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1.
 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   

2.
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   

3.
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   

4.
29Si magic-angle spinning nuclear magnetic resonance (NMR) spectra are presented for seven crystalline phases of the nepheline group: natural nephelines from a plutonic environment (Bancroft, Ontario) and a volcanic deposit (Mt. Somma, Italy), kalsilite, synthetic pure Na nepheline, carnegieite, and two samples of orthorhombic KAlSiO4. In all phases, nearly all of the Si sites have four Al neighbors, indicating nearly complete Al-Si ordering. Excess Si over the 1:1 stoichiometric Si/Al ratio appears to substitute randomly for Al on an ordered lattice, adding Si sites with 3 and 0 Al neighbors in a 3:1 ratio. Various types of structural disorder, including Al-Si disorder, that are reported from some x-ray diffraction studies are probably long range in nature and are due to the presence of ordered domains. In naturally occurring nepheline, the relative abundance of T sites with three-fold local symmetry is maintained at the ideal stoichiometric value of 1/4, even when the K/(K+Na) ratio is substantially lower. This is in agreement with conclusions reached about the average structure from x-ray data. The distinction between the two sites, at least in terms of the local structure that is reflected in 29Si NMR chemical shifts, is lost in a pure Na nepheline sample.  相似文献   

5.
We present the variation in trace element partition coefficients measured at the interface between rapidly cooled clinopyroxene crystals and co-existing melts. Results indicate that, as the cooling rate is increased, clinopyroxene crystals are progressively depleted in Si, Ca and Mg counterbalanced by enrichments in Al (mainly tetrahedral Aliv), Na and Ti. Partition coefficients (Ds) for rare earth elements (REE), high field strength elements (HFSE) and transition elements (TE) increase with increasing cooling rate, in response to clinopyroxene compositional variations. The entry of REE into the M2 site is facilitated by a coupled substitution where either Na substitutes for Ca on the M2 site or Aliv substitutes for Si in the tetrahedral site. The latter substitution reflects an increased ease of locally balancing the excess charge at M2 as the number of surrounding Aliv atoms increases. Due to the lower concentration of Ca in rapidly cooled clinopyroxenes, divalent large ion lithophile elements (LILE) on M2 decrease at the expense of monovalent cations. Conversely, higher concentrations of HFSE and TE on the M1 site are facilitated as the average charge on this site increases with the replacement of divalent-charged cations by Alvi. Although crystallization kinetics modify clinopyroxene composition, deviations from equilibrium partitioning are insufficient to change the tendency of a trace element to be compatible or incompatible. Consequently, there are regular relationships between ionic radius, valence of the trace element and D. At both equilibrium and cooling rate conditions, Ds for isovalent cations define parabola-like curves when plotted against ionic radius, consistent with the lattice strain model, demonstrating that the partitioning of trace elements is driven by charge balance mechanisms; cation substitution reactions can be treated in terms of the energetics of the various charge-imbalanced configurations.  相似文献   

6.
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   

7.
Silicon-29 “magic angle spinning” nuclear magnetic resonance (NMR) spectroscopy has been used to study the changes in local Si environment during Al, Si ordering in synthetic cordierite, Mg2Al4Si5O18. In the most disordered form, crystallized from a glass, eight distinct tetrahedral sites for silicon can be identified and assigned, while there are only two distinguishable Si sites in the well-annealed ordered form. This allows the changes in the Si site environments to be determined as a function of annealing time for the transformation from the disordered to the ordered form. The first crystallized state has a considerable degree of partitioning between T1 and T2 sites with the following site occupancies: T1 ? Al:Si=0.80:0.20, T2?Al:Si=0.27:0.73 The changes in Si environment are approximately linear with log time. The measured values of 29Si isotropic chemical shift do not fit well to previously determined correlations of shift with various structural parameters.  相似文献   

8.
The dissolution rate of illite, a common clay mineral in Australian soils, was studied in saline-acidic solutions under far from equilibrium conditions. The clay fraction of Na-saturated Silver Hill illite (K1.38Na0.05)(Al2.87Mg0.46Fe3+0.39Fe2+0.28Ti0.07)[Si7.02Al0.98]O20(OH)4 was used for this study. The dissolution rates were measured using flow-through reactors at 25 ± 1 °C, solution pH range of 1.0-4.25 (H2SO4) and at two ionic strengths (0.01 and 0.25 M) maintained using NaCl solution. Illite dissolution rates were calculated from the steady state release rates of Al and Si. The dissolution stoichiometry was determined from Al/Si, K/Si, Mg/Si and Fe/Si ratios. The release rates of cations were highly incongruent during the initial stage of experiments, with a preferential release of Al and K over Si in majority of the experiments. An Al/Si ratio >1 was observed at pH 2 and 3 while a ratio close to the stoichiometric composition was observed at pH 1 and 4 at the higher ionic strength. A relatively higher K+ release rate was observed at I = 0.25 in 2-4 pH range than at I = 0.01, possibly due to ion exchange reaction between Na+ from the solution and K+ from interlayer sites of illite. The steady state release rates of K, Fe and Mg were higher than Si over the entire pH range investigated in the study. From the point of view of the dominant structural cations (Si and Al), stoichiometric dissolution of illite occurred at pH 1-4 in the higher ionic strength experiments and at pH ?3 for the lower ionic strength experiments. The experiment at pH 4.25 and at the lower ionic strength exhibited lower RAl (dissolution rate calculated from steady state Al release) than RSi (dissolution rate calculated from steady state Si release), possibly due to the adsorption of dissolved Al as the output solutions were undersaturated with respect to gibbsite. The dissolution of illite appears to proceed with the removal of interlayer K followed by the dissolution of octahedral cations (Fe, Mg and Al), the dissolution of Si is the limiting step in the illite dissolution process. A dissolution rate law showing the dependence of illite dissolution rate on proton concentration in the acid-sulfate solutions was derived from the steady state dissolution rates and can be used in predicting the impact of illite dissolution in saline acid-sulfate environments. The fractional reaction orders of 0.32 (I = 0.25) and 0.36 (I = 0.01) obtained in the study for illite dissolution are similar to the values reported for smectite. The dissolution rate of illite is mainly controlled by solution pH and no effect of ionic strength was observed on the dissolution rates.  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.
Computer simulation is used to investigate the short range ordering around an isolated oxygen vacancy in sillimanite. The static lattice energy with the use of empirical potentials is calculated, for different Al/Si distributions around a vacancy in a supercell of sillimanite. A parametrisation of the total energy is built up and used to deduce the best Al/Si ordering around the oxygen vacancy. It is found that a strong ordering about the vacancy occurs. In the ab-plane two sets of aluminium cluster are found besides the vacancy, surrounded above and below by silicon atoms, a configuration that promotes local charge balance. By placing two vacancies on sites directly adjacent to the same oxygen, the central oxygen site is bonded to four cations: this situation is found to be energetically unfavourable.  相似文献   

11.
《地学前缘(英文版)》2020,11(4):1353-1367
Chronologically well-constrained loess-palaeosols(recorded glacial and inter-glacial climate) revealed pedogenesis induced ionic substitutions,caused end-member compositional deviations in illite and chlorite,linked to widespread climatic changes occurred during Late Pleistocene.Further,micro-level climatic resolution is yet to be resolved.Thus,layer-wise X-ray diffraction analyses of clay separates,followed by Rietveld refinement revealed varied cell parameters and interatomic distances.Obtained values for detrital and pedogenic illite and chlorite when plotted against stratigraphic succession show notable changes in the crystallographic axes.The illite lattices associated with inadequately pedogenized palaeosols have been altered into illite/smectite mixed layers,but,the chlorite lattices represent expansion of a-,b-and contraction of c-axes with much greater amount of distortions,suggestive of warm-humid and acidic environment.The detrital 48,44 and 83,74 bonded illite and chlorite with2 sub-types each,when pedogenized retained 48,44 and 34;and 83 and 74 bonds(in their neo-formed 3 and 2 sub-types),respectively.The Al-O bond shows expansion,but,unchanged Si-O and decreased Si-K and K-O bonds show loss of Al and retention of Si and K ions in the illite lattices.The illite with 32 atoms and 48 bonds represent contraction of K-O,Si-K,Al-O and Si-O bonds caused bond reinforcement;however,loss of Al~(3+)reflects all-out illite alteration.Owing to Al-O and K-O bond expansion,major K~+ and Al~(3+) ionic loss occurred during the LGM,however,further ionic loss depends upon the magnitude of the loess-palaeosol weathering that they have suffered.The clilate sensitive Fe,Mg and Al ionic losses for Fe-O,Mg-O and Al_(11)-O_9 bond length expansions were recognized in the chlorite lattices.Such ionic losses are common,but,complete distortion is attributed to Al,Si,Fe and Mg ionic losses,followed by weakening of Al-O,Si-O,Fe-O and Mg-O bonds.Though,Si-O_4 and Fe_1-O_4 bonds,and Si and Fe_(1 st) ions remain intact.Thus,three major glacial episodes of ～5 ka each occurred under alkaline environment,but,intervened by two successive cycles of 55 ka each,encompassing three alternate warm and cold climatic sub-cycles of 12-15 ka.But,the coldness increases with each warm-cold sub-cycle that attained the glacial maxima.Further,these events correlate well with the deep-sea records of the North Atlantic(MIS-1 to MIS-5 e) and CLP loess-palaeosols(～127 ka).  相似文献   

12.
Phases with the composition K2MgSi5O12, belonging to the leucite structure group were synthesised under dry and hydrothermal conditions and studied using 29Si NMR. The 29Si spectrum for the dry-crystallised material (which is cubic) consists of a single broad line, suggesting a high degree of disorder. The hydrothermally crystallised material (which is probably monoclinic, with a distorted leucite lattice) has a 29Si spectrum which consists of ten lines of equal intensity, two of which have small chemical shift anisotropies and are therefore assigned to Q4(4Si) sites. These data have been interpreted in terms of a structure with 12 distinct tetrahedral sites over which 2 Mg atoms and 10 Si atoms are fully ordered. A 2-dimensional COSY spectrum shows correlations between some Q4(3Si) silicon atoms and two other Q4(3Si) silicon atoms. This fully constrains the topology of the unit cell. Two schemes of Si/Mg ordering over the unit cell can give good fits to the COSY spectrum. Using the tetrahedral (T) site notation defined for natural tetragonal leucite, the first of these arrangements involves Mg and Q4(4Si) silicon atoms each occupying one T1-type site and one T3-type site, and Q4(3Si) silicons occupying the remaining sites, i.e. four T2-type sites, two T1-type sites and two T3-type sites. In the second arrangement, the T2-type sites are occupied by Mg atoms and Q4(4Si) silicon atoms and all the T1-and T3-type sites are occupied by Q4(3Si) atoms.  相似文献   

13.
The X- and W-band single-crystal electron paramagnetic resonance spectra of an electron-irradiated natural quartz permit quantitative analysis of a 29Si hyperfine structure (A ~12.6 MHz) and an 27Al hyperfine structure (A ≤ 0.8 MHz) for a previously reported hole-like center. The 29Si hyperfine structure arises from interaction with two equivalent Si atoms and is characterized by the direction of the unique A axis close to a Si–O bond direction. The 27Al hyperfine structure, confirmed by pulsed electron nuclear double resonance and electron spin echo envelope modulation spectra, is characterized by the unique A axis approximately along a twofold symmetry axis. These 29Si and 27Al hyperfine data, together with published theoretical results on peroxy radicals in SiO2 as well as our own density functional theory (DFT) calculations on model peroxy centers, suggest this hole-like center to have the unpaired spin on a pair of oxygen atoms linked to two symmetrically equivalent Si atoms and a substitutional Al3+ ion across the c-axis channel, a first peroxy radical in quartz. The nuclear quadrupole matrix P also suggests that the Al3+ ion corresponds closely to the diamagnetic precursor to the [AlO4]0 center. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

14.
The chemical interaction between fluorine and highly polymerized sodium aluminosilicate melts [Al/(Al+Si)= 0.125–0.250 on the join NaAlO2-SiO2] has been studied with Raman spectroscopy. Fluorine is dissolved to form F– ions that are electrically neutralized with Na+ or Al3+. There is no evidence for association of fluorine with either Si4+ or Al3+ in four-fold coordination and no evidence of fluorine in six-fold coordination with Si4+ in these melt compositions. Upon solution of fluorine nonbridging oxygens are formed and are a part of structural units with nonbridging oxygen per tetrahedral cations (NBO/T) about 2 and 1. The proportions of these two depolymerized units in the melts increase systematically with increasing F/(F+O) at constant Al/(Al+Si) and with decreasing Al/(Al+Si) at constant F/(F+O). Depolymerization (increasing NBO/T) of silicate melts results from a fraction of aluminum and alkalies (in the present study; Na+) reacting to form fluoride complexes. In this process an equivalent amount of Na+ (orginally required for Al-3+charge-balance) or Al3+ (originally required Na+ to exist in tetrahedral coordination) become network-modifiers.The structural data have been used to develop a method for calculating the viscosity of fluorine-bearing sodium aluminosilicate melts at 1 atm. Where experimental viscosity data are available, the calculated and measured values are within 5% of each other.A method is also suggested by which the liquidus phase equilibria of fluorine-bearing aluminosilicate melts may be predicted. In accord with published experimental data it is suggested, for example, that — on the basis of the determined solubility mechanism of fluorine in aluminosilicate melts — with increasing fluorine content of feldspar-quartz systems, the liquidus boundaries between aluminosilicate minerals (e.g., feldspars) and quartz shift away from silica.  相似文献   

15.
The ordering of Al and Si in Mg cordierite Mg2Al4Si5O18 is considered using computer simulation. First the enthalpy of interaction J ij between sites is derived by computer modelling 101 different Al/Si configurations and analysing their energies. They are compared with similar results for three other minerals and with ab initio calculations to assess the whole approach. Secondly the ordering process is studied using Monte Carlo simulation applied to the J ij . The ordering phase transition temperature T c is found as 1800°C in reasonable agreement with the experimental estimate of 1450° C. These are much lower than the estimate T c(ABW)≈7600°C obtained from Bragg-Williams theory. Strong short-range order sets in below T c(ABW), and the reasons for much lower temperature T c of long-range ordering are discussed. Strong short-range also sets in very rapidly in a simulated anneal, in agreement with experiment. Thirdly an attempt is made to compare our calculated enthalpies directly with the results of NMR and calorimetry experiments, not completely successfully. A free energy ΔG≈4.6 eV for the activation barrier for ordering is suggested.  相似文献   

16.
Microsommite, ideal formula [Na4K2(SO4)] [Ca2Cl2][Si6Al6O24], is a rare feldspathoid that occurs in volcanic products of Vesuvius. It belongs to the cancrinite–davyne group of minerals, presenting an ABAB… stacking sequence of layers that contain six-membered rings of tetrahedra, with Si and Al cations regularly alternating in the tetrahedral sites. The structure was refined in space group P63 to R=0.053 by means of single-crystal X-ray diffraction data. The cell parameters are a=22.161?Å=√3a dav, c=5.358?Å=c dav; Z=3. The superstructure arises due to the long-range ordering of extra-framework ions within the channels of the structure. This ordering progressively decreases with rising temperature until it is completely lost and microsommite transforms into davyne. The order–disorder transformation has been monitored in several crystals by means of X-ray superstructure reflections and the critical parameters T c?≈?750?°C and β?≈?0.12 were obtained. The kinetics of the ordering process were followed at different temperatures and the activation energy was determined to be about 125?kJ?mol?1. The continuous order–disorder phase transition in microsommite has been discussed on the basis of a two-dimensional Ising model in a triangular lattice with nn (nearest neighbours) and nnn (next-nearest neighbours) interactions. Such a model was simulated using a Monte Carlo technique. The theoretical model well matches the experimental data; two phase transitions were indicated by the simulated runs: at low temperature only one of the three sublattices begins to disorder, whereas the second transition involves all three sublattices.  相似文献   

17.
The adverse impacts of the inorganic labile monomeric Al (Ali) fraction on aquatic organisms have meant that Al (Altot) determination and even speciation has become a routine part of environmental monitoring and assessment. However, if samples are not filtered prior to analysis then particulate Al (Altot(p)) could influence the determination of Altot, and therefore the determination of the more toxicologically important (Ali), both when it is measured analytically or modelled from Altot. This paper shows that the Al/DOC ratio in unfiltered samples can identify the Altot(p) fraction, and thus improve the speciation of Ali. These findings are based on data from a study in a 67 km2 catchment in northern Sweden during the snowmelt-driven spring flood of two consecutive years. Filtered and unfiltered samples were studied to determine the spatial and temporal patterns in Altot(p). The concentrations of Altot(p) were greatest in larger downstream sites where significant silt deposits are located. The sites with no silt in their drainage area showed a mean difference between filtered (Altot(f)) and unfiltered (Altot(uf)) samples of 6%, while sites with silt deposits had a mean difference of 65%. The difference between filtered and unfiltered samples was greatest at peak flow. Spikes in Altot(p) did not behave consistently during fractionation with a cation exchange column, resulting in increases in either measured Ali(f) or non-labile monomeric Al (Alo(f)). Altot(p) spikes were associated with sharp increases in the Al:DOC ratio. The baseflow Al:DOC ratio could be used to model filtered Altot from DOC with a Spearman rho of 0.75.  相似文献   

18.
The compositions of coexisting hornblendes and biotites from amphibolite and granulite facies gneisses from the south coast of Western Australia were controlled by host rock composition, paragenesis, metamophic grade, pressure, and oxygen fugacity. The Mg/(Mg + Fe2+) and Mn/Fe2+ ratios in both minerals and possibly the Alvi contents of the hornblendes are related to host rock compositions. Metamorphic grade appears to influence, perhaps only indirectly, the Ti, Mn, and Fe3+ contents of both minerals and possibly the hornblende Ca content. The higher Ti and lower Mn contents of the granulite facies hornblendes and biotites are attributed to their coexistence with pyroxenes, whereas their lower Fe3+/(Fe2+ + Fe3+) ratios are probably due to lower oxygen fugacity in the granulite facies environment. Grade-related colour variations in both minerals were controlled by their Ti/Fe2+ and Fe3+/(Fe2+ + Fe3+ ratios. The relatively low Alvi contents of the hornblendes suggest low- to moderate-pressure metamorphism.Variations in element distribution coefficients are related to variations in mineral compositions rather than metamorphic grade. Thus KD(Aliv ?Si) is related to the Aliv andedenite alkali contents of the hornblendes, KD(Fe2+ ?Mg) to the distributions of Aliv ?Si and Alvi + Ti + Fe3+, KD(Mn) to the Mn contents of both minerals, and KD(Alvi) to the Alvi contents of the biotites.  相似文献   

19.
The 29-Si NMR spectra of natural and synthetic leucites (KAlSi2O6) are found to contain a number of resonances which are interpreted in terms of the known structure of low-temperature (tetragonal) leucite. Computer simulation of the spectra suggests that the most distorted tetrahedral lattice site T 1 contains a higher proportion of Al than the other two tetrahedral sites. The occurrence of some ordering of the tetrahedral Si and Al in leucite is confirmed by Mossbauer studies of synthetic iron-containing leucites, including the fully ferric end-member KFeSi2O6, in which the three tetrahedral sites can be distinguished. On replacement of about half this Fe by Al, the most distorted of these sites is lost from the spectrum, reflecting the preference of Al for this site. A linear relationship is found between the unit cell dimensions of all these leucites and their iron content.  相似文献   

20.
Single crystals of emerald synthesized by means of the flux method were adopted for crystallographic analyses. Emerald crystals with a wide range of Cr3+-doping content up to 3.16 wt% Cr2O3 were examined by X-ray single crystal diffraction refinement method. The crystal structures of the emerald crystals were refined to R 1 (all data) of 0.019–0.024 and wR 2 (all data) of 0.061–0.073. When Cr3+ substitutes for Al3+, the main adjustment takes place in the Al-octahedron and Be-tetrahedron. The effect of substitution of Cr3+ for Al3+ in the beryl structure results in progressively lengthening of the Al–O distance, while the length of the other bonds remains nearly unchanged. The substitution of Cr3+ for Al3+ may have caused the expansion of a axis, while keeping the c axis unchanged in the emerald lattice. As a consequence, the Al–O–Si and Al–O–Be bonding angles are found to decrease, while the angle of Si–O–Be increases as the Al–O distance increases during the Cr replacement.  相似文献   

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