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1.
The vibrational interactions of oxide tetrahedra and the correlation off vibrational bands with structural entities in variously polymerized idealized silicate structural units have been investigated with normal-coordinate calculations. Raman and infrared intensities as well as frequencies were calculated, yielding good qualitative agreement with observed glass and crystal spectra. Structures considered include those in the binary metal-oxide-silica systems, and analogous structures with Al or Ge replacing every other Si. The fitted force constants for stretching of bonds from Si to bridging oxygens (Obr) and non-bridging oxygens (Onb) show close correlation with bond lengths in crystals. In all-silicate units the high-frequency Raman modes due to symmetric T-Onb stretching show essentially no coupling between unlike types of tetrahedra (O-types), classified on the basis of numbers of Obr versus Onb. Calculated spectra of units with more than one O-type (e.g. double chain) have multiple high-frequency Raman modes. Interconnection of different O-types in glasses is very probable on the basis of unsplit low/mid-frequency Si-Obr stretching bands in the measured spectra of all-Si glasses. In Si-Al and Si-Ge structures oscillation of bridging oxygens may play an important role in the high-frequency Raman. There may be more high-frequency modes than there are different types of tetrahedra, and behavior in such modes is complex, not readily described by “one-mode/two-mode” terminology. High frequency antisymmetric (both infrared- and Raman-active) and low- to mid-frequency symmetric (Raman active) T-O stretching modes may sometimes be identified with individual T-O-T groups, although this depends on the degree of polymerization.  相似文献   

2.
Aluminosilicate glasses with compositions along the joins silica-calcium aluminate, silica sodium aluminate and silica-potassium aluminate have been prepared by conventional and solar melting techniques and studied by Raman spectroscopy. The Raman spectra of crystalline calcium aluminate, anorthite and silica polymorphs are discussed in relation to their crystal structures, and compared with the spectra of the corresponding glasses. The glass and crystal spectra are generally comparable, suggesting similar vibrational structures. These crystals have structures based on tetrahedral aluminosilicate frameworks, and a similar molecular structure is suggested for the glasses, although it is noted that the Raman spectra do not directly characterize the aluminate polyhedra. Within the three glass series, our interpretation of the unresolved high-frequency bands shows the appearance of discrete bands near 1120, 1000, 930 and 890 cm?1 as the silica content is decreased. This is compared with the behaviour of high-frequency bands in simple silicate systems, and used to suggest that the four bands in the aluminosilicate systems are due to stretching vibrations of silicate tetrahedra bound to one, two, three and four aluminium atoms. The spectra of calcium, sodium, potassium and lithium aluminosilicate glasses with similar silica contents are compared, and interpreted by the above model. This is used to construct a simple model for the effect of metal cation on aluminosilicate molecular groups in the glass structure, consistent with the results of calorimetric studies on similar systems.  相似文献   

3.
Normal coordinate calculations have been carried out on partially polymerized simple silicate crystals, including Li and Na di- and metasilicates, Li and Gd pyrosilicates, thortveitite and rankinite. In the antisymmetric Si-O stretching modes which are active at 800–1200 cm?1 in infrared spectra, Si-Obr vibrations occur at higher frequencies than Si-Onb vibrations if the bonds have equivalent strengths. However, this relationship is usually reversed when bridging oxygens are overbonded and non-bridging oxygens are underbonded in terms of Pauling bond strengths, a situation which is generally more common in crystals. An observed bimodality of the high-frequency envelope in infrared spectra of glasses in the alkali oxide-silica systems may be somewhat fortuitous, with the high frequency component (ca. 1100 cm?1) representing underbonded non-bridging oxygens and saturated bridging oxygens, and the lower-frequency component (ca. 1000 cm?1) mainly oversaturated bridging oxygens. Significant differences between crystals and glasses in the number and location of the main high-frequency infrared peaks suggest that there are short-range bonding rearrangements in the glasses, and that crystallite models are not applicable. Mid-frequency (600–800 cm?1) infrared modes in silicates more polymerized than the pyrosilicate (Si2O7) appear to be mostly antisymmetric modes in which Si rattles against bridging oxygens, rather than symmetric stretching modes.  相似文献   

4.
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   

5.
Configurational changes with temperature are important for the thermodynamic and transport properties of most aluminosilicate melts, but in general are not well understood. Here, we present high-resolution 27Al and 17O NMR data on several calcium aluminosilicate glasses prepared with varying quench rates and thus with fictive temperatures that span ranges up to about 200 K. In all compositions the content of five-coordinated aluminum increases with fictive temperature, in agreement with recent high temperature NMR data on melts. In a glass of CaAl2Si2O8 (“anorthite”) composition, the content of non-bridging oxygens also increases with temperature; however this effect was not observed in a sample with a much higher CaO/Al2O3 ratio. We present a consistent notation for reactions among structural species in these systems that clarify why in some cases, high-coordinated network cations may appear on the same side of the reaction, while in others they occur on the opposite sides: the key difference is in accounting for all coordination changes for oxygens. Mixing of non-bridging oxygens and of high-coordinated aluminum make significant contributions to the overall configurational entropy and heat capacity of the melts, as does the mixing of various bridging oxygens and of tetrahedral network cations. Other, less well known, types of increase in disorder with temperature may be important as well.  相似文献   

6.
The results of a detailed TEM study of the low frequency modes of distortion of high temperature SiO2-tridymite and their relationship to the extensive structural polymorphism of tridymite are presented. It is found that low energy modes of distortion of the ideal tridymite tetrahedral framework structure give rise to a strong and extremely characteristic diffuse intensity distribution (which can be broken into two component types) for the two highest temperature polymorphs of tridymite. Experimental results strongly suggest that this observed diffuse distribution is not a result of irreversible beam damage but is rather an intrinsic property of the ideal tridymite tetrahedral framework structure. The diffuse intensity distribution is closely related to the lower temperature polymorphs of tridymite — in particular, the primary modulation wave-vectors of these low temperature polymorphs always fall on the higher temperature diffuse distribution. The first type of diffuse distribution appears to result from coupled tetrahedral edge rotation of 〈110〉 columns of corner-connected SiO4 tetrahedra (uncorrelated from column to column as a result of the tetrahedral connectivity of the ideal tridymite framework structure). The real space structural origin of the second curved type of diffuse distribution, however, remains unclear.  相似文献   

7.
The local, up to second nearest neighbor, around Si atoms in alkali and alkaline earth-silicate glasses has been characterized by SiKβ X-ray emission spectroscopy. Principally two types of Si atoms can be distinguished. These are Si atoms with only other Si atoms as second nearest neighbours, and those with one or more alkali or alkaline earth atoms in their second coordination sphere. The spectroscopic results indicate that the lower molecular weight alkali and alkaline earth-silicate glasses tend towards a bimodal distribution of local Si environments, which is designated Q4-Q0 following the assignment by Engelhardtet al. (1975) for silica species in aqueous solutions. From a different perspective the outcome of these experiments suggests that, though the concentration of bridging oxygens (O(br)) and non-bridging oxygens (O(nbr)) is fixed by the stoichiometry of the glass, the distribution of O(nbr) in the glass varies according to the kind of alkali or alkaline earth atom present. From observed nucleation data on R2O-SiO2 (R = Li, Na, K) glasses it is inferred that a bimodal Q distribution and in particular the presence of Q0 species dominates the internal nucleation process in the alkali and alkaline earth-silicate glasses studied. Using this inference rationalizations can readily be found to explain the observed resistance to thermal shock and devitrification rates of these glasses.  相似文献   

8.
Tecto-aluminosilicate and peraluminous glasses have been prepared by conventional and laser heating techniques, respectively, in the CaO-Al2O3-SiO2 system. The structure of these glasses were studied using Raman spectroscopy, X-ray absorption at the Al K-edge and 27Al NMR spectroscopy with two different high fields (400 and 750 MHz). Raman spectroscopy and X-ray absorption are techniques sensitive to the network polymerization and, in particular, show different signal as a function of silica content. However, these two techniques are less sensitive than NMR to describe the local aluminium environment. For tectosilicate glasses, aluminium in five-fold coordination, [5]Al, was found and a careful quantification allows the determination of a significant amount of [5]Al (7% in the anorthite glass). The proportion of [5]Al increases for the peraluminous glasses with small amounts (<2%) of six-fold coordination, [6]Al. The presence of [5]Al agrees with previous observations of the existence of nonbridging oxygens (NBOs) in tectosilicate compositions. However, the proportion of [5]Al in the present study indicates that no major proportion of triclusters (oxygen coordinated to three (Si,Al)O4 tetrahedra) is required to explain these NBOS.  相似文献   

9.
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   

10.
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   

11.
?-Na2Si2O5, ζ-Na2Si2O5, Na2Si[Si2O7], and Na6Si3[Si9O27] have been synthesized using an MA6/8 superpress. Densification in high-pressure sodium silicates is effected largely by changes in packing. In the relaxed (1 bar) structures, cation polyhedra and thermal/displacement parameters are similar to those of low-pressure silicates, but the extra-framework cation positions are oversized. The two mixed [4]Si and [6]Si framework silicates of known structure (Na2Si[Si2O7] and Na6Si3[Si9O27]) belong to the limited homologous series Na2mSim[Sin–mO2n+m], with m<n. The structure-composition relationships of wadeite-type, A2Ge4O9-type, and Na6Si3[Si9O27] silicates and germanates depend on T-O distance and size of the large extra-framework cation. Characteristic features of the SiO4 tetrahedral units are present in micro-Raman spectra of mixed [4]Si and [6]Si framework silicates, but bands uniquely attributable to SiO6 octahedra are weak or obscured. However, [6]Si has a profound indirect influence on the Raman spectra, resulting in intense and complex low-frequency bands, assigned to symmetric bending modes with coupled displacements at both bridging oxygens and nonbridging oxygens bonded to [6]Si, and a shift to higher frequency and reduction in intensity of the high-frequency bands assigned to symmetric [4]Si-Onbr stretching vibrations. Raman spectroscopy does not appear to be a useful structural probe for small amounts of [6]Si in silicate glasses and melts.  相似文献   

12.
The short range distribution of interatomic distances in three feldspar glasses has been determined by X-ray radial distribution analysis. The resulting radial distribution functions (RDF's) are interpreted by comparison with RDF's calculated for various quasi-crystalline models of the glass structure.The experimental RDF's of the alkali feldspar glasses were found to be inconsistent with the four-membered rings of tetrahedra associated with crystalline feldspars; the structures of these glasses are probably based on interconnected six-membered rings of the type found in tridymite, nepheline, or kalsilite. In contrast, the RDF of calcic feldspar glass is consistent with a four-membered ring structure of the type found in crystalline anorthite. T-O bond lengths (T = Si,Al) increase from 1.60 Å in SiO2 glass [J. H. Konnert and J. Karle (1973) Acta Cryst.A29, 702–710] to 1.63 Å in the alkali feldspar glasses to 1.66 Å in the calcic feldspar glass due to the substitution of Al for Si in the tetrahedra] sites. The T-O-T bond angles inferred from the RDF peak positions are 151° in SiO2 glass (see reference above), 146° in the alkali feldspar glasses, and 143° in the calcic feldspar glass. Detail in the RDF at distances greater than 5 Å suggests that the alkali feldspar glasses have a higher degree of long range order than the calcic feldspar glasses.Assuming that the structural details of our feldspar glasses are similar to those of the melts, the observed structural differences between the alkali feldspar and calcic feldspar glasses helps explain the differences in crystallization kinetics of anhydrous feldspar composition melts. Structural interpretations of some thermodynamic and rheologic phenomena associated with feldspar melts are also presented based on these results.  相似文献   

13.
The aim of this study was to use Monte Carlo simulations to provide atomic-level insights into the dissolution behavior of borosilicate and aluminoborosilicate glasses in dilute aqueous solutions. In the first part of this work, the effects of different structural features, such as the presence of non-bridging oxygens (NBO) or the formation of boroxol rings, on glass dissolution were evaluated separately and led to the following conclusions. (1) The dependence of the dissolution rate on the amount of NBO was found to be linear at all Si/B ratios and the accelerating effect of NBO was shown to increase with increasing Si/B ratio. (2) The formation of boroxol rings and of clusters of boroxol rings resulted in an increase of the dissolution rate at all Si/B ratios and, again, the extent of the rate increase was strongly dependent on the Si/B ratio. (3) For aluminosilicate glasses, the implementation of the aluminum avoidance rule was found to increase the rate of dissolution relative to that obtained for a random distribution.In the second part of this work, the dissolution of the NeB glasses studied by Pierce et al. (2010) was modeled in dilute aqueous solutions. Pierce et al. concluded from their study that either the rupture of the Al-O bonds or that of the Si-O bonds was the rate-limiting step controlling the dissolution of the NeB glasses. The simulations refined this conclusion and showed that, at low B/Al ratios, the rupture of both Al-O-Si and Si-O-Si linkages contributed to the dissolution rate whereas, at high B/Al ratios, the dissolution rate was independent of the rupture of Al-O-Si linkages and was controlled by S1 sites (silicon sites at the glass-water interface with one connection to nearest-neighbor sites) and dissolution via detachment of clusters.  相似文献   

14.
Sedimentary biogenic silica from Redeyef in Gafsa basin (southern Tunisia) was analysed for its 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra and complemented by X-ray diffraction and SEM observations. The 29Si MAS NMR spectrum is characterized by the abundance of hydroxylated silicon, displayed in resonance intensities and reflects a clear tendency towards dissolution of diatomaceous amorphous silica and the occurrence of the hydrated silica, which is the main component that ensures the diagenetic transition via the mechanism of dissolution–precipitation to other more crystalline silica phases, after the lost of its hydroxyls groups (water) by heating (burial). 27Al MAS NMR reveals two coordinations of Al; the octahedrally coordinated Al suggests the presence of clay relics trapped during crystal growth or a microcrystalline zeolite (clinoptilolite detected by SEM observations), while the tetrahedrally coordinated Al suggests the presence of minor quantities of minerals with tetrahedral Al, such as an Al-rich fluid and/or minerals such as feldspars.  相似文献   

15.
Estimation of the framework connectivity and the atomic structure of depolymerized silicate melts and glasses (NBO/T > 0) remains a difficult question in high-temperature geochemistry relevant to magmatic processes and glass science. Here, we explore the extent of disorder and the nature of polymerization in binary Ca-silicate and ternary Ca-aluminosilicate glasses with varying NBO/T (from 0 to 2.67) using O-17 NMR at two different magnetic fields of 9.4 and 14.1 T in conjunction with quantum chemical calculations. Non-random distributions among framework cations (Si and Al) are demonstrated in the variation of relative populations of oxygen sites with NBO/T. The proportion of non-bridging oxygen (NBO, Ca-O-Si) in the binary and ternary aluminosilicate glasses increases with NBO/T. While the trend is consistent with predictions from composition, the detailed fractions apparently deviate from the predicted values, suggesting further complications in the nature of polymerization. The proportion of each bridging oxygen in the glasses also varies with NBO/T. The fractions of Al-O-Si and Al-O-Al increase with increasing polymerization as CaO is replaced with Al2O3, while that of Si-O-Si seems to decrease, implying that activity of silica may decrease from calcium silicate to polymerized aluminosilicates (XSiO2=constant). Quantum chemical molecular orbital calculations based on density functional theory show that a silicate chain with Al-NBO (Ca-O-Al) has an energy penalty (calculated cluster energy difference) of about 108 kJ/mol compared with the cluster with Ca-O-Si, consistent with preferential depolymerization of Si-networks, reported in an earlier O-17 NMR study [Allwardt, J., Lee, S.K., Stebbins, J.F., 2003. Bonding preferences of non-bridging oxygens in calcium aluminosilicate glass: Evidence from O-17 MAS and 3QMAS NMR on calcium aluminate glass. Am. Mineral.88, 949-954]. These prominent types of non-randomness in the distributions suggest significant chemical order in silicate glasses that leads to a decrease in silica activity coefficient and will be useful in modeling transport properties of melts.  相似文献   

16.
29Si MAS-NMR spectra were obtained for vermiculite with tetrahedral composition x=Al/(Al+Si)=0.28, a synthetic mica with x=0.43, and margarite with x=0.5. Comparison between the observed and Monte-Carlo simulated spectra was used to test different Al-Si distribution schemes in the tetrahedral sheet. The results of this analysis are interpreted including earlier data corresponding to lower Al for Si substitutions (0.12≤x≤0.28), and it is shown that for all samples (8 compositions in the range 0.1≤x≤0.5) the Al-Si distribution complies with the principle of homogeneous dispersion of charges (HDC). Thus, Al-Si distribution models for any particular x can be predicted on the basis of a simple crystallochemical criterion. HDC is a short range order concept and implies that, within the restrictions imposed by the principle, charges be distributed at random. For increasing values of x, however, the observance of HDC implies a coupling between adjacent hexamer rings which restricts the degrees of freedom for random distribution and is equivalent to a progressive extension of order. This is reflected by the frequencies of meta and para substitutions in the hexamer rings, which vary in a characteristical, continuous way with x, until the fully ordered distribution scheme of margarite (x=0.5) is reached. Partial long-range order for x<0.5, with alternating Si and (Al+Si) tetrahedra and all substitutions in meta disposition, could not be confirmed.  相似文献   

17.
Mizuhiko Akizuki 《Lithos》1983,16(4):249-254
Spherulites consisting of fibrous alkali feldspar and silica minerals are produced by devitrification of rhyolite glass under hydrothermal conditions. The alkali feldspars (Ab72.5Or23.0An4.5, Ab81.7Or14.0An4.3) in spherulites from two localities in Japan consist of triclinic anorthoclase showing fine cross-hatched twinning and monoclinic sanidine showing fine cross-hatching not attributable to twinning. The cross-hatching, which corresponds to albite and pericline twinning, is produced in the process of transition from a monoclinic to a triclinic phase. The spherulite may develop at a temperature lower than about 200°C because the co-existing silica mineral is not quartz, but metastable tridymite. According to the phase diagram of the alkali feldspars by MacKenzie (1952), the alkali feldspars should have been triclinic during growth. However, the textures show that the alkali feldspar grew as a disordered monoclinic phase. Because of the high growth rate, the Al/Si disordered structure was produced during growth and afterwards transformed into a triclinic structure with cross-hatched twinning.  相似文献   

18.
Al X-ray absorption K-edge spectra were calculated for atom clusters which simulate Al surrounded by oxygens in tetrahedral and octahedral environments in five minerals, and are compared with experimental Al K-edge spectra from the same minerals. The experimental data show simple edges, for Al tetrahedra, that are shifted to lower energy by 2 eV compared with more complicated edges for Al octahedra. The calculated edge itself is due to a strong 1s to 3p resonance in the μ0 for Al. The shifting of the calculated edges is due to different molecular potentials used and different types of atomic scattering. The calculated edge features for tetrahedral Al are all similar, and are due primarily to single scattering from the tetrahedral oxygens. Calculated edge features for octahedral Al are more complicated and are dominated by multiple scattering. The calculated edge features resemble the experimental data, but are much larger in amplitude than their experimental counterparts.  相似文献   

19.
Short and medium range order of silica and sodium silicate glasses have been investigated from a quantitative analysis of 29Si MAS NMR and 23Na, 17O MQMAS NMR spectra. The method described enables the extraction of the underlying 17O NMR parameter distributions of bridging oxygens (BOs) and non-bridging oxygens (NBOs), and yields site populations which are confirmed by 29Si NMR data. The extracted NMR parameter distributions and their variations with respect to the glass chemical composition can then be analyzed in terms of local structural features (bond angles and bond lengths, coordination numbers) with the help of molecular dynamics simulations combined with first-principles calculations of NMR parameters. Correlations of relevant structural parameters with 23Na, 29Si and 17O NMR interactions (isotropic chemical shift δiso, quadrupolar coupling constant CQ and quadrupolar asymmetry parameter ηQ) are re-examined and their applicability is discussed. These data offer better insights into the structural organization of the glass network, including both chemical and topological disorder. Adding sodium to pure silica significantly diminishes the Si-O-Si bond angles and leads to a longer mean Si-O bond length with a slight decrease of the mean Na-O bond length. Moreover, the present data are in favor of a homogeneous distribution of Na around both oxygen species in the silicate network. Finally, our approach was found to be sensitive enough to investigate the effect of addition of a small quantity of molybdenum oxide (about 1 mol%) on the 17O MAS spectrum, opening new possibilities for investigating the Mo environment in silicate glasses.  相似文献   

20.
The structural environments of Mn2+ and Sr2+ at concentrations of 0.2–0.8 wt% in dry and hydrous silicate glasses have been studied using X-ray absorption spectroscopy (EXAFS and XANES). The environment of Mn in hydrous silica glasses containing 4.5 and 6.0 wt% H2O appears to be close to an undistorted octahedral site, whereas Mn in other compositions, both dry and hydrous, occupies either a distribution of octahedral and tetrahedral sites, or distorted, non-centrosymmetric sites. The fraction of non-centrosymmetric sites (including tetrahedral) or the average degree of distortion of the sites decreases with both increasing dissolved water concentration and number of non-bridging oxygens in the glass. For Sr the changes as a function of polymerisation and water concentration are less clear, but it appears that the average Sr-O distances are shorter than in crystalline silicates and decrease further with decreasing polymerisation. The structural data obtained in this study are not readily integrated with previous partitioning data for Mn, suggesting that changes in the first-shell coordination geometry of Mn do not play a direct role in controlling the melt compositional dependence of trace element partition coefficients. It is tentatively suggested that distortions of the framework resulting from the incorporation of Mn (i.e. a second shell effect) control the partitioning behaviour.  相似文献   

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