首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
Water solubility in pyrope to 100 kbar   总被引:14,自引:0,他引:14  
The solubility and incorporation mechanism of water in natural, almost pure pyrope from Dora Maira, Western Alps was investigated. The infrared spectrum of the natural, untreated sample (58 ppm water) shows several exceptionally sharp bands in the OH-stretching region, including a single band at 3601.9 cm−1 and a band system with main components at 3640.5, 3650.8 and 3660.6 cm−1. High-temperature and high-pressure infrared spectra suggest that the two absorption features arise from almost free OH groups in sites with different compressibility and thermal expansivity, with the site causing the 3601.9 cm−1 band being much stiffer. Pyrope samples were annealed in a piston-cylinder or multi-anvil apparatus for several days in the presence of excess water, excess SiO2 and excess Al2SiO5 to determine the equilibrium solubility of water in pyrope to 100 kbar. Total solubility increases with pressure, however, this is exclusively due to the high-frequency band system, while the intensity of the low-frequency band decreases with pressure. At 1000 °C and the oxygen fugacity of the Ni-NiO buffer, the bulk solubility can be described by the equation c OH =Af H2O 0.5exp(−PΔV/RT) with A = 0.679 ppm/bar0.5 and ΔV = 5.71 cm3/mol. This equation implies the incorporation of water in the crystal as isolated OH groups. With increasing temperature, solubility appears to decrease with ΔH = − 14 kJ/mol. At Fe-FeO buffer conditions, solubility is 30 to 50% lower than with the Ni-NiO buffer, suggesting that the incorporation of OH is not coupled to the reduction of Fe3+. Possibly, the 3601.9 cm−1 band is associated with the tetrahedral OH B defect and the high-frequency system with the dodecahedral OH Li defect. Based on the experimentally established solubility model, it is estimated that garnet in a hot subducted slab will transport 170 ppm of water into the mantle beyond the breakdown limit of amphibole. In a cold slab, 470 ppm of water can be incorporated into garnet at the breakdown limit of phengite. These numbers imply that a significant fraction of the total water in the hydrosphere has been recycled into the mantle since the Proterozoic. Received: 6 January 1997 / Accepted: 27 March 1997  相似文献   

2.
 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⋯·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., Tz). The energies of the librational modes are about 210 cm−1 for Rx and 190 cm−1 for Ry. Received: 8 April 1999 / Accepted: 5 April 2000  相似文献   

3.
 High-pressure and high-temperature Raman spectra of CaGeO3 tetragonal garnet have been collected to 11.5 GPa and 1225 K, respectively, in order to investigate possible intrinsic anharmonic behaviour in this phase. The Raman peak positions were observed to vary linearly with pressure and temperature within the ranges studied, with the higher-energy peaks showing larger P- and T-induced shifts than the low energy modes. The observed T-induced shifts are similar to those reported for grossular and andradite, while the observed P-induced shifts are generally larger than those of aluminosilicate and MgSiO3 majorite garnets (Gillet et al. 1992; Rauch et al. 1996) due to the larger bulk modulus of CaGeO3 garnet. The observed mode shifts of CaGeO3 garnet were used to determine the isothermal and isobaric mode Grüneisen parameters for this phase. These parameters are similar in value to those reported previously for grossular and andradite (Gillet et al. 1992). The calculated intrinsic anharmonic parameters, a i , for CaGeO3 garnet were determined to be nonzero, indicating significant anharmonic behaviour for this phase. These values, which range from −3.8 × 10−5 K−1 to −1.3 × 10−5 K−1, are also similar to those reported for andradite and grossular, but smaller than those determined for pyrope (Gillet et al. 1992). Hence, we expect MgSiO3 majorite to show greater anharmonicity than the germanate analogue studied by us. The anharmonic parameters determined for CaGeO3 tetragonal garnet may now be introduced into quasiharmonic vibrational heat capacity models to account for the observed anharmonic behaviour. Received: 21 April 1999 / Revised, accepted: 11 September 1999  相似文献   

4.
 Infrared absorption spectra of brucite Mg (OH)2 were measured under high pressure and high temperature from 0.1 MPa 25 °C to 16 GPa 360 °C using infrared synchrotron radiation at BL43IR of Spring-8 and a high-temperature diamond-anvil cell. Brucite originally has an absorption peak at 3700 cm−1, which is due to the OH dipole at ambient pressure. Over 3 GPa, brucite shows a pressure-induced absorption peak at 3650 cm−1. The pressure-induced peak can be assigned to a new OH dipole under pressure. The new peak indicates that brucite has a new proton site under pressure and undergoes a high-pressure phase transition. From observations of the pressure-induced peak under various PT condition, a stable region of the high-pressure phase was determined. The original peak shifts to lower wavenumber at −0.25 cm−1 GPa−1, while the pressure-induced peak shifts at −5.1 cm−1 GPa−1. These negative dependences of original and pressure-induced peak shifts against pressure result from enhanced hydrogen bond by shortened O–H···O distance, and the two dependences must result from the differences of hydrogen bond types of the original and pressure-induced peaks, most likely from trifurcated and bent types, respectively. Under high pressure and high temperature, the pressure-induced peak disappears, but a broad absorption band between 3300 and 3500 cm−1 was observed. The broad absorption band may suggest free proton, and the possibility of proton conduction in brucite under high pressure and temperature. Received: 16 July 2001 / Accepted: 25 December 2001  相似文献   

5.
Summary The infrared absorption spectrum of amethyst in the region of stretching vibrations of X–OH groups reveals several bands that have been used for the separation of natural from synthetic amethyst. The intensity and shape of these bands have been measured as a function of crystallographic orientation. Using a resolution of 0.5 cm−1 the 3595 cm−1 band is present in all infrared spectra of natural amethyst and in some rare synthetic ones. If present in synthetic amethyst, its full width at half maximum (FWHM) is about 7 cm−1 whereas it is about 3 cm−1 in all natural samples. This new criterion, unlike the previous ones, seems appropriate to separate natural from synthetic amethyst in all cases.  相似文献   

6.
 Single crystals of synthetic vanadium-, chromium- and cobalt-bearing garnets, Pyr:V0.06, Pyr:V0.13, Pyr:Cr0.04, Pyr:Co0.10, and Gt:Co3.00, and a natural vanadium-bearing grossular, Gross:V0.07 (Cr3+ < 0.005), were studied by electronic absorption spectroscopy in the wavenumber range 35 000–5000 cm−1 under ambient conditions and at temperatures up to 600 K and pressures up to 8 GPa. The T and P behavior of the absorption band energies and intensities shows the following for the different transition metal-bearing garnets: Cr: The thermal expansion of chromium octahedra are similar to and the Racah parameter the same in synthetic Cr-doped pyrope, αpoly≅ 1.3 × 10−5 K−1, and in natural pyrope, αpoly≅ 1.5 × 10−5 K−1, and B=655 cm−1, respectively. Ca2+[8]-free garnets have a slightly stronger crystal field at the Y[6] site and, therefore, the energies of the two spin-allowed Cr3+ dd bands are ca. 300 cm−1 higher in Mg-pyrope than in natural Ca-bearing pyrope. Co: Increasing temperature causes only a small thermal expansion of the cobalt dodecahedra. Increasing pressure gives rise to appreciable compression, which is similar to that of the Fe2+-dodecahedra in almandine, where k=125 ± 25 GPa. T and P dependence of the Co band intensities may be caused by strong spin-orbit coupling. V: Occurs in at least two valence states and structural sites: (1) V3+ in octahedral sites gives rise to two spin-allowed bands, at 17 220 cm−1 and 24 600 cm−1, whose temperature dependence is typical for spin-allowed dd transitions in centrosymmetric sites. (2) V4+, which causes a set of dd absorption bands similar to those observed in the spectrum of V4+-doped Zr[SiO4]. The P behavior of the V absorption bands indicates an interaction between V3+ and V4+ species. Received: 27 June 2001 / Accepted: 19 December 2001  相似文献   

7.
A suite of more than 200 garnet single crystals, extracted from 150 xenoliths, covering the whole range of types of garnet parageneses in mantle xenoliths so far known from kimberlites of the Siberian platform and collected from nearly all the kimberlite pipes known in that tectonic unit, as well as some garnets found as inclusions in diamonds and olivine megacrysts from such kimberlites, were studied by means of electron microprobe analysis and single-crystal IR absorption spectroscopy in the v OH vibrational range in search of the occurrence, energy and intensity of the v OH bands of hydroxyl defects in such garnets and its potential use in an elucidation of the nature of the fluid phase in the mantle beneath the Siberian platform. The v OH single-crystal spectra show either one or a combination of two or more of the following major v OH bands, I 3645–3662 cm−1, II 3561–3583 cm−1, III 3515–3527 cm−1, and minor bands, Ia 3623–3631 cm−1, IIa 3593–3607 cm−1. The type of combination of such bands in the spectrum of a specific garnet depends on the type of the rock series of the host xenolith, Mg, Mg-Ca, Ca, Mg-Fe, or alkremite, on the xenolith type as well as on the chemical composition of the respective garnet. Nearly all garnets contain band systems I and II. Band system III occurs in Ti-rich garnets, with wt% TiO2 > ca. 0.4, from xenoliths of the Mg-Ca and Mg-Fe series, only. The v OH spectra do not correspond to those of OH defects in synthetic pyropes or natural ultra-high pressure garnets from diamondiferous metamorphics. There were no indications of v OH from inclusions of other minerals within the selected 60 × 60 μm measuring areas in the garnets. The v OH spectra of pyrope-knorringite- and pyrope-knorringite-uvarovite-rich garnets included in diamonds do not show band systems I to III. Instead, they exhibit one weak, broad band (Δv OH 200–460 cm−1) near 3570 cm−1, a result that was also obtained on pyrope-knorringite-rich garnets extracted from two olivine megacrysts. The quantitative evaluation, on the basis of relevant existing calibrational data (Bell et al. 1995), of the sum of integral intensities of all v OH bonds of the garnets studied yielded a wide range of “water” concentrations within the set of the different garnets, between values below the detection limit of our single-crystal IR method, near 2 × 10−4 wt%, up to 163 × 10−4 wt%. The “water” contents vary in a complex manner in garnets from different xenolith types, obviously depending on a large number of constraints, inherent in the crystal chemistry as well as the formation conditions of the garnets during the crystallization of their mantle host rocks. Secondary alteration effects during uplift of the kimberlite, play, if any, only a minor role. Despite the very complex pattern of the “water” contents of the garnets, preventing an evaluation of a straightforward correlation between “water” contents of the garnets and the composition of the mantle's fluid phase during garnet formation, at least two general conclusions could be drawn: (1) the wide variation of “water” contents in garnets is not indicative of regional or local differences in the composition of the mantle's fluid phase; (2) garnets formed in the high-pressure/high-temperature diamond-pyrope facies invariably contain significantly lower amounts of “water” than garnets formed under the conditions of the graphite-pyrope facies. This latter result (2) may point to significantly lower f H2O and f O2 in the former as compared to the latter facies. Received: 25 November 1997 / Accepted: 9 March 1998  相似文献   

8.
 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   

9.
A selected set of five different kyanite samples was analysed by electron microprobe and found to contain chromium between <0.001 and 0.055 per formula unit (pfu). Polarized electronic absorption spectroscopy on oriented single crystals, R1, R2-sharp line luminescence and spectra of excitation of λ3- and λ4-components of R1-line of Cr3+-emission had the following results: (1) The Fe2+–Ti4+ charge transfer in c-parallel chains of edge connected M(1) and M(2) octahedra shows up in the electronic absorption spectra as an almost exclusively c(||Z′)-polarized, very strong and broad band at 16000 cm−1 if <, in this case the only band in the spectrum, and at an invariably lower energy of 15400 cm−1 in crystals with  ≥ . The energy difference is explained by an expansion of the Of–Ok, and Ob–Om edges, by which the M(1) and M(2) octahedra are interconnected (Burnham 1963), when Cr3+ substitutes for Al compared to the chromium-free case. (2) The Cr3+ is proven in two greatly differing crystal fields a and b, giving rise to two sets of bands, derived from the well known dd transitions of Cr3+ 4A2g4T2g(F)(I), →4T1g(F)(II), and →4T1g(P)(III). Band energies in the two sets a and b, as obtained by absorption, A, and excitation, E, agree well: I: 17300(a, A), 17200(a, E), 16000(b, A), 16200(b, E); II: 24800(a, A), 24400(a, E); 22300(b, A), 22200(b, E); III: 28800(b,A) cm−1. Evaluation of crystal field parameters from the bands in the electronic spectra yield Dq(a)=1730 cm−1, Dq(b)=1600 cm−1, B(a)=790 cm−1, B(b)=620 cm−1 (errors ca. ±10 cm−1), again in agreement with values extracted from the λ3, λ4 excitation spectra. The CF-values of set a are close to those typical of Cr3+ substituting for Al in octahedra of other silicate minerals without constitutional OH as for sapphirine, mantle garnets or beryl, and are, therefore, interpreted as caused by Cr3+ substituting for Al in some or all of the M(1) to M(4) octaheda of the kyanite structure, which are crystallographically different but close in their mean Al–O distances, ranging from 1.896 to 1.919 A (Burnham 1963), and slight degrees of distortion. Hence, band set a originates from substitutive Cr3+ in the kyanite structural matrix. The CF-data of Cr3+ type b, expecially B, resemble those of Cr3+ in oxides, especially of corundum type solid solutions or eskolaite. This may be interpreted by the assumption that a fraction of the total chromium contents might be allocated in a precursor of a corundum type exsolution. Received: 3 January 1997 / Revised, accepted: 2 May 1997  相似文献   

10.
 Raman spectra of hydrous β-Mg2SiO4 (1.65 wt% H2O) have been measured in a diamond-anvil cell with helium as a pressure-transmitting medium at room temperature to 50 GPa. We observe three OH-stretching modes, a doublet with components at 3329 and 3373 cm−1, which decrease linearly with pressure, and a single mode at 3586 cm−1, which remains nearly constant up to 24 GPa before decreasing at higher pressures. Assessment of the mode frequencies and their pressure dependence, together with previous results from X-ray and IR data, are consistent with protonation of the O1 site in agreement with previous studies. Strict assignment of Raman activity awaits detailed structural models. The nature of the protonation in wadsleyite may require more specific experimental probes for full solution of the hydrogen-site problem. Received: 18 July 2000 / Accepted: 22 November 2000  相似文献   

11.
 We have determined the quenched cation ordering states of five orthopyroxene crystals collected from the marginal border group and the lower zone a and b of the Skaergaard intrusion, and modeled these data to retrieve their closure temperatures (T c) of Fe–Mg ordering and cooling rates. According to existing thermal models for the Skaergaard pluton, conductive cooling dominated the high and low temperature regimes, which were separated by an intermediate temperature regime in which the cooling was controlled primarily by convective fluid circulation. The cooling rates retrieved from the quenched cation ordering states of the orthopyroxene crystals strictly apply to temperatures around the closure temperatures of the ordering states, ∼340–400° C, which fall at the transition from convective to the lower temperature conductive cooling. The cooling rates obtained from the cation ordering states of orthopyroxene vary from ∼1 to 270 K/ka. These results agree well with a thermal model calculated using an assumed average permeability of 10-12 cm2 for the pluton, but not completely with a model calculated on the basis of an average permeability of 10-13 cm2, although both values produced shifts of δ18O that are comparable to those observed in the pluton. Received: 27 February 1995/Accepted: 20 July 1995  相似文献   

12.
Near-infrared (NIR) absorption bands related to total water (4000 and 7050 cm−1), OH groups (4500 cm−1) and molecular H2O (5200 cm−1) were studied in two polymerised glasses, a synthetic albitic composition and a natural obsidian. The water contents of the glasses were determined using Karl Fischer titration. Molar absorption coefficients were calculated for each of the bands using albitic glasses containing between 0.54 and 9.16 wt.% H2O and rhyolitic glasses containing between 0.97 and 9.20 wt.% H2O. Different combinations of baseline type and intensity measure (peak height/area) for the combination bands at 4500 and 5200 cm−1 were used to investigate the effect of evaluation procedure on calculated hydrous species concentrations. Total water contents calculated using each of the baseline/molar absorption coefficient combinations agree to within 5.8% relative for rhyolitic and 6.5% relative for albitic glasses (maximum absolute differences of 0.08 and 0.15 wt.% H2O, respectively). In glasses with water contents >1 wt.%, calculated hydrous species concentrations vary by up to 17% relative for OH and 11% relative for H2O (maximum absolute differences of 0.33 and 0.43 wt.% H2O, respectively). This variation in calculated species concentrations is typically greater in rhyolitic glasses than albitic. In situ, micro-FTIR analysis at 300 and 100 K was used to investigate the effect of varying temperature on the NIR spectra of the glasses. The linear and integral molar absorption coefficients for each of the bands were recalculated from the 100 K spectra, and were found to vary systematically from the 300 K values. Linear molar absorption coefficients for the 4000 and 7050 cm−1 bands decrease by 16–20% and integral molar absorption coefficients by up to 30%. Depending on glass composition and baseline type, the integral molar absorption coefficients for the absorption bands related to OH groups and molecular H2O change by up to −5.8 and +7.4%, respectively, while linear molar absorption coefficients show less variation, with a maximum change of ∼4%. Using the new molar absorption coefficients for the combination bands to calculate species concentrations at 100 K, the maximum change in species concentration is 0.08 wt.% H2O, compared with 0.39 wt.% which would be calculated if constant values were assumed for the combination band molar absorption coefficients. Almost all the changes in the spectra can therefore be interpreted in terms of changing molar absorption coefficient, rather than interconversion between hydrous species. Received: 17 December 1998 / Revised, accepted 8 July 1999  相似文献   

13.
The thermal behaviour of ripidolite, an iron-rich chlorite, has been studied in situ by infrared emission spectroscopy up to 800 °C. The more di,trioctahedral nature due to significant amounts of Fe3+ is reflected, in addition to the two bands around 3420 and 3560 cm−1, by an extra band around 3345 cm−1. This extra band is absent in pure dioctahedral chlorites without Fe3+. These bands have been assigned to (AlAl)O-OH, (SiAl)O-OH and (SiSi)O-OH stretching modes with increasing frequencies. The bands disappear upon dehydroxylation around 650 °C. A similar behaviour is observed for the corresponding libration modes around 716, 759 and 802 cm−1. The stretching and bending modes of the inner-OH of the octahedral sheet in the 2:1 clay-like layer are observed around 3645, 943 and 904 cm−1. Although the bands decrease in intensity, they remain present up to 800 °C as dehydroxylation of the octahedral sheet is not yet complete at this temperature. The presence of two bending modes is explained as being due to a differentiation between Mg-OH and Fe-OH modes. At 650 °C a new sharp band is observed around 502 cm−1 assigned to a (Fe,Mg)-O-Al bending mode caused by the formation of a spinel-like interlayer phase after dehydroxylation. Received: 4 June 1999 / Accepted: 6 August 1999  相似文献   

14.
Mineral inclusions in pyrope crystals from Garnet Ridge in the Navajo Volcanic Field on the Colorado Plateau are investigated in this study with emphasis on the oxide minerals. Each pyrope crystal is roughly uniform in composition except for diffusion halos surrounding some inclusions. The pyrope crystals have near constant Ca:Fe:Mg ratios, 0.3 to 5.7 wt% Cr2O3, and 20 to 220 ppm H2O. Thermobarometric calculations show that pyrope crystals with different Cr contents formed at different depths ranging from 50 km (where T ≈ 600 °C and P = 15 kbar) to 95 km (where T ≈ 800 °C and P = 30 kbar) along the local geotherm. In addition to previously reported inclusions of rutile, spinel and ilmenite, we discovered crichtonite series minerals (AM21O38, where A = Sr, Ca, Ba and LREE, and M mainly includes Ti, Cr, Fe and Zr), srilankite (ZrTi2O6), and a new oxide mineral, carmichaelite (MO2−x(OH)x, where M = Ti, Cr, Fe, Al and Mg). Relatively large rutile inclusions contain a significant Nb (up to 2.7 wt% Nb2O5), Cr (up to ∼6 wt% Cr2O3), and OH (up to ∼0.9 wt% H2O). The Cr and OH contents of rutile inclusions are positively related to those of pyrope hosts, respectively. Needle- and blade-like oxide inclusions are commonly preferentially oriented. Composite inclusions consisting mainly of carbonate, amphibole, phlogopite, chlorapatite, spinel and rutile are interpreted to have crystallized from trapped fluid/melt. These minerals in composite inclusions commonly occur at the boundaries between garnet host and large silicate inclusions of peridotitic origin, such as olivine, enstatite and diopside. The Ti-rich oxide minerals may constitute a potential repository for high field strength elements (HFSE), large ion lithophile elements and light rare earth elements (LREE) in the upper mantle. The composite and exotic oxide inclusions strongly suggest an episode of metasomatism in the depleted upper mantle beneath the Colorado Plateau, contemporaneous with the formation of pyrope crystals. Our observations show that mantle metasomatism may deplete HFSE in metasomatic fluids/melts. Such fluids/melts may subsequently contribute substantial trace elements to island arc basalts, providing a possible mechanism for HFSE depletion in these rocks. Received: 20 December 1997 / Accepted: 15 October 1998  相似文献   

15.
Raman spectra of silicate garnets   总被引:2,自引:1,他引:1  
The single-crystal polarized Raman spectra of four natural silicate garnets with compositions close to end-members almandine, grossular, andradite, and uvarovite, and two synthetic end-members spessartine and pyrope, were measured, along with the powder spectra of synthetic pyrope-grossular and almandine-spessartine solid solutions. Mode assignments were made based on a comparison of the different end-member garnet spectra and, in the case of pyrope, based on measurements made on additional crystals synthesized with 26Mg. A general order of mode frequencies, i.e. R(SiO4)>T(metal cation)>T(SiO4), is observed, which should also hold for most orthosilicates. The main factors controlling the changes in mode frequencies as a function of composition are intracrystalline pressure (i.e. oxygen-oxygen repulsion) for the internal SiO4-vibrational modes and kinematic coupling of vibrations for the external modes. Low frequency vibrations of the X-site cations reflect their weak bonding and dynamic disorder in the large dodecahedral site, especially in the case of pyrope. Two mode behavior is observed for X-site cation vibrations along the pyrope-grossular binary, but not along the almandine-spessartine join. Received: 3 December 1996 / Revised, accepted: 13 April 1997  相似文献   

16.
Polarized absorption spectra, σ and π, in the spectral range 30000–400 cm−1 (3.71–0.05 eV) were obtained on crystal slabs // [001] of deep blue rutile at various temperatures from 88 to 773 K. The rutile crystals were grown in Pt-capsules from carefully dried 99.999% TiO2 rutile powder at 50 kbar/1500 °C using graphite heating cells in a belt-type apparatus. Impurities were below the detection limits of the electron microprobe (about 0.005 wt% for elements with Z≥13). The spectra are characterized by an unpolarized absorption edge at 24300 cm−1, two weak and relatively narrow (Δν1/2≈3500–4000 cm−1), slightly σ-polarized bands ν1 at 23500 cm−1 and ν2 at 18500 cm−1, and a complex, strong band system in the NIR (near infra red) with sharp weak peaks in the region of the OH stretching fundamentals superimposed on the NIR system in the σ-spectra. The NIR band system and the UV edge produce an absorption minimum in both spectra, σ and π, at 21000 cm−1, i.e. in the blue, which explains the colour of the crystals. Bands ν1 and ν2 are assigned to dd transitions to the Jahn-Teller split upper Eg state of octahedral Ti3+. The NIR band system can be fitted as a sum of three components. Two of them are partly π-polarized, nearly Gaussian bands, both with large half widths 6000–7000 cm−1, ν3 at 12000 cm–1 and the most intense ν4 at 6500 cm−1. The third NIR band ν5 of a mixed Lorentz-Gaussian shape with a maximum at 3000 cm−1 forms a shoulder on the low-energy wing of ν4. Energy positions, half band widths and temperature behaviour of these bands are consistent with a small polaron type of Ti3+Ti4+ charge transfer (CT). Polarization dependence of CT bands can be explained on the basis of the structural model of defect rutile by Bursill and Blanchin (1983) involving interstitial titanium. Two OH bands at 3322 and 3279 cm−1 in σ-spectra show different stability during annealing, indicating two different positions of proton in the rutile structure, one of them probably connected with Ti3+ impurity. Total water concentration in blue rutile determined by IR spectroscopy is 0.10 wt-% OH. The EPR spectra measured in the temperature interval 20–295 K show the presence of an electron centre at temperatures above 100 K and Ti3+ ions in more than one structural position, but predominantly in compressed interstitial octahedral sites, at lower temperatures. These results are in good agreement with the conclusions based on the electronic absorption data. Received: 24 March 1997 / Revised, accepted: 14 October 1997  相似文献   

17.
Summary  Transmission M?ssbauer spectra of synthetic Ca-free P21/c Mg0.22Fe0.78SiO3 clinopyroxene were collected at temperatures in the range 4.2 to 745 K and in an external magnetic field of 60 kOe at 180 K. The magnetic order-disorder transition temperature was determined by M?ssbauer thermoscanning to be 21 ± 3 K. Above this temperature, all M?ssbauer spectra consist of a superposition of two doublets, respectively produced by Fe2+ ions at an almost regular octahedral M1 site and at a more distorted octahedral M2 site. The temperature variation of the Fe2+ center shifts were analyzed using the Debye model for the lattice vibrations. The characteristic M?ssbauer temperatures were found to be 356 K ± 35 K for M1 and 333 K ± 25 K for M2. From the external field (60 kOe) M?ssbauer spectrum recorded at 180 K, the principal component V zz of the electric field gradient (EFG) was determined to be positive for both sites but precise values for the magnitudes of the asymmetry parameters η of the EFG could not be determined. The temperature variations of the M1 and M2 quadrupole splittings ΔE Q(T ) are consistent with the higher distortion of the M2 octahedra. Using the crystal-field model to interpret ΔE Q(T ), the energy gaps δ1 and δ2 of the first excited electronic states within the 5D orbital term were estimated to be 410 ± 50 cm−1 and 730 ± 50 cm−1 for M1, and δ1 = 1050 ± 75 cm−1 for M2. Received May 29, 2000;/revised version accepted July 13, 2001  相似文献   

18.
 The infrared spectra and electron paramagnetic resonance (EPR) of channel constituents in beryls synthesized hydrothermally in the presence of NH4Cl were investigated. Two forms of ammonium ion were observed to be incorporated into the c -channel. IR-spectra show the double band at 3295 and 3232 cm−1 and two broad bands between 2600 and 3000 cm−1 which were assigned to the NH3 molecule and NH4 + ion, respectively. Similar N–H stretching vibrations are also observed in Regency hydrothermal synthetic beryls and can be used to separate these synthetic beryls from their natural counterparts. After γ-irradiation of hydrothermally grown samples at 77 K, the EPR of the NH3 +(I) radical was observed. The NH3 +(II) radical replaces the NH3 +(I) radical when the sample is heated to room temperature. Both the NH3 molecule and the NH3 + radical have their C3 symmetry axes perpendicular to the crystal c-axis. The spin Hamiltonian parameters of the NH3 +(I) are axial-symmetric due to the rapid rotation of the radical about the c-axis. The NH3 +(II) radical has a low symmetry and shows a hindered rotation because of its shift from the c-axis position and an interaction with the proton in the near neighbourhood. Possible models for the paramagnetic centres are discussed. Received: 16 May 2000 / Accepted: 5 July 2001  相似文献   

19.
 The solubility of hydroxyl in coesite was investigated in multianvil experiments performed at 1200 °C over the nominal pressure range 5–10 GPa, at an f O2 close to the Ni-NiO buffer. The starting material for each experiment was a cylinder of pure silica glass plus talc, which dehydrates at high P and T to provide a source of water and hydrogen (plus enstatite and excess SiO2). Fourier-transform infrared (FTIR) spectra of the recovered coesite crystals show five sharp bands at 3606, 3573, 3523, 3459, and 3299 cm−1, indicative of structurally bonded hydrogen (hydroxyl). The concentration of hydrogen increases with pressure from 285 H/106 Si (at 5 GPa) to 1415 H/106 Si (at 10 GPa). Assuming a model of incorporation by (4H)Si defects, the data are fit well by the equation C OH=Af 2 H2<\INF>Oexp(−PΔV/RT), with A=4.38 H/106 Si/GPa, and ΔV=20.6 × 10−6 m3 mol−1. An alternative model entailing association of hydrogen with cation substitution can also be used to fit the data. These results show that the solubility of hydroxyl in coesite is approximately an order of magnitude lower than in olivines and pyroxenes, but comparable to that in pyropic garnet. However, FTIR investigations on a variety of ultrahigh pressure metamorphic rocks have failed in all cases to detect the presence of water or hydrogen in coesite, indicating either that it grew in dry environments or lost its hydrogen during partial transformation to quartz. On the other hand, micro-FTIR investigations of quartz crystals replacing coesite show that they contain varying amounts of H2O. These results support the hypothesis that preservation of coesite is not necessarily linked to fast exhumation rates but is crucially dependent on limited fluid infiltration during exhumation. Received: 23 August 1999 / Accepted: 10 April 2000  相似文献   

20.
 Structural parameters and cation ordering are determined for four compositions in the synthetic MgGa2O4-Mg2GeO4 spinel solid solution (0, 8, 15 and 23 mol% Mg2GeO4; 1400 °C, 1 bar) and for spinelloid β-Mg3Ga2GeO8 (1350 °C, 1 bar), by Rietveld refinement of room-temperature neutron diffraction data. Sample chemistry is determined by XRF and EPMA. Addition of Mg2GeO4 causes the cation distribution of the MgGa2O4 component to change from a disordered inverse distribution in end member MgGa2O4, [4]Ga = x = 0.88(3), through the random distribution, toward a normal cation distribution, x = 0.37(3), at 23 mol% Mg2GeO4. An increase in ao with increasing Mg2GeO4 component is correlated with an increase in the amount of Mg on the tetrahedral site, through substitution of 2 Ga3+⇄ Mg2++Ge4+. The spinel exhibits high configurational entropy, reaching 20.2 J mol−1 (four oxygen basis) near the compositional upper limit of the solid solution. This stabilizes the spinel in spite of positive enthalpy of disordering over the solid solution, where ΔH D  = αx + βx 2, α = 22(3), β = −21(3) kJ mol−1. This model for the cation distribution across the join suggests that the empirically determined limit of the spinel solid solution is correlated with the limit of tetrahedral ordering of Mg, after which local charge-balanced substitution is no longer maintained. Spinelloid β-Mg3Ga2GeO8 has cation distribution M1[Mg0.50(2)Ga0.50(2)] M2[Mg0.96(2)Ga0.04(2)] M3[Mg0.77(2) Ga0.23(2)]2 (Ge0.5Ga0.5)2O8 (tetrahedral site occupancies are assumed). Octahedral site size is correlated to Mg distribution, where site volume, site distortion, and Mg content follow the relation M1<M3<M2. The disordered cation distribution provides local electrical neutrality in the structure, and stabilization through increased configurational entropy (27.6 J mol−1; eight oxygen basis). Comparison of the crystal structures of Mg1+ N Ga2−2 N Ge N O4 spinel, β-Mg3Ga2GeO8, and Mg2GeO4 olivine reveals β-Mg3Ga2GeO8 to be a true structural intermediate. Phase transitions across the pseudobinary are necessary to accommodate an increasing divergence of cation size and valence, with addition of Mg2GeO4 component. Octahedral volume increases while tetrahedral volume decreases from spinel to β-Mg3Ga2GeO8 to olivine, with addition of Mg and Ge, respectively. Furthermore, M-M distances increase regularly across the join, suggesting that changes in topology reduce cation-cation repulsion. Received: 9 November 1998 / Revised, accepted: 3 August 1999  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号