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1.
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   

2.
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   

3.
The Raman spectra of albite glasses with 4.5 and 6.6 weight percent water have been obtained, and are compared with that of a dry sample. The hydrous glasses show bands near 3600 cm?1 due to O-H stretching, and a previously unreported weak band near 1600 cm?1 due to bending of molecular H2O. Other weak spectral features are discussed, and the effect of dissolved water on the aluminosilicate framework vibrations is considered.  相似文献   

4.
Inelastic neutron scattering (INS) was used to study the vibrational dynamics of the hydrogen atoms in natural trioctahedral phlogopite, K0.93Na0.03(Mg2.47Fe0.22Al0.16Fe0.04Tl0.06)[Si2.84Al1.16]O10OH1.71F0.28Cl0.01, within the 50–1,000?cm?1 energy range. The INS spectra collected using direct geometry spectrometer SEQUOIA (ORNL) were interpreted by means of the solid-state DFT calculations covering both normal mode analysis and molecular dynamics. To optimize the structure and to calculate the vibrational modes under harmonic approximation, both a hybrid PBE0 and the AM05 functional were used, while the molecular dynamics calculations (60?ps/1?fs) were performed only with the computationally less-demanding AM05 functional. The main contributions to the dominant band within ~750–550?cm?1 are symmetric and antisymmetric Mg–O–H bending modes, overlapping with the skeletal stretching and bending modes causing weaker secondary movements of H atoms of inner hydroxyl groups. Signatures of the Mg–O–H bending modes appear down to ~400?cm?1, where a region of octahedra deformation modes starts. These deformations cause just shallow movements of the hydrogen atoms and are mirrored by the modes with close vibrational energies. The region from ~330?cm?1 down to the low-energy end of the spectrum portrays induced vibrations of the H atoms caused by deformation of individual polyhedra, translational vibrations of the parts of the 2:1 layer relative one to another, and librational and translational vibrations of the layer. The main difference between the INS spectrum of dioctahedral Al-muscovite and trioctahedral Mg-phlogopite is that the Mg–O–H modes are all assigned to in-plane vibrations of the respective hydrogen atoms.  相似文献   

5.
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   

6.
Optical absorption spectra obtained on glasses quenched from sodium silicate melts show Mn3+ to be the dominant species for melts heated in air and Mn2+ to be the dominant species for melts heated at Po2 = 10?17 bar. The absorption spectrum of Mn3+ consists of an intense band at 20,000cm?1 with a 15,000cm?1 satellite possibly arising from the Jahn-Teller effect. The independence of the spectrum from melt composition and the high band intensity is offered as evidence for a distinct Mn3+ complex in the melt. The spectrum of Mn2+ is weak and many expected bands are not observed. A two-band luminescence spectrum from Mn2+ has been tentatively interpreted as due to Mn2+ in interstitial sites in the network and Mn2+ coordinated by non-bridging oxygens.  相似文献   

7.
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   

8.
The influence of melt composition and structure on the oxygen isotope fractionation was studied for the multicomponent (SiO2 ± TiO2 + Al2O3 ± Fe2O3 + MgO ± CaO) system at 1500°C and 1 atm. The experiments show that significant oxygen isotope effects can be observed in silicate melts even at such high temperature. It is shown that the ability of silicate melt to concentrate 18O isotope is mainly determined by its structure. In particular, an increase of the NBO/T ratio in the experimental glasses from 0.11 to 1.34 is accompanied by a systematic change of oxygen isotope difference between melt and internal standard by values from–0.85 to +1.29‰. The obtained data are described by the model based on mass-balance equations and the inferred existence of O0, O–, and O2– (bridging, non-bridging, and free oxygen) ions in the melts. An application of the model requires the intra-structure isotope fractionation between bridging and non-bridging oxygens. Calculations show that the intra-structure isotope fractionation in our experiments is equal to 4.2 ± 1.0‰. To describe the obtained oxygen isotope effects at the melts relatively to temperature and fraction of non-bridging oxygen a general equation was proposed.  相似文献   

9.
 Cordierite precursors were prepared by a sol-gel process using tetraethoxysilane, aluminum sec.-butoxide, and Mg metal flakes as starting materials. The precursors were treated by 15-h heating steps in intervals of 100 °C from 200 to 900 °C; they show a continuous decrease in the analytical water content with increasing preheating temperatures. The presence of H2O and (Si,Al)–OH combination modes in the FTIR powder spectra prove the presence of both H2O molecules and OH groups as structural components, with invariable OH concentrations up to preheating temperatures of 500 °C. The deconvolution of the absorptions in the (H2O,OH)-stretching vibrational region into four bands centred at 3584, 3415, 3216 and 3047 cm−1 reveals non-bridging and bridging H2O molecules and OH groups. The precursor powders remain X-ray amorphous up to preheating temperatures of 800 °C. Above this temperature the precursors crystallize to μ-cordierite; at 1000 °C the structure transforms to α-cordierite. Close similarities exist in the pattern of the 1400–400 cm−1 lattice vibrational region for precursors preheated up to 600 °C. Striking differences are evident at preheating temperatures of 800 °C, where the spectrum of the precursor powder corresponds to that of conventional cordierite glass. Bands centred in the “as-prepared” precursor at 1137 and 1020 cm−1 are assigned to Si–O-stretching vibrations. A weak absorption at 872 cm−1 is assigned to stretching modes of AlO4 tetrahedral units and the same assignment holds for a band at 783 cm−1 which appears in precursors preheated at 600 °C. With increasing temperatures, these bands show a significant shift to higher wavenumbers and the Al–O stretching modes display a strong increase in their intensities. (Si,Al)–O–(Si,Al)-bending modes occur at 710 cm−1 and the band at 572 cm−1 is assigned to stretching vibrations of AlO6 octahedral units. A strong band around 440 cm−1 is essentially attributed to Mg–O-stretching vibrations. The strongly increasing intensity of the 872 and 783 cm−1 bands demonstrates a clear preference of Al for a fourfold-coordinated structural position in the precursors preheated at high temperatures. The observed band shift is a strong indication for increasing tetrahedral network condensation along with changes in the Si–O and Al–O distances to tetrahedra dimensions similar to those occurring in crystalline cordierite. These structural changes are correlated to the dehydration process starting essentially above 500 °C, clearly demonstrating the inhibiting role of H2O molecules and especially of OH groups. Received: 1 March 2002 / Accepted: 26 June 2002  相似文献   

10.
A detailed evaluation of the assignments given to the infrared (IR) vibrations in the lattice stretching region is presented here based on observations of the effects of various chemical substitutions in synthetic analogues of phlogopite, KMg3(AlSi3)O10(OH)2. As in previous studies, this study has confirmed that the 995, 960, and 460 cm?1 vibrations are influenced by Si, the 822 and 760 cm?1 vibrations by Al, the 915 and 725 cm?1 vibrations by Al and Si, and the 592 cm?1 vibration by OH. Contrary to previous studies, it is shown here that the 690, 495, and 375 cm?1 vibrations are strongly linked with Mg and not just Si. The 655 cm?1 band in phlogopite is attributed to an in-plane Al-O vibration rather than an Al-O-Si vibration. As a check on the band assignments made here, IR spectra were obtained for synthetic clintonite, CaMg2Al(Al3Si)O10(OH)2, as well as its chemical analogues and compared with the IR spectrum of phlogopite. The band intensities for the Si-O, Al-O, and Si-O-Mg vibrations changed in accord with the composition of clintonite. The most intense band in clintonite at 660 cm?1 appears to be associated only with Al and is assigned here to a tetrahedral Al-O-Al vibration which must be present, if not dominant, in this mineral. The near coincidence of an in-plane Al-O vibration at 655 cm?1 (phlogopite) and an in-plane Al-O-Al vibration at 660 cm?1 (clintonite) makes the identification of tetrahedral Al-Si order-disorder in trioctahedral layered silicates by IR spectroscopy very difficult. The ratio of the 822/995 cm?1 bands may, however, prove to be very useful for discerning the amount of tetrahedrally coordinated Al in these types of minerals.  相似文献   

11.
A single-crystal of topaz was studied by Raman spectroscopy to assign the internal modes of the high-frequency range and to compare with infrared data. All active modes exhibit an important Davydov splitting (150 cm?1) but we have found a small Bethe splitting (14.5 cm?1) consistent with a very regular SiO4 tetrahedron. Because of a high value of v 1 (～920 cm?1) the Raman active modes present a mixed v 1/v 3 character. Finally the substitution of OH for F splits an A g internal mode and lead to some proper modes at 3650 cm?1, 3639 cm?1 and 1165 cm?1.  相似文献   

12.
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   

13.
The infrared (IR) spectra of gem-quality olivine crystals from Pakistan, formed in serpentinised dunitic rocks, are characterised by strongly pleochroic absorption bands at 3,613, 3,597, 3,580 and 3,566 cm?1. These bands are assigned to O-H stretching vibrations of OH point defects corresponding to H2O concentrations of about 35 wt ppm. Unlike other olivine spectra, the dominating bands are strongly polarised parallel to the b-axis. The unusual spectra type, excludes the presence of planar defects. This finding is supported by transmission electron microscopy. The 3,613 cm?1 band is related to vacant Si sites, the slightly lower energetic bands preferentially to vacant M2 sites. The exclusive presence of these bands is not only a characteristic feature of olivines treated under high P,T conditions equivalent to mantle environment, the presence of these bands in untreated natural olivine also indicates formation conditions equivalent to crustal rocks.  相似文献   

14.
安徽马鞍山磷铝石宝石矿物学特征研究   总被引：3，自引：1，他引：2  
周彦  亓利剑  戴慧  张青  蒋小平 《岩矿测试》2014,33(5):690-697
近年来在安徽马鞍山地区所在的绿松石矿体附近,相继发现一种绿色、半透明的磷铝石,部分达到宝石级别。本文采用电子探针、X射线粉晶衍射仪、扫描电镜、傅里叶变换红外光谱仪、紫外可见光谱仪等测试技术,对该地区磷铝石的化学成分、矿物成分、微观结构和光谱特征进行对比验证和综合分析,研究其水的赋存形式,进而对磷铝石的呈色机制作了深入探讨。电子探针分析显示该地区磷铝石的化学成分主要以Al、P元素组成,含微量的Fe、V元素。X射线粉晶衍射与红外吸收光谱分析表明主要矿物为磷铝石,基本不含有其他杂质矿物;磷铝石是一种水合磷酸盐矿物,含有结晶水以及少量结构水的矿物,且结晶水与结构水多与Al3+(Fe3+)相结合的形式存在。偏光显微镜和扫描电镜观察显示磷铝石整体以鳞片状集合体产出,微观上多以短柱状及板片状堆积,单个晶体显示斜方晶系结晶生长习性。紫外可见吸收光谱中639 nm处吸收谱峰由Fe3+与V3+联合所致,300、423、864 nm处吸收峰由Fe3+所致,说明Fe3+与V3+的共同作用是马鞍山地区磷铝石呈现绿色的主要原因。本研究对于认识该类磷铝石的宝石矿物学性质以及颜色成因具有一定意义。  相似文献   

15.
Powder IR spectra of synthetic richterite-tremolite and Sr-tremolite-tremolite solid solutions were obtained in the spectral range between 1400 and 600?cm?1. Under the consideration of the crystal structure and the Wykoff positions of the atoms in the primitive unit cell, the number, type and symmetry of vibrational modes were deduced. The space group of tremolite C2h was used as the factor group leading to 16 theoretical stretching vibrations in the IR range caused by the Si4O11 ∞-ribbon. The energy of the internal vibrations of the Si4O11 ∞-ribbon is a function of the relative bond strengths and masses of nearby ions. For the amphiboles a one-mode behavior was observed for all the Si-O, Si-O-Si and O-Si-O stretching vibrations, indicating no clustering in the two solid solution series. In both solid solution series the vibrational energy of the stretching vibrations is a linear function of composition. In the system richterite-tremolite a shift of the stretching frequencies of the Si4O11 ∞-ribbon over the whole compositional range of up to 30?cm?1 was observed. In contrast, for Sr-tremolite-tremolite the maximum shift was only 5?cm?1. These quite small band shifts allow the (Si4O11)∞-ribbon to be treated as an isolated entity for factor group analysis. Nevertheless, by the two exchange mechanisms, Ca(M4)???Sr(M4) and □(A) Ca(M4)???Na(A)Na(M4), the FWHHs increased and the amplitudes decreased, indicating a slight distortion of the ribbon. For Sr-tremolite-tremolite only a linear expansion of the lattice was observed. In the series richterite-tremolite individual bond angles of the SiO4 tetrahedra are additionally changed, causing the higher energy shift of the bands. The strongest and sharpest bands were observed for the end member tremolite. The one-mode behavior of the Si4O11-double chain indicates that there is no short-range order of Na/Ca and Ca/Sr at the M4 sites of these amphiboles.  相似文献   

16.
The infrared spectroscopic properties of selected OH defects in zircon are investigated by first-principles calculations. The explicit treatment of the coupled nature of OH motions in the stretching modes, together with the calculation of the intensity and polarization of absorption bands, makes it possible to directly compare theoretical and experimental data. The bands observed at 3,420 cm?1 (polarization parallel to c axis) and 3,385 cm?1 (polarization perpendicular to c axis) in natural and synthetic samples correspond to the IR-active vibrational modes of the hydrozircon defect, that is, fully protonated Si vacancy. The broad band observed at 3,515 cm?1 in the spectrum of zircon crystals grown in F-rich environments is consistent with the occurrence of composite (OH,F) tetrahedral defects. Calculations also show that the band observed at 3,200 cm?1 in the spectrum of synthetic undoped samples can be ascribed to fully protonated Zr vacancies. The theoretical values of integrated absorption coefficients indicate that general correlations can be reasonably used to determine the concentration of OH groups in zircon.  相似文献   

17.
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   

18.
The crystal structure of Bi2Al4−x Fe x O9 compounds (x = 0–4) has striking similarities with the crystal structure of mullite. A complete substitution of Al by Fe3+ in both octahedral and tetrahedral sites is a particular structural feature. The infrared (IR) spectra of the Bi2M4O9 compounds (M = Al, Fe3+) are characterised by three band groups with band maxima in the 900–800, 800–600 and 600–400 cm−1 region. Based on the spectroscopic results obtained from mullite-type phases, the present study focuses on the composition-dependent analysis of the 900–800 cm−1 band group, which is assigned to Al(Fe3+)–O stretching vibrations of the corner-sharing MO4 tetrahedra. The Bi2Al4O9 and Bi2Fe4O9 endmembers display single bands with maxima centred at 922 and 812 cm−1, respectively. Intermediate Bi2Al4−x Fe x O9 compounds exhibit a distinct splitting into three relatively sharp bands, which is interpreted in terms of ordering effects within the tetrahedral pairs. Thereby the high-energy component band of the band triplet relates to Al–O–Al conjunctions and the low-energy component band to Fe–O–Fe conjunctions. The intermediate band is assigned to stretching vibrations of Al–O–Fe or Fe–O–Al configurations of the corner-sharing tetrahedral pairs. Bands in the 800–600 cm−1 range are assigned to low-energy stretching vibrations of the MO4 tetrahedra and to M–O–M bending vibrations of the tetrahedral pairs. Absorptions in the 600–400 cm−1 range are essentially determined by M–O stretching modes of the M cations in octahedral coordination.  相似文献   

19.
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   

20.
Far-infrared, mid-IR, and Raman powder spectra were measured on six phases (bromellite, chrysoberyl, phenakite, bertrandite, beryl, and euclase) in the system BeO-Al2O3-SiO2-H2O. A single-crystal absorption spectrum of IR fundamentals in beryl is also presented, which more closely resembles the powder absorption spectrum than it does absorption spectra calculated from single-crystal reflection data. Assignments of the SiO4 and BeO4 internal vibrations are made in accordance with each mineral's symmetry and composition and by comparison to structural analogs. Heat capacities C v calculated for these partial band assignments agree with C v derived from experimental C p for all six phases, provided that Kieffer's (1979c) model is slightly modified to correctly enumerate both Si-O and Be-O stretching modes in the high frequency region (>750 cm?1). Si-O stretching bands were found to out-number Be-O stretching modes in the high-energy region of the vibrational spectra with two exceptions: (1) For those phases containing oxygen ions not coordinated to silicon, vibrations occurring at v>1,080 cm?1 that are attributable to Be-O (H) stretching must be treated separately in the model in order to calculate C v accurately. (2) Minerals consisting entirely of interlocking Si and Be tetrahedra (i.e., phases without Al or OH) can be modeled by one optic continuum representing all optical modes. These results, along with the occurrence of very low energy lattice vibrations for Be-silicates within Al, suggests that although Be-O bonds are generally weaker than neighboring Si-O bonds, Be mimics the network-forming characteristic of Si to a limited extent.  相似文献   

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