Premelting and calcium mobility in gehlenite (Ca2Al2SiO7) and pseudowollastonite (CaSiO3)     

M. A. Bouhifd  G. Gruener  B. O. Mysen  P. Richet 《Physics and Chemistry of Minerals》2002,29(10):655-662

 Premelting effects in gehlenite (Ca₂Al₂SiO₇) have been studied by Raman spectroscopy and calorimetry, and in gehlenite and pseudowollastonite (CaSiO₃) by electrical conductivity. The enthalpy of premelting of gehlenite is 17.3 kJ mol⁻¹ and represents 9% of the reported enthalpy of fusion, which is in the range of the reported fraction of other minerals. The Raman and electrical conductivity experiments at high temperatures, for gehlenite and pseudowollastonite, show that the premelting effects of both compositions are associated with enhanced dynamics of calcium atoms near the melting point. This conclusion agrees with the results obtained for other minerals like diopside, but contrasts with those found for sodium metasilicate in which the weaker bonding of sodium allows the silicate framework to distort near the melting temperature and deform in such a way to prefigure the silicate entities present in the melt. Received: 30 April 2002 / Accepted: 7 August 2002 Acknowledgements We thank Y. Linard for help with DSC measurements and two anonymous reviewers for their constructive comments. This work has been partly supported by the EU Marie-Curie fellowship contract no. HPMF-CT-1999-00329, the CNRS-Carnegie Institution of Washington program PICS no.192, and the NSF grants EAR-9614432 and EAR-9901886 to B.O.M.  相似文献   

A hitherto unrecognised band in the Raman spectra of silica rocks: influence of hydroxylated Si–O bonds (silanole) on the Raman moganite band in chalcedony and flint (SiO<Subscript>2</Subscript>)     

Patrick?SchmidtEmail author  Ludovic?Bellot-Gurlet  Aneta?Slodczyk  Fran?ois?Fr?hlich 《Physics and Chemistry of Minerals》2012,39(6):455-464

Chalcedony is a spatial arrangement of hydroxylated nanometre-sized α-quartz (SiO₂) crystallites that are often found in association with the silica mineral moganite (SiO₂). A supplementary Raman band at 501 cm⁻¹ in the chalcedony spectrum, attributed to moganite, has been used for the evaluation of the quartz/moganite ratio in silica rocks. Its frequency lies at 503 cm⁻¹ in sedimentary chalcedony, representing a 2 cm⁻¹ difference with its position in pure moganite. We present a study of the 503 cm⁻¹ band’s behaviour upon heat treatment, showing its gradual disappearance upon heating to temperatures above 300 °C. Infrared spectroscopic measurements of the silanole (SiOH) content in the samples as a function of annealing temperature show a good correlation between the disappearance of the 503 cm⁻¹ Raman band and the decrease of structural hydroxyl. Thermogravimetric analyses reveal a significant weight loss that can be correlated with the decreasing of this Raman band. X-ray powder diffraction data suggest the moganite content in the samples to remain stable. We propose therefore the existence of a hitherto unknown Raman band at 503 cm⁻¹ in chalcedony, assigned to ‘free’ Si–O vibrations of non-bridging Si–OH that oscillate with a higher natural frequency than bridging Si–O–Si (at 464 cm⁻¹). A similar phenomenon was recently observed in the infrared spectra of chalcedony. The position of this Si–OH-related band is nearly the same as the Raman moganite band and the two bands may interfere. The actually observed Raman band in silica rocks might therefore be a convolution of a silanole and a moganite vibration. These findings have broad implications for future Raman spectroscopic studies of moganite, for the assessment of the quartz/moganite ratio, using this band, must take into account the contribution from silanole that are present in chalcedony and moganite.  相似文献   

The orientation and vibrational states of H2O in synthetic alkali-free beryl     

B. A. Kolesov  C. A. Geiger 《Physics and Chemistry of Minerals》2000,27(8):557-564

 The polarized single-crystal Raman spectra of synthetic H₂O-containing alkali-free beryl were recorded at room and low temperatures, and the polarized single-crystal IR spectra at room temperature. The H₂O molecule in the channel cavities is characterized by a Raman-active symmetric stretching vibration (ν₁) at 3607 cm⁻¹ and an IR-active asymmetric stretch (ν₃) at 3700 cm⁻¹ at room temperature. At low temperatures this ν₃ mode is observed in the Raman. Weak ν₁ and ν₃ modes of a second type of H₂O are also observed in the Raman spectra but only at 5 K. The H⋯&middot;H vector of the most abundant type of H₂O is parallel to the channel axis of beryl along [0 0 0 1]. The components of the polarizability tensor of the ν₁ mode of H₂O are similar to, but not exactly the same as, those of a free H₂O molecule. The Raman measurements indicate that the H₂O molecule is rotationally disordered around [0 0 0 1]. External translation and librational modes of H₂O could be observed as overtones with the internal H₂O-stretching modes. In the case of the librational motions, normal modes could also be observed directly in the Raman spectra at ∼200 cm⁻¹. The energies of the translational modes can be determined from an analysis of the overtones and are about 9 cm⁻¹ in energy (i.e., T_z). The energies of the librational modes are about 210 cm⁻¹ for R_x and 190 cm⁻¹ for R_y. Received: 8 April 1999 / Accepted: 5 April 2000  相似文献   

LiFeSi<Subscript>2</Subscript>O<Subscript>6</Subscript> and NaFeSi<Subscript>2</Subscript>O<Subscript>6</Subscript> at low temperatures: an infrared spectroscopic study     

M. Zhang  G. J. Redhammer  E. K. H. Salje  M. Mookherjee 《Physics and Chemistry of Minerals》2002,29(9):609-616

 Synthetic aegirine LiFeSi₂O₆ and NaFeSi₂O₆ were characterized using infrared spectroscopy in the frequency range 50–2000 cm⁻¹, and at temperatures between 20 and 300 K. For the C2/c phase of LiFeSi₂O₆, 25 of the 27 predicted infrared bands and 26 of 30 predicted Raman bands are recorded at room temperature. NaFeSi₂O₆ (with symmetry C2/c) shows 25 infrared and 26 Raman bands. On cooling, the C2/c–P2₁/c structural phase transition of LiFeSi₂O₆ is characterized by the appearance of 13 additional recorded peaks. This observation indicates the enlargement of the unit cell at the transition point. The appearance of an extra band near 688 cm⁻¹ in the monoclinic P2₁/c phase, which is due to the Si–O–Si vibration in the Si₂O₆ chains, indicates that there are two non-equivalent Si sites with different Si–O bond lengths. Most significant spectral changes appear in the far-infrared region, where Li–O and Fe–O vibrations are mainly located. Infrared bands between 300 and 330 cm⁻¹ show unusually dramatic changes at temperatures far below the transition. Compared with the infrared data of NaFeSi₂O₆ measured at low temperatures, the change in LiFeSi₂O₆ is interpreted as the consequence of mode crossing in the frequency region. A generalized Landau theory was used to analyze the order parameter of the C2/c–P2₁/c phase transition, and the results suggest that the transition is close to tricritical. Received: 21 January 2002 / Accepted: 22 July 2002  相似文献   

Raman spectra of hydrous γ -Mg2SiO4 at various pressures and temperatures     

L. Liu  C.-C. Lin  T. P. Mernagh  T. Inoue 《Physics and Chemistry of Minerals》2002,29(3):181-187

 One well-defined OH Raman band at 3651 ± 1 cm⁻¹ and one weak feature near 3700 ± 5 cm⁻¹ are recognized for the hydrous γ-phase of Mg₂SiO₄. Like the hydrous β-phase, the H₂O content in the γ-phase shifts most of the corresponding silicate modes towards lower frequencies. Variations in Raman spectra of the hydrous γ-phase were investigated up to about 200 kbar at room temperature and in the range 81–873 K at atmospheric pressure. Unlike the anhydrous γ-phase, which remains intact up to at least 873 K, the hydrous γ-phase sometimes converts to a defective forsterite structure above 800 K. Although the hydrous γ-phase remains intact up to at least 800 K, Raman signals of the OH bands disappear completely above 423 K. The Raman frequency of the well-defined OH band decreases linearly with increasing temperature between 81 and 423 K. In the region of the silicate vibrations, the Raman frequencies of the two most intense bands increase nonlinearly with increasing pressure, and decrease with increasing temperature. The frequencies for all other weak bands, however, decreased linearly with increasing temperature. The latter most likely reflects the larger scatter of the data for the weak bands. Received: 27 April 2001 / Accepted: 12 September 2001  相似文献   

Thermodynamics of diopside, anorthite, pseudowollastonite, CaMgGeO4 olivine, and åkermanite up to near the melting point     

P. Courtial  C. Téqui  P. Richet 《Physics and Chemistry of Minerals》2000,27(4):242-250

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High-pressure Raman spectroscopic study of Fo90 hydrous wadsleyite     

Annette K. Kleppe  Andrew P. Jephcoat  Joseph R. Smyth 《Physics and Chemistry of Minerals》2006,32(10):700-709

Raman spectra of monoclinic Fo₉₀ hydrous wadsleyite with 2.4 wt% H₂O have been measured in a diamond-anvil cell with helium as a pressure-transmitting medium to 58.4 GPa at room temperature. The most intense, characteristic wadsleyite modes, the Si–O–Si symmetric stretch at 721 cm⁻¹ and the symmetric stretch of the SiO₃ unit at 918 cm⁻¹, shift continuously to 58.4 GPa showing no evidence of a first order change in the crystal structure despite compression well beyond the stability field of wadsleyite in terms of pressure. The pressure dependence of these two modes is nearly identical for Fo₉₀ hydrous and Fo₁₀₀ anhydrous wadsleyite. A striking feature in the high-pressure Raman spectra of Fo₉₀ hydrous wadsleyite is the appearance of new Raman modes above 9 GPa in the mid-frequency range (300–650 cm⁻¹ at 1-bar and shifted to 500–850 cm⁻¹ at 58.4 GPa) accompanied by a significant growth in their intensities under further compression. In the OH stretching frequency range Fo₉₀ hydrous wadsleyite exhibits a larger number of modes than the Mg end-member phase. The higher number of modes may be due to either additional protonation sites or simply that we observe a different subset of all possible OH modes for each sample. The high-pressure behaviour of the OH stretching modes of Fo₉₀ and Fo₁₀₀ hydrous wadsleyite is consistent: OH stretching modes with frequencies <3,530 cm⁻¹ decrease with increasing pressure whereas the higher-frequency OH modes show a close to constant pressure dependence to at least 13.2 GPa. The approximately constant pressure dependence of the OH modes above 3,530 cm⁻¹ is consistent with protons being located at the O1···O edges around M3.  相似文献   

Melting and premelting of silicates: Raman spectroscopy and X-ray diffraction of Li2SiO3 and Na2SiO3     

Pascal Richet  Bjorn O. Mysen  Denis Andrault 《Physics and Chemistry of Minerals》1996,23(3):157-172

The isostructural lithium (Li₂SiO₃) and sodium (Na₂SiO₃) metasilicates have been investigated from room temperature up to the melting point by single-crystal Raman spectroscopy and energy-dispersive X-ray powder diffraction. The unit-cell parameters and Raman frequencies of Li₂SiO₃ vary regularly with temperature up to the melting point, which is consistent with the lack of premelting effects in calorimetric measurements. In contrast, Na₂SiO₃ undergoes a transition at about 850 K from orthorhombic Cmc 2₁ symmetry, to a lower symmetry (possibly Pmc 2₁), and shows near 1200 K changes in the Raman spectra that correlate well with the premelting effects as determined from calorimetry observations. In both compounds, a high alkali mobility likely sets in several hundreds of degrees below the melting point. Premelting in Na₂SiO₃ is associated with extensive deformation of the silicate chains as evidenced near the melting point by similarities in the Raman spectra of the crystalline and liquid phases.  相似文献   

Mechanisms of OH defect incorporation in naturally occurring,hydrothermally formed diopside and jadeite     

M. Andrut  M. Wildner  J. Ingrin  A. Beran 《Physics and Chemistry of Minerals》2007,34(8):543-549

The IR spectrum of an alpine, hydrothermally formed diopside containing 17 wt ppm H₂O consists of three main OH absorption bands centred at 3647, 3464 and 3359 cm⁻¹. Jadeite from a Californian vein occurrence is characterised by bands at 3616 and 3557 cm⁻¹ and contains about 197 wt ppm H₂O. Based on the pleochroic scheme of the OH absorption bands in diopside, OH defect incorporation models are derived on the basis of fully occupied cation sites and under the assumption of M1 and M2 site vacancies; OH defects replacing O2 oxygen atoms are most common. The less pronounced OH pleochroism and the broad band absorption pattern of jadeite indicate a high degree of OH defect disordering. The pleochroic scheme of the main absorption bands at 3616 and 3557 cm⁻¹ implies partial replacement of O2 oxygen atoms by OH dipoles pointing to vacant Si sites. Under the assumption of M1 and M2 site vacancies, O1–H and O2–H defects are also derivable. OH incorporation modes assuming Si-vacancies should be considered for jadeite-rich clinopyroxenes formed in deep crust and upper mantle regions.  相似文献   

Raman spectroscopy, vibrational analysis, and heating of buergerite tourmaline     

David A. McKeown 《Physics and Chemistry of Minerals》2008,35(5):259-270

Polarized Raman spectra were collected for single crystal buergerite (NaFe₃Al₆(BO₃)₃Si₆O₁₈(O_0.92(OH)_0.08)₃F) from room temperature to near 1,375°C. Vibrational assignments to features in the room temperature spectra were determined by lattice dynamics calculations, where internal BO₃ motions dominate modes near 1,300 cm⁻¹, internal SiO₄ displacements dominate modes between 900 and 1,200 cm⁻¹, while less localized displacements within the isolated Si₆O₁₈ ring mix with motions within Na, Fe, Al, F, and BO₃ environments for fundamental modes below 780 cm⁻¹. At elevated temperatures, most buergerite Raman features broaden and shift to lower frequencies up to 900°C. Above this temperature, the lattice mode peaks evolve into broad bands, while OH stretch modes near 3,550 cm⁻¹ disappear. According to Raman spectroscopy, X-ray diffraction, differential thermal analysis, and scanning electron microscopy, buergerite undergoes a complex transition that starts near 700°C and extends over a 310°C interval, where initially, Al and Fe probably become disordered within the Y- and Z-sites, and most F and all OH are later liberated. A reversible crystal-to-amorphous transition is seen by Raman for buergerite fragments heated as high as 930°C. Buergerite becomes permanently altered when heated to temperatures greater than 930°C; after cooling to room temperature, these altered fragments are comprised of mullite and Fe-oxide crystals suspended in an amorphous borosilicate matrix.  相似文献   

Dehydration process and structural development of cordierite ceramic precursors derived from FTIR spectroscopic investigations     

D. Voll  A. Beran 《Physics and Chemistry of Minerals》2002,29(8):545-551

 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   

Spectroscopic evidence for pressure-induced phase transitions in diopside     

A. Chopelas  G. Serghiou 《Physics and Chemistry of Minerals》2002,29(6):403-408

 Raman spectra of diopside were collected from atmospheric pressure to 71 GPa. The pressure dependences of 22 modes were determined. Changes occurred in the spectra at three different pressures. First, at approximately 10 GPa, the two Raman modes at 356 and 875 cm⁻¹ disappeared, while the mode at 324 cm⁻¹ split into two modes, diverging at this pressure with significantly different pressure shifts; second, at approximately 15 GPa, a small (1 to 2 cm⁻¹) drop in several of the frequencies was observed accompanied by changes in the pressure dependency of some of the modes; and third, above 55 GPa, the modes characteristic of chains of tetrahedrally coordinated silicon disappeared, while those for octahedrally coordinated silicon appeared. The first change at 10 GPa appears to be a C2/c to C2/c transition involving a change in the Ca coordination. The third change above 55 GPa appears to be a change in the silicon coordination. At 15 GPa, it is suggested that a change in compressional mechanism takes place. Received: 14 November 2000 / Accepted: 9 January 2002  相似文献   

Phosphorus solubility mechanisms in haplogranitic aluminosilicate glass and melt: effect of temperature and aluminum content   总被引：1，自引：0，他引：1  

Bjorn O. Mysen 《Contributions to Mineralogy and Petrology》1998,133(1-2):38-50

The solubility behavior of phosphorus in glasses and melts in the system Na₂O-Al₂O₃-SiO₂-P₂O₅ has been examined as a function of temperature and Al₂O₃ content with microRaman spectroscopy. The Al₂O₃ was added (2, 4, 5, 6, and 8 mol% Al₂O₃) to melts with 80 mol% SiO₂ and ∼2 mol% P₂O₅. The compositions range from peralkaline, via meta-aluminous to peraluminous. Raman spectra were obtained of both the phosphorus-free and phosphorous-bearing glasses and melts between 25 and 1218 °C. The Raman spectrum of Al-free, P-bearing glass exhibits a characteristic strong band near 940 cm⁻¹ assigned to P=O stretching in orthophosphate complexes together with a weaker band near 1000 cm⁻¹ assigned P₂O₇ complexes. With increasing Al content, the proportion of P₂O₇ initially increases relative to PO₄ and is joined by AlPO₄ complexes which exhibit a characteristic P-O stretch mode slightly above 1100 cm⁻¹. The latter complex appears to dominate in meta-aluminosilicate glass and is the only phosphate complex in peraluminous glasses. When P-bearing peralkaline silicate and aluminosilicate glasses are transformed to supercooled melts, there is a rapid decrease in PO₄/P₂O₇ so that in the molten state, PO₄ units are barely discernible. The P₂O₇/AlPO₄ abundance ratio in peralkaline compositions increases with increasing temperature. This decrease in PO₄/P₂O₇ with increasing temperature results in depolymerization of the silicate melts. Dissolved P₂O₅ in peraluminous glass and melts forms AlPO₄ complexes only. This solution mechanism has no discernible influence on the aluminosilicate melt structure. There is no effect of temperature on this solution mechanism. Received: 7 October 1997 / Accepted: 11 May 1998  相似文献   

Infrared spectroscopic characterization of dehydration and accompanying phase transition behaviors in NAT-topology zeolites     

Hsiu-Wen?WangEmail author  David?L.?Bish 《Physics and Chemistry of Minerals》2012,39(4):277-293

Relative humidity ( P_{\textH 2 \textO} P_{{{\text{H}}_{ 2} {\text{O}}}} , partial pressure of water)-dependent dehydration and accompanying phase transitions in NAT-topology zeolites (natrolite, scolecite, and mesolite) were studied under controlled temperature and known P_{\textH 2 \textO} P_{{{\text{H}}_{ 2} {\text{O}}}} conditions by in situ diffuse-reflectance infrared Fourier transform spectroscopy and parallel X-ray powder diffraction. Dehydration was characterized by the disappearance of internal H₂O vibrational modes. The loss of H₂O molecules caused a sequence of structural transitions in which the host framework transformation path was coupled primarily via the thermal motion of guest Na⁺/Ca²⁺ cations and H₂O molecules. The observation of different interactions of H₂O molecules and Na⁺/Ca²⁺ cations with host aluminosilicate frameworks under high- and low- P_{\textH 2 \textO} P_{{{\text{H}}_{ 2} {\text{O}}}} conditions indicated the development of different local strain fields, arising from cation–H₂O interactions in NAT-type channels. These strain fields influence the Si–O/Al–O bond strength and tilting angles within and between tetrahedra as the dehydration temperature is approached. The newly observed infrared bands (at 2,139 cm⁻¹ in natrolite, 2,276 cm⁻¹ in scolecite, and 2,176 and 2,259 cm⁻¹ in mesolite) result from strong cation–H₂O–Al–Si framework interactions in NAT-type channels, and these bands can be used to evaluate the energetic evolution of Na⁺/Ca²⁺ cations before and after phase transitions, especially for scolecite and mesolite. The 2,176 and 2,259 cm⁻¹ absorption bands in mesolite also appear to be related to Na⁺/Ca²⁺ order–disorder that occur when mesolite loses its Ow4 H₂O molecules.  相似文献   

A Raman spectroscopic investigation of Fe-rich sphalerite: effect of fe-substitution     

Sherif Kharbish 《Physics and Chemistry of Minerals》2007,34(8):551-558

A Raman spectroscopic study of Fe-rich sphalerite (Zn_{1 − x} Fe _x S) has been carried out for six samples with 0.10 ≤ x ≤ 0.24. Both the intensities and frequencies of the TO and LO modes of sphalerite are approximately independent of Fe concentration. However, the substitution of Zn by Fe results in five additional bands with frequencies between the TO (271 cm⁻¹) and LO (350 cm⁻¹) modes. Three of these bands are attributed to resonance modes (i.e. Y ₁, Y ₂ and Y ₃ modes). The fourth band (B mode) is assigned to a breathing mode of the nearest-neighbor sulfur atoms around the Fe atoms. The band at 337 cm⁻¹ is attributed to the presence of Fe³⁺. The excellent correlations between the normalized intensities of these five different modes and x _Fe show that these modes depend on Fe-content. Another extra mode at 287 cm⁻¹ is assigned to the presence of Cd in sphalerite.  相似文献   

Orbital interactions in phyllosilicates: Perturbations of an idealized two-dimensional,infinite silicate frame     

William F. Bleam  Roald Hoffmann 《Physics and Chemistry of Minerals》1988,15(4):398-408

The _∞ ² [Si₂O ₅ ²⁻ ] frame in phyllosilicate minerals is distorted through the rotation and tilting of the silicate tetrahedra and interacts with octahedral cations through its apical oxygens. Qualitative perturbation theory and extended Hückel band structure calculations demonstrate that rotation and tilting distortions of the _∞ ² [Si₂O ₅ ²⁻ ] frame have little influence on orbital interactions within the frame. The effects which are observed can be traced to next-nearestneighbor, oxygen-oxygen interactions. Analysis of band widths and crystal-orbital-overlap-populations demonstrate the importance of O(2s) orbitals in the silicate bond. Interactions between Si(3s, 3p) and O(2s) atomic orbitals account for about half of the bonding overlap in the Si-O bond. Crystal orbitals within the _∞ ² [Si₂O ₅ ²⁻ ] frame are perturbed in kaolinite, lizardite, pyrophyllite and talc through interactions of the apical oxygens with octahedrally coordinated Al(III) and Mg(II). These interactions appear to involve states that are non-bonding in an isolated frame, having little effect on the Si-O_apical bond while significantly reducing the apical-oxygen atomic population.  相似文献   

The charge-density distribution, its multipole refinement and the antiferromagnetic structure of dioptase, Cu6[Si6O18]·6H2O     

E. L. Belokoneva  Yu. K. Gubina  J. B. Forsyth  P. J. Brown 《Physics and Chemistry of Minerals》2002,29(6):430-438

 The chemical bonding in the ring silicate mineral dioptase is investigated on the basis of accurate single-crystal X-ray diffraction data. A multipole model is used in the refinements. Static deformation electron density is mapped for the silicon tetrahedron, Cu-octahedron and water molecule in different sections. The silicon tetrahedron exhibits peaks resulting from σ-bonds between Si–sp³ hybrid orbitals and O–p orbitals. The excess density is located on bonds between the Si atom and bridge (in ring) O(1)-, O(1′)-oxygens and across the interior of the Si–O–Si angle. In the Jahn-Teller distorted Cu octahedron, in addition to peaks which result from single Cu–O σ-bonds, there are peaks which are due to 3d electrons. The analysis of crystal-field influence on the Cu charge distribution is made using the tetragonal D _{4 d} approximation for the low-symmetry (C1) Cu octahedron. The calculation of the occupancies of the 3d atomic orbitals shows that the Cu non-bonding orbitals are most populated (˜20%) and the bonding orbitals least populated (14%), as is typical for the Jahn-Teller octahedron. The effective atomic charge on the Cu atom in dioptase determined from the multipoles is +1.23e: closer to the Cu⁺¹ than to the Cu⁺² state. The charge on the Si atom has a value +1.17e, which is in the range typical for Si atoms already determined by this method. The accumulation of density on bridge oxygens and across the interior of the Si–O–Si angle may be explained by additional strain in the bond with the decrease of the Si–O–Si angle in dioptase to 132°. The same effect was found earlier in coesite. A single-crystal neutron diffraction study shows that dioptase becomes antiferromagnetic below a Néel temperature of 15.9(1) K, in contrast to the previously reported specific heat anomaly at 21 K. The magnetic propagation vector is (0, 0, 3/2) on the hexagonal triple cell or (1/2, 1/2, 1/2) in rhombohedral indices. The relation between the antiferromagnetic and the charge-density models for dioptase is discussed. The less occupied Cu d _x2−y2 orbitals are responsible for the magnetic properties. These lie in the Cu–O squares, which are approximately perpendicular to c _hex, but which are alternately inclined to it by a small angle. The magnetic moments of 0.59(1)μ _B on the Cu ions in the same level are ordered ferromagnetically, but between ions in alternate levels the coupling is antiferromagnet. Within experimental error the magnetic moments are perpendicular to the square planes, which make an angle ±13(3)° to the triad axis. Received: 8 June 2001 / Accepted: 10 January 2002  相似文献   

Vibrational states of H<Subscript>2</Subscript>O in beryl: physical aspects     

Boris Kolesov 《Physics and Chemistry of Minerals》2008,35(5):271-278

Single-crystal polarized Raman spectra (3,000–4,000 cm⁻¹ at 3 ≤ T ≤ 300 K) were measured for synthetic alkali-free and natural beryl, Be₂Al₃Si₆O₁₈·xH₂O, to determine the behavior of H₂O molecules of both Type I and Type II in the cavities. At low temperature, the H₂O molecules of Type I displace from the center of cavity and give rise to very weak hydrogen bonding with the host lattice. The H₂O Type I translational motion is characterized by substantial anharmonicity and looks like a motion of “a particle in the box” with a frequency of 6.3 cm⁻¹. Water Type II is characterized by a free rotation with respect to the C ₂ molecule axis, and it makes possible the water nuclear isomers (i.e. ortho- and para-) to be observed at low temperature.

Boris KolesovEmail:

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Thermodynamic mixing behavior of synthetic Ca-Tschermak–diopside pyroxene solid solutions: III. An analysis of IR line broadening and heat of mixing behavior     

Katja Etzel  Artur Benisek 《Physics and Chemistry of Minerals》2008,35(7):399-407

A series of synthetic Ca-Tschermak–diopside (CaAlAlSiO₆–CaMgSi₂O₆) clinopyroxenes were investigated by powder infrared spectroscopy at room temperature in the wavenumber range 80–2,000 cm⁻¹. Measurable local structural heterogeneities in the crystals are suggested by the line broadening parameter, Δcorr that are observed for intermediate solid-solution compositions. The broadening is most pronounced in the high wavenumber region of the IR spectra that contains stretching modes involving the TO₄ polyhedra. The effective line widths for three selected wavenumber regions deviate positively from linear behavior. This is also observed for the enthalpy of mixing of this solid solution. The relationship between “excess Δcorr”, δΔcorr, and heat of mixing, ΔH _mix, behavior was investigated for this clinopyroxene series and for several other binary silicate solid solutions. The ΔH _mix versus δΔcorr slope values show a linear relationship with respect to the integrated excess volume of the various solid solutions.  相似文献   

Equilibrium density and expansivity of silicate melts in the glass transition range     

M. J. Toplis  P. Richet 《Contributions to Mineralogy and Petrology》2000,139(6):672-683

Equilibrium volumes and expansivities of three liquids in the system anorthite (CaAl₂Si₂O₈)–diopside (CaMgSi₂O₆) have been derived from dilatometric measurements of the equilibrium length of samples in the glass transition range. The typical temperature range of 40 K for the measurements is limited at low temperature by the very long times necessary to reach structural equilibrium and at high temperature by the penetration of the rod used to measure sample dilatation. Despite such narrow intervals, the expansivities are determined to better than 3% thanks to the high precision with which length changes are measured. The coefficient of volume thermal expansion (1/V dV/dT) of the fully relaxed liquid just above the glass transition is found to decrease linearly from diopside composition (139 ± 4 × 10⁻⁶ K⁻¹) to anorthite composition (59 ± 2 × 10⁻⁶ K⁻¹). These values are greater than those determined for the same liquids at superliquidus temperatures, demonstrating that expansivities of silicate melts may decrease markedly with increasing temperature. A predictive model based upon partial molar volumes which vary as a linear function of the logarithm of temperature is proposed. Received: 25 February 2000 / Accepted: 29 May 2000  相似文献