match suggests that the cation ordering seen in experiments corresponds
to a configuration with considerable short-range order but no long-range order, similar to a state that is at a temperature
just above an ordering phase transition.
Received: 28 August 2000 / Accepted: 12 March 2001 相似文献
8.
The electronic structure of the three polymorphs of Al2SiO5, andalusite, sillimanite, and kyanite, is studied by linearized-augmented-plane-wave (LAPW) calculations using the WIEN code.
Total energy calculations verify, in agreement with recent pseudopotential calculations, that andalusite is the most stable
phase, followed by sillimanite and kyanite.We determine the electronic charge density distribution and find strong polarizations
on all oxygen ions. We identify different polarizations due to Al or Si neighbors which depend on their respective distances
to the oxygen atom. The chemical bonding is not purely ionic in nature but has important covalent contributions. Electric
field gradients (EFGs) at all sites are calculated and agree well (within 10%) with available experimental data on Al. We
identify the origin of the EFGs and demonstrate its relation to the nearest-neighbor coordination and the resulting anisotropy
of the electronic charge distribution.
Received: 22 March 2000 / Accepted: 3 August 2000 相似文献
9.
Al-containing MgSiO3 perovskites of four different compositions were synthesized at 27 GPa and 1,873 K using a Kawai-type high-pressure apparatus:
stoichiometric compositions of Mg0.975Si0.975Al0.05O3 and Mg0.95Si0.95Al0.10O3 considering only coupled substitution Mg2+ + Si4+ = 2Al3+, and nonstoichiometric compositions of Mg0.99Si0.96Al0.05O2.985 and Mg0.97Si0.93Al0.10O2.98 taking account of not only the coupled substitution but also oxygen vacancy substitution 2Si4+ = 2Al3+ + VO¨. Using the X-ray diffraction profiles, Rietveld analyses were performed, and the results were compared between the stoichiometric
and nonstoichiometric perovskites. Lattice parameter–composition relations, in space group Pbnm, were obtained as follows. The a parameters of both of the stoichiometric and nonstoichiometric perovskites are almost constant in the X
Al range of 0–0.05, where X
Al is Al number on the basis of total cation of two (X
Al = 2Al/(Mg + Si + Al)), and decrease with further increasing X
Al. The b and c parameters of the stoichiometric perovskites increase linearly with increasing Al content. The change in the b parameter of the nonstoichiometric perovskites with Al content is the same as that of the stoichiometric perovskites within
the uncertainties. The c parameter of the nonstoichiometric perovskites is slightly smaller than that of the stoichiometric perovskites at X
Al of 0.10, though they are the same as each other at X
Al of 0.05. The Si(Al)–O1 distance, Si(Al)–O1–Si(Al) angle and minimum Mg(Al)–O distance of the nonstoichiometric perovskites
keep almost constant up to X
Al of 0.05, and then the Si(Al)–O1 increases and both of the Si(Al)–O1–Si(Al) angle and minimum Mg(Al)–O decrease with further
Al substitution. These results suggest that the oxygen vacancy substitution may be superior to the coupled substitution up
to X
Al of about 0.05 and that more Al could be substituted only by the coupled substitution at 27 GPa. The Si(Al)–O1 distance and
one of two independent Si(Al)–O2 distances in Si(Al)O6 octahedra in the nonstoichiometric perovskites are always shorter than those in the stoichiometric perovskite at the same
Al content. These results imply that oxygen defects may exist in the nonstoichiometric perovskites and distribute randomly. 相似文献
10.
Potassic cordierites with the chemical composition K
x
Mg2Al4+x
Si5–xO18 (x = 0.00, 0.10, 0.20, and 0.25) were synthesized by annealing glasses at 1290° C for different lengths of time. The procedure resulted in cordierites with different states of Al,Si-order for the tetrahedral sites in the structure. The dependence between the potassium-content and the state of order on one side and between annealing time and the state of order on the other side was then studied using 29Si MAS nuclear magnetic resonance (NMR) spectroscopy. The spectra show that the state of order is a continuous function of annealing time for all compositions considered, but the rate of ordering decreases with increasing K-content. Since the substitution K+Al Si leads to higher Al/Si-ratios; the lower rate of ordering is discussed as a consequence of changed statistics for Al, Si site exchanges. The Al atoms replacing silicon in the structure to balance the charge of potassium cations are not located close to the potassium ion but at a maximum distance from it. This is shown to be a consequence of an improvement in coordination of all oxygen atoms in the cordierite framework. 相似文献
11.
Susanne Flux Niranjan D. Chatterjee Klaus Langer 《Contributions to Mineralogy and Petrology》1984,86(3):294-297
Pure, synthetic 2M1-muscovites (Ms) and 2M1-paragonites (Pg) prepared at 7, 13.5, and 18 kbar
have been subjected to careful X-ray powder diffractometry and IR powder spectrometry. The results of X-ray studies show
that the lattice constants of these micas are independent of the pressure employed in synthesizing them. Similarly, the IR
powder spectra of Ms and Pg remain unaffected by the synthesis pressure. Neither the Al[4]-O-Al[4] vibrations decrease in relative intensity nor do the Si-O-Al[4] vibrations increase in intensity with increasing pressure of synthesis, as they should, if pressure-induced (Al, Si)[4] ordering had been operative. Finally, the v
OH bands in Ms and Pg are also found to retain their fine structural details, regardless of the pressures employed in synthesizing
these phases. In particular, the half widths of v
OH in these (Al,Si)[4]-disordered micas remain constant at around 60 cm−1, compared to ∼ 10 cm−1 in (Al,Si)[4]-ordered mica margarite.
In the light of this study, shortening of the b lattice parameter and progressive (Al,Si)[4] ordering with increasing pressure of synthesis of muscovites reported by Velde (1980) would appear to be anomalous. Consequently,
any discussion of the effect of (Al,Si)[4] ordering on the stability of muscovite is a moot point at present. 相似文献
12.
This paper is an extension of the earlier one dealing with kyanite in which the best fitting value of the oxygen ligand distance for Cr3+ is adopted to study the spectroscopic properties of Cr3+ ions doped at the two possible Al sites in the other two polymorphs of the aluminosilicate group (Al2O3 · SiO2), namely, andalusite and sillimanite. The superposition model and the crystal field analysis package recently developed for 3d ions doped at arbitrary low symmetry sites in crystals are used to predict energy levels and statevectors within the whole 3d
3 configuration. Then the values of the ground state zerofield splitting for Cr3+ ions at each Al sites in the two crystals are obtained. The splittings of the lower excited states 2
E and 4
T
2 as well as the admixture of 4
T
2 into 2
E have also been predicted. Comparison of our results with the available experimental data enable us to correlate the optical and EPR Spectroscopic properties with the substitutional Cr3+ sites. The conclusion is that in andalusite and sillimanite only the Al sites with nearly-octahedral six-fold coordination seem to be occupied by Cr3+ ions. 相似文献
13.
J. D. C. McConnell A. De Vita S. D. Kenny V. Heine 《Physics and Chemistry of Minerals》1997,25(1):15-23
Ab initio total energy calculations based on a new optimised oxygen pseudopotential has been used to determine the enthalpy
of disorder for the exchange of Al and Si in tetrahedral coordination in simple derivative aluminosilicate structures based
on the high temperature tridymite structure. The problem has been studied as a function of defect interaction, and defect
concentration, and the results indicate that the energy for Al/Al neighbouring tetrahedra can be assigned primarily to two
effects, the first, a coulombic effect, associated with the disturbed charge distribution, and the second associated with
the strain related to misfit due to the very different dimensions of the Si and Al containing tetrahedra. In practice each
of these effects contributes approximately 0.2 eV per Al−Al neighbour to the overal disorder enthalpy. These simple results
were obtained after a careful study of possible chemical interaction between adjacent Al/Si containing tetrahedra which showed
that chemical interaction was effectively absent. Since individual Al/Si tetrahedra proved to be discrete entities that are
individually heavily screened by the shared oxygens it follows that coulombic and strain effects in disorder effectively account
for the whole of the disorder enthalpy. The complete set of results have been used to establish new criteria for the structure
and disorder enthalpies of the feldspar group of minerals and their long period derivatives.
Received: October 7, 1996 / Revised, accepted: March 17, 1997 相似文献
14.
The main driving force behind Al/Si ordering in tetrahedral framework aluminosilicates is nearest-neighbour Al/Al avoidance.
Computer simulation is used to explore the direct consequences of such Al/Al avoidance. The main result is that the order-disorder
transition temperature T
c
falls dramatically as the concentration x of Al in the structure is reduced, and if the only interactions are those associated with nearest-neighbour Al/Al avoidance,
T
c
becomes zero for x less than some critical value x
c
, where x
c
=0.31 for the feldspar framework and x
c
=0.34 for cordierite. Also a large degree of short range order is found above T
c
. Both results differ radically from the standard Bragg-Williams model. Plots of entropy and enthalpy of ordering are given
as functions of x and T, which may be used to interpret experimental data or for extrapolation into ranges of x and T inaccessible to experiment.
Received: 14 May 1997 / Revised, accepted: 2 June 1997 相似文献
15.
A method of determining the number of Al-O-Al bonds per unit cell from 29Si nuclear magnetic resonance (NMR) data of synthetic cordierites with increasing Si, Al order is described. The number of Al-O-Al bonds is found to vary linearly with the logarithm of the annealing time. This may be correlated with previously published heat of solution data on similar samples (Carpenter et al. 1983) to determine the enthalpy change Δh, associated with a single Al?Si interchange in cordierite. Δh is found to be 8.1 kcal/mole. The NMR data show that the short range Al, Si order cannot be described in terms of twin domains of ordered orthorhombic cordierite. An ordering model derived from group theoretical constraints on possible Al, Si distributions within the hexagonal symmetry of the cordierite is found to provide a better fit to the NMR data. 相似文献
16.
Hard Mode Infrared Spectroscopy (HMIS) is used to correlate the line shifts Δω, the intensity changes ΔA and the variations of spectral line widths Γ of infrared absorption bands with the degree of Al, Si ordering, Q od, in kinetically disordered Na-feldspar. A simple relationship Δω∝ΔA∝ΔΓ∝Q od 2 was found with the phonon band around 650 cm?1 being particularly sensitive to small changes of the degree of Al, Si ordering. It is shown that the average degree of Al, Si order can be determined from HMIS with an accuracy of ca. 8 percent using 50 mg of Na-feldspar. The experimental results agree well with recent X-ray determinations using identical samples. The significance of HMIS for the study of kinetic processes in minerals is explained. 相似文献
17.
Paul McMillan Andrew Putnis Michael A. Carpenter 《Physics and Chemistry of Minerals》1984,10(6):256-260
A number of previous investigations have examined the ordering behavior of magnesium cordierite using X-ray diffraction, transmission electron microscopy, infrared spectroscopy and solution calorimetry. In the present investigation, one series of samples from the above studies has been examined by Raman spectroscopy. Systematic modifications in the spectra with annealing time at 1,200° C are consistent with a continuous ordering of the average Al/Si distribution from 4 h to at least 64 h, and which may begin earlier. Spectral changes are first definitely observed when the ordered domains are around 100 Å across, suggesting that Raman spectroscopy is sensitive to this distance scale. The spectra of samples annealed at 1,200° C are compared with samples annealed at 1,400°; C where ordering proceeds much faster, and the possible use of Raman spectroscopy in characterization of Al/Si order in cordierite is discussed. Finally, the Raman spectrum of Mg2Al4Si5O18 with a stuffed β-quartz structure has been obtained. Comparison of its spectrum with that of cordierite glass suggests similar structures for both, which seem different to that of disordered cordierite. 相似文献
18.
B. J. Hensen 《Contributions to Mineralogy and Petrology》1986,92(3):362-367
The theoreticalP-T grid for stability relations of the phases cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si), and quartz (Qz) of Hensen (1971), has proved useful in the interpretation of metamorphic mineral assemblages formed at low oxygen fugacity. Both experimental data and evidence from natural rocks indicate that at high oxygen fugacity compatability relations change as a result of the enlargement of the stability field of spinel, which causes a topological inversion and the stabilisation of the invariant points [Sa], [Ga], and [Cd]. This implies the stable existence of the univariant equilibria (for
buffered conditions): Sp+Qz=Ga+Hy+Si+O2 (Sa, Cd), Cd+Sp+Qz=Hy+Si+O2 (Sa, Ga) and Sa+Sp+Qz=Hy+Si+O2 (Ga, Cd) and the divariant reaction: Sp+Qz=Hy+Si+O2 (Sa, Ga, Cd). These redox equilibria are restricted to conditions of high oxygen fugacity. The proposed theoreticalP-T grids, for both low and high oxygen fugacity, satisfactorily explain all experimental data and metamorphic mineral assemblages so far found in granulites. 相似文献
19.
A computational study of oxygen diffusion in olivine 总被引：1，自引：1，他引：0
Atomistic modelling techniques are used to study the rate-determining steps that limit diffusion of oxygen in forsterite. The activation energies for diffusion parallel to all three crystallographic axes by the vacancy and interstitial mechanisms are calculated. The activation energy for extrinsic vacancy diffusion is predicted to be isotropic with a barrier height of 119 kJ mol–1. Conversely, in the interstitial case it is found to be anisotropic, with extrinsic activation energies that range between 94 and 178 kJ mol–1. The effect of intrinsic defects and two typical impurities, iron and hydrogen, upon diffusion is also considered. We find that the migration energy is slightly higher in iron-rich fayalite compared with forsterite and that the presence of hydrogen defects will not affect the diffusion mechanism. These observations lead us to reinterpret existing experimental results on oxygen diffusion in natural olivine. We suggest that at low oxygen partial pressure the mechanism observed is a vacancy mechanism, while at high oxygen partial pressure the mechanism is interstitial. We believe that this change in mechanism is mediated by iron redox reactions. Taking this process into account, we derive activation energies in excellent agreement with those found experimentally in natural samples of olivine. The anisotropy of activation barriers and hence the change in diffusion rates with temperature could be used to distinguish between the two mechanisms in future experimental work. 相似文献
20.
Oxygen deficient perovskites in the system CaSiO3–CaAlO2.5 and implications for the Earth’s interior
U. W. Bläß F. Langenhorst D. J. Frost F. Seifert 《Physics and Chemistry of Minerals》2007,34(6):363-376
Oxygen deficient perovskites of the system CaSiO3–CaAlO2.5 have been synthesised at high-pressure and -temperature conditions relevant to the Earth’s transition zone in order to investigate
their stabilities in the Earth’s mantle and determine structural properties associated with vacancy incorporation. Two polysomes
of thermodynamically stable defect perovskites with Ca(Al0.4Si0.6)O2.8 and Ca(Al0.5Si0.5)O2.75 stoichiometry have been identified. The ordering of oxygen defects into pseudo-cubic (111) layers results in well-ordered
ten- or eightfold superstructures, respectively. At all other compositions examined, a metastable formation of perovskites
has been observed instead, which are assumed to grow initially disordered. These are now characterised by tiny domains, formed
due to subsequent ordering of vacancies along various pseudo-cubic {111} layers. Both ordered defect perovskites show a large
P–T stability field ranging from about 9–18 GPa and 4–12 GPa, respectively. Microstructural TEM analyses revealed the presence
of growth and ferroelastic twins, which indicate a phase transition from rhombohedral to monoclinic symmetry during quenching.
Electron energy loss spectroscopy of Si and Al K edges point at the presence of tetrahedral, octahedral and maybe some pentacoordinated silicon, whereas aluminium is predominantly
octahedrally coordinated with minor fractions in lower coordination. Observed properties are interpreted in terms of a new
structural model, explaining the observed phase transition and formation of different twin laws as well as giving reasons
for the development of such large superstructures. With respect to phase relations of the transition zone, the potential occurrence
of such defect perovskites in the Earth’s interior is discussed. 相似文献