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1.
Thermoluminescence, electron paramagnetic resonance and optical absorption properties of rhodonite, a natural silicate mineral, have been investigated and compared to those of synthetic crystal, pure and doped. The TL peaks grow linearly for radiation dose up to 4 kGy, and then saturate. In all the synthetic samples, 140 and 340°C TL peaks are observed; the difference occurs in their relative intensities, but only 340°C peak grows strongly for high doses. Al2O3 and Al2O3 + CaO-doped synthetic samples presented several decades intenser TL compared to that of synthetic samples doped with other impurities. A heating rate of 4°C/s has been used in all the TL readings. The EPR spectrum of natural rhodonite mineral has only one huge signal around g = 2.0 with width extending from 1,000 to 6,000 G. This is due to Mn dipolar interaction, a fact proved by numerical calculation based on Van Vleck dipolar broadening expression. The optical absorption spectrum is rich in absorption bands in near-UV, visible and near-IR intervals. Several bands in the region from 540 to 340 nm are interpreted as being due to Mn3+ in distorted octahedral environment. A broad and intense band around 1,040 nm is due to Fe2+. It decays under heating up to 900°C. At this temperature it is reduced by 80% of its original intensity. The pink, natural rhodonite, heated in air starts becoming black at approximately 600°C.  相似文献   

2.
The photoluminescence and excitation spectra of sodalites from Greenland, Canada and Xinjiang (China) are observed at 300 and 10 K in detail. The features of the emission and excitation spectra of the orange-yellow fluorescence of these sodalites are independent of the locality. The emission spectra at 300 and 10 K consist of a broad band with a series of peaks and a maximum peak at 648 and 645.9 nm, respectively. The excitation spectra obtained by monitoring the orange-yellow fluorescence at 300 and 10 K consist of a main band with a peak at 392 nm. The luminescence efficiency of the heat-treated sodalite from Xinjiang is about seven times as high as that of untreated natural sodalite. The emission spectrum of the S2 − center in sodalite at 10 K consists of a band with a clearly resolved structure with a series of maxima spaced about 560 cm−1 (20–25 nm) apart. Each narrow band at 10 K shows a fine structure consisting of a small peak due to the stretching vibration of the isotopic species of 32S34S−, a main peak due to that of the isotopic species of 32S2 − and five peaks due to phonon sidebands of the main peak.  相似文献   

3.
Summary The temperature dependence of photoluminescence emission of a natural fluorite has been studied in the wavelength region of 380–500 nm and in the temperature range of 17.5–300 K. The emission spectra of the sample show a broad emission band between 380 and 500 nm for temperatures above 100 K. At 100 K and below, vibronic lines appear on the emission band at approximately 413.3, 418.1, 419.3, 420.2, 423.9 and 427.1 nm. This broad emission band and the vibronic lines in fluorite are usually associated with phonon-coupled electronic transitions from 4f65d to 4f7 in the Eu2+ ion. Temperature dependences of the peak energy, intensity and full-width at half-maximum of the broad emission band are discussed, and the behaviour explained in terms of a configurational coordinate model. The excited state vibrational energy was obtained to be 0.023 ± 0.001 eV and this is lower than the LO phonon energy of 0.062 eV in pure fluorite. The activation energy of thermal quenching of the photoluminescence intensity was found to be 0.022 ± 0.002 eV.  相似文献   

4.
The thermoluminescence properties of nine CO chondrites have been measured. With the exception of Colony and Allan Hills A77307 (ALHA 77307), whose maximum induced TL emission is at approximately 350°C, CO chondrites exhibit two TL peaks, one at 124 ± 7°C (130°C peak) and one at 252 ± 7°C (250°C peak). The 130°C peak shows a 100-fold range in TL sensitivity (0.99 ± 0.21 for Isna to 0.010 ± 0.004 for Colony), and correlates with various metamorphism-related phenomena, such as silicate heterogeneity, metal composition and McSween's metamorphic subtypes. The peak at 250°C does not show these correlations and, Colony excepted, varies little throughout the class (0.3 to 0.07, Colony 0.018 ± 0.004). Mineral separation experiments, and a series of annealing experiments on Isna, suggest that the TL properties for CO chondrites reflect the presence of feldspar in two forms, (1) a form produced during metamorphism, and analogous to the dominant form of feldspar in type 3 ordinary chondrites, and (2) a primary, metamorphism-independent form, perhaps associated with the amoeboid inclusions. If this interpretation is correct, then the CO chondrites have not experienced temperatures above the order/disorder temperature for feldspar (500–600°C) and they cooled more slowly than comparable (i.e. type <3.5) type 3 ordinary chondrites. Colony and ALHA 77307 have atypical TL properties, including very low TL sensitivity, suggesting that phosphors other than feldspar are important. They have apparently experienced less metamorphism than the others, and may have also been aqueously altered.  相似文献   

5.
Unpolarized absorption spectra of single crystals of Cr3+ doped Al2O3 (synthetic ruby) have measured using a new, time-resolving, dispersive, streak photographic system over the range ～350 to ～700 nm during a series of shock loading experiments. The crystal field absorptions assigned to the transition 4 A 2g→4 T 2g were observed to shift in a series of experiments from 555±1 nm at atmospheric pressure to 503±5 nm at 46 GPa. In a single experiment at 32 GPa the 4 A 2g→4 T 1g transition was observed to shift from 405±1 to 386±5 nm. The present data extrapolate downwards in compression toward the 10 GPa data of Stephens and Drickamer (1961) although both crystal field absorption energies increase considerably less with compression than predicted by the simple ionic point charge model. The single datum observed for the Racah parameter B, 588±38 cm?1 at 32 GPa, is consistant with previous results to 10 GPa and the trend of decreasing B, with compression expected from the divergence of the data from the point charge model due to increasing covalancy.  相似文献   

6.
Summary Thermoluminescence (TL) and radioluminescence (RL) wavelength spectra of aragonite have been investigated. Despite the different origin of the samples used, a number of common luminescence features emerges. The spectra for TL peaks appearing near 50, 70 and 170 K are identical with a prominent and broad band centered near 400 nm. In the case of TL peaks appearing at about 350, 380, 420 and 455 K, the onset of new emission bands is due to the characteristic emission of some cation impurities (i.e., Mn2+, Pb2+). Below 200 K, all emission bands encountered for a given TL peak are also present in the RL spectrum obtained at the temperature of the glow peak. The electron-hole recombination mechanisms involved in aragonite appear to be related with those found in rhombohedric carbonates.
Thermolumineszenz und Radiolumineszenz in Aragonit
Zusammenfassung Wellenlängen-Spektra der Thermolumineszenz (TL) and der Radiolumineszenz (RL) von Aragonit wurden untersucht. Trotz des unterschiedlichen Ursprungs der verwen-deten Proben zeigte die Lumineszenz eine Reihe gemeinsamer Züge. Die bei ungefähr 50, 70 und 170 K auftretenden TL-Spektra sind ident und zeigen eine dominierende und breite Bande mit Zentrum bei ca. 400 nm. Die TL-Maxima, die bei ungefähr 350, 380, 420 und 250 K auftreten, gehören zu Spektren, deren wesentliche Züge Kationen-Verunreinigungen wie Mn2+ und Pb2+ zugeordnet werden können. Die TL- und RL-Spektra sind ident, wenn die Temperatur des Kristalls kleiner als 200 K ist. Die Elektron-Loch Rekombinationsmechanismen in Aragonit scheinen mit jenen in den rhomboedrischen Karbonaten verwandt zu sein.
Resumen Dos aragonitos de distinta procedencia se han investigado con et fin de analizar espectralmente sus respectivas emisiones de termoluminiscencia (TL) y radioluminiscencia (RL). A pesar de su distinto origen, ambos aragonitos presentan caracteristicas luminiscentes comunes. Los mâximos de TL que aparecen a 50, 70 y 170 K presentan espectros idénticos. Su principal caracteristica común es la presencia de una prominente y ancha banda cuyo máximo se situa cerca de 400 nm. Los máximos de TL que aparecen alrededor de 350, 380, 420 y 450 K presentan espectros cuya principal caracteristica es la de contener bandas asignables a iones-impureza tales como Mn y Pb. Los espectros de TL y RL son coincidentes siempre que la temperatura del cristal sea inferior a 200 K. Los mecanismos de recombination electron-hueco en et aragonito presentan una cierta similitud con los de los carbonatos romboédricos.

With 6 figures  相似文献   

7.
The optical absorption and luminescence spectra of MgAl2O4:Cr3+ natural spinel (from Ural) have been measured at 77 K and 293 K. The luminescent emission from 4 T 2g , 2 E g covers wide region of 600–750 nm. The emission spectrum at 77 K shows a very rich vibrational structure which can be mainly explained through the vibrational modes of the oxygen octahedron.  相似文献   

8.
Transposed temperature drop calorimetry at 1000 °C was performed on natural zircons (ZrSiO4) from Sri Lanka that were partially to completely metamict due to α-decay event damage (0.06 to 11.7×1015 α-decay events/mg). The enthalpy of annealing at room temperature (ΔHanneal) varies sigmoidally as a function of radiation dose. ΔHanneal reaches a saturation plateau at radiation doses greater than 5×1015 α-decay events/mg. The annealing of several samples to a crystalline structure with broadened diffraction peaks does not significantly affect the enthalpy of annealing. The large magnitude of the enthalpy of annealing plateau, ?59±3 kJ/mol, suggests that the damage to the structure is pervasive on the scale of Angstroms, consistent with the loss of mid-range order characteristic of a glass. The energetics are consistent with, but do not require, chemical heterogeneity caused by micro-domains of amorphous SiO2-rich and ZrO2-rich regions in the metamict state.  相似文献   

9.
Enthalpies of solution of synthetic pentlandite Fe4.5Ni4.5S8, natural violarite (Fe0.2941Ni0.7059)3S4 from Vermillion mine, Sudbury, Ontario, synthetic pyrrhotite, FeS, synthetic high temperature NiS, synthetic vaesite, NiS2, synthetic pyrite, FeS2, Ni and Fe have been measured in a Ni0.6S0.4 melt at 1,100 K. Using these data and the standard enthalpies of formation of binary sulfides, given in literature, standard enthalpies of formation of pentlandite and violarite were calculated. The following values are reported: ΔH f o, Pent =?837.37±14.59 kJ mol?1 and ΔH f o, Viol =?378.02±11.81 kJ mol?1. While there are no thermo-chemical data for pentlandite with which our new value can be compared, an equilibrium investigation of stoichiometric violarite by Craig (1971) gives a significantly less negative enthalpy of formation. It is suggested that the difference may be due to the higher degree of order in the natural sample.  相似文献   

10.
Thermochemical properties have been either measured or estimated for synthetic monazite, LaPO4, and dissakisite, CaLaMgAl2(SiO4)3OH, the Mg-equivalent of allanite. A dissakisite formation enthalpy of ?6,976.5 ± 10.0 kJ mol?1 was derived from high-temperature drop-solution measurements in lead borate at 975 K. A third-law entropy value of 104.9 ± 1.6 J mol?1 K?1 was retrieved from low-temperature heat capacity (C p) measured on synthetic LaPO4 with an adiabatic calorimeter in the 30–300 K range. The C p values of lanthanum phases were measured in the 143–723 K range by differential scanning calorimetry. In this study, La(OH)3 appeared as suitable for drop solution in lead borate and represents an attractive alternative to La2O3. Pseudo-sections were calculated with the THERIAK-DOMINO software using the thermochemical data retrieved here for a simplified metapelitic composition (La = ∑REE + Y) and considering monazite and Fe-free epidotes along the dissakisite-clinozoïsite join, as the only REE-bearing minerals. Calculation shows a stability window for dissakisite-clinozoïsite epidotes (T between 250 and 550°C and P between 1 and 16 kbar), included in a wide monazite field. The P–T extension of this stability window depends on the bulk-rock Ca-content. Assuming that synthetic LaPO4 and dissakisite-(La) are good analogues of natural monazite and allanite, these results are consistent with the REE-mineralogy sequence observed in metapelites, where (1) monazite is found to be stable below 250°C, (2) around 250–450°C, depending on the pressure, allanite forms at the expense of monazite and (3) towards amphibolite conditions, monazite reappears at the expense of allanite.  相似文献   

11.
Natural hydrothermal ZnCO3 crystal aggregates are nominally anhydrous phases with interfacial water, with substitutional divalent cations and decarbonation c. 300°C. All these common features must be involved during the experimental heating of a thermoluminescence (TL) glow curve up to 500°C: dehydration–dehydroxylation, phase transition and ion transition of point defects in Zn2+ positions. A representative specimen of natural smithsonite was analysed by X-ray fluorescence spectrometry, field emission scanning electron microscopy (FESEM) with a chemical probe of energy dispersive spectrometer, high temperature in situ X-ray diffraction, differential scanning calorimetry, differential thermal analyses coupled to thermogravimetry, TL, radioluminescence and high resolution spectra thermoluminescence (3DTL), to gain an overview of the spectra emission and defects linkages modified by heating from room temperature up to 500°C. The ZnCO3 specimen contains minor amounts of Ca, Cu, Cr, Cd, Pb, Ce, Co, Ni, Mn and Fe. Under FESEM, it displays CaCO3 clusters and oscillatory zoning distribution with lamellae ranging from Ca0.11Zn0.89 to Ca0.19Zn0.81. The analytical results suggest assignments of defects and processes to measured 3DTL emission bands, as follows: (1) peak at ~260°C, ~360 nm bonds, breaking during the thermal decarbonation process; (2) peak ~120°C, ~340 nm: non-bridging oxygen centres associated to a complex dehydration–dehydroxylation process and (3) peak at ~170°C, ~650 nm, crystal field effects on the thermoluminescence of Mn2+ centres and associated transitional elements in the ZnCO3 phase. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

12.
Extremely U-depleted (<1 ppm) zircons from H8 banded ores in the East Orebody of the Bayan Obo REE–Nb–Fe deposit are presented, with mineral compositions, textures, 232Th–208Pb SHRIMP ages and petrological context. Cores of East Orebody zircon contain up to 7 wt% HfO2 and are zoned, depicting bipyramidal crystal forms. A distinct generation of patchy, epitaxial rim zircon, similarly depleted in U, is intergrown with rare earth ore minerals (bastnäsite, parisite, monazite). Overprinting aegirine textures indicate paragenetically late, reactive Na-rich fluids. Chondrite-normalized REE patterns without Eu anomalies match closely with those from the Mud Tank and Kovdor carbonatitic zircons. Increased HREE in rims ((Lu/Gd)N 43–112) relative to cores ((Lu/Gd)N 6–7.5) and the localized presence of xenotime are attributable to reactive, mineralizing fluid compositions enriched in Y, REE and P. Cathodoluminescence further reveals HREE fractionation in rims, evidenced by a narrow-band Er3+ emission at 405 nm. The extreme depletion of U in core and rim zircon is characteristic for this mineral deposit and is indicative of a persistent common source. U depletion is also a characteristic for zircons from carbonatitic or kimberlitic systems. 232Th–208Pb (SHRIMP II) geochronological data reveal the age of zircon cores as 1,325 ± 60 Ma and a rim-alteration event as 455.6 ± 28.27 Ma. The combined findings are consistent with a protolithic igneous origin for zircon cores, from a period of intrusive, alkaline–carbonatitic magmatism. Fluid processes responsible for the REE–Nb mineralizations affected zircon rim growth and degradation during the widely reported Caledonian events, providing a new example in a localized context of HREE enrichment processes.  相似文献   

13.
The thermoelastic parameters of Ca3Cr2Si3O12 uvarovite garnet were examined in situ at high pressure up to 13 GPa and high temperature up to 1100 K by synchrotron radiation energy-dispersive X-ray diffraction within a 6-6-type multi-anvil press apparatus. A least-square fitting of room T data to a third-order Birch–Murnaghan (BM3) EoS yielded K0 = 164.2 ± 0.7 GPa, V0 = 1735.9 ± 0.3 Å3 (K’0 fixed to 4.0). P–V–T data were fitted simultaneously by a modified HT-BM3 EoS, which gave the isothermal bulk modulus K0 = 163.6 ± 2.6 GPa, K’0 = 4.1 ± 0.5, its temperature derivative (?K0,T/?T)P = –0.014 ± 0.002 GPa K?1, and the thermal expansion coefficients a0 = 2.32 ± 0.13 ×10?5 K?1 and b0 = 2.13 ± 2.18 ×10?9 K?2 (K’0 fixed to 4.0). Our results showed that the Cr3+ enrichment in natural systems likely increases the density of ugrandite garnets, resulting in a substantial increase of mantle garnet densities in regions where Cr-rich spinel releases chromium through a metasomatic reaction.  相似文献   

14.
Synthetic ringwoodite γ-(Mg1?x Fe x )2SiO4 of 0.4 ≤ x ≤ 1.0 compositions and variously colored micro-grains of natural ringwoodite in shock metamorphism veins of thin sections of two S6-type chondrites were studied by means of microprobe analysis, TEM and optical absorption spectroscopy. Three synthetic samples were studied in addition with Mössbauer spectroscopy. The Mössbauer spectra consist of two doublets caused by VIFe2+ and VIFe3+, with IS and QS parameters close to those established elsewhere (e.g., O’Neill et al. in Am Mineral 78:456–460, 1993). The Fe3+/Fetotal ratio evaluated by curve resolution of the spectra, ranges from 0.04 to 0.1. Optical absorption spectra of all synthetic samples studied are qualitatively very similar as they are directly related to the iron content. They differ mostly in the intensity of the observed absorption features. The spectra consist of a very strong high-energy absorption edge and a series of absorption bands of different width and intensity. The three strongest and broadest absorptions of them are attributed to splitting of electronic spin-allowed 5 T 2g → 5 E g transitions of VIFe2+ and intervalence charge-transfer (IVCT) transition between ferrous and ferric ions in adjacent octahedral sites of the ringwoodite structure. The spin-allowed bands at ca. 8,000 and 11,500 cm?1 weakly depend on temperature, whilst the Fe2+/Fe3+ IVCT band at ~16,400 cm?1 displays very strong temperature dependence: i.e., with increasing temperature it decreases and practically disappears at about 497 K, a behavior typical for bands of this type. With increasing pressure the absorption edge shifts to lower energies while the spin-allowed bands shift to higher energy and strongly decreases in intensity. The IVCT band also strongly weakens and vanishes at about 9 GPa. We assigned this effect to pressure-induced reduction of Fe3+ in ringwoodite. By analogy with synthetic samples three broad bands in spectra of natural (meteoritic) blue ringwoodite are assigned to electronic spin-allowed transitions of VIFe2+ (the bands at ~8,600 and ~12,700 cm?1) and Fe2+/Fe3+ IVCT transition (~18,100 cm?1), respectively. Spectra of colorless ringwoodite of the same composition consist of a single broad band at ca. 12,000 cm?1. It is assumed that such ringwoodite grains are inverse (Fe, Mg)2SiO4-spinels and that the single band is caused by the split spin-allowed 5 E → 5 T 2 transition of IVFe2+. Ringwoodite of intermediate color variations between dark-blue and colorless are assumed to be partly inversed ringwoodite. No glassy material between the grain boundaries in the natural colored ringwoodite aggregates was found in our samples and disprove the cause of the coloration to be due to light scattering effect (Lingemann and Stöffler in Lunar Planet Sci 29(1308), 1998).  相似文献   

15.
Four samples of synthetic chromium-bearing spinels of (Mg, Fe2+)(Cr, Fe3+)2O4 composition and four samples of natural spinels of predominantly (Mg, Fe2+)(Al, Cr)2O4 composition were studied at ambient conditions by means of optical absorption spectroscopy. Synthetic end-member MgCr2O4 spinel was also studied at pressures up to ca. 10 GPa. In both synthetic and natural samples, chromium is present predominantly as octahedral Cr3+ seen in the spectra as two broad intense absorption bands in the visible range caused by the electronic spin-allowed 4 A 2g  → 4 T 2g and 4 A 2g  → 4 T 1g transitions (U- and Y-band, respectively). A distinct doublet structure of the Y-band in both synthetic and natural spinels is related to trigonal distortion of the octahedral site in the spinel structure. A small, if any, splitting of the U-band can only be resolved at curve-fitting analysis. In all synthetic high-chromium spinels, a couple of relatively narrow and weak bands of the spin-allowed transitions 4 A 2g  → 2 E g and 4 A 2g  → 2 T 1g of Cr3+, intensified by exchange-coupled interaction between Cr3+ and Fe3+ at neighboring octahedral sites of the structure, appear at ~14,400 and ~15,100 cm?1. A vague broad band in the range from ca. 15,000 to 12,000 cm?1 in synthetic spinels is tentatively attributed to IVCr2+ + VICr3+ → IVCr3+ + VICr2+ intervalence charge-transfer transition. Iron, mainly as octahedral Fe3+, causes intense high-energy absorption edge in near UV-range (ligand–metal charge-transfer O2? → Fe3+, Fe2+ transitions). As tetrahedral Fe2+, it appears as a strong infrared absorption band at around 4,850 cm?1 caused by electronic spin-allowed 5 E → 5 T 2 transitions of IVFe2+. From the composition shift of the U-band in natural and synthetic MgCr2O4 spinels, the coefficient of local structural relaxation around Cr3+ in spinel MgAl2O4–MgCr2O4 system was evaluated as ~0.56(4), one of the lowest among (Al, Cr)O6 polyhedra known so far. The octahedral modulus of Cr3+ in MgCr2O4, derived from pressure-induced shift of the U-band of Cr3+, is ~313 (50) GPa, which is nearly the same as in natural low-chromium Mg, Al-spinel reported by Langer et al. (1997). Calculated from the results of the curve-fitting analysis, the Racah parameter B of Cr3+ in natural and synthetic MgCr2O4 spinels indicates that Cr–O-bonding in octahedral sites of MgCr2O4 has more covalent character than in the diluted natural samples. Within the uncertainty of determination in synthetic MgAl2O4 spinel, B does not much depend on pressure.  相似文献   

16.
本文从五台地区滹沱群豆村亚群四集庄组、东冶亚群纹山组和郭家寨亚群西河里组地层中共采集了5件浅变质砂岩样品,并对其进行了La-MC-ICPMS锆石U-Pb年龄测定。分析结果显示,四集庄组2件砂岩样品碎屑锆石207Pb/206Pb年龄主要集中于~2.5Ga和2.1~2.2Ga两个峰值,其中~2.5Ga碎屑锆石来自新太古代五台群和五台地区花岗质杂岩;2.1~2.2Ga碎屑锆石获得207Pb/206Pb加权平均年龄2134±5Ma,限定了四集庄组砂岩沉积下限为2134Ma。结合四集庄组火山岩形成时代(2140±10Ma)和四集庄组底部发育厚层砾岩,我们认为滹沱群初始形成时代为~2.2Ga,即早元古代中期。东冶亚群纹山组底部砂岩中碎屑锆石207Pb/206Pb年龄主要集中于2050~2122Ma之间,其中64粒相对年轻的锆石获得207Pb/206Pb加权平均年龄2068±3Ma,代表了东冶亚群形成时代下限为2070Ma左右。综合豆村亚群青石村组火山岩形成时代2087±9Ma,我们认为东冶亚群初始形成于2070Ma左右。郭家寨亚群中最年轻碎屑锆石207Pb/206Pb年龄为1958±10Ma,表明郭家寨亚群开始沉积时代小于1.95Ga,为早元古代晚期/末期。区域上,早元古代末期是华北最终克拉通阶段,而郭家寨亚群与东冶亚群呈明显的角度不整合接触关系,两者记录了明显不同的地质过程。因此,我们建议郭家寨亚群应从滹沱群中解体出来并独立命名为郭家寨群,且郭家寨群可能沉积于华北克拉通化过程中/之后,开始沉积的时代为1.9~1.8Ga。  相似文献   

17.
1 Introduction According to recent researches, the North China Craton consists of three parts: the eastern block, western block and central zone (Zhao, 2001; Wilde et al., 2002). Paleoarchean continental blocks and zircon residuals have only been found in a few regions, such as Anshan, East Liaoning (Liu et al., 1992; Song et al., 1996; Wan et al., 2002, 2005), Caozhuang, East Hebei (Liu et al., 1992) and Xinyang, West Henan (Zheng et al., 2004), which are mainly distributed in the east…  相似文献   

18.
The shock-metamorphosed quartz exhibits thermal luminescence (TL) with maxima at 365 nm, 470 nm and 610–680 nm. By electron paramagnetic resonance (EPR) analysis E1 type electron centers and hole centers have been found which originate from vacancies including those from the substitution of Al3+ and/or Fe3+, for Si4+. The EPR and TL spectra may be interpreted mainly in terms of vacancy type defects associated with dislocations in the crystal structure of quartz.  相似文献   

19.
Synthetic, flux-grown uvarovite, Ca3Cr2 [SiO4]3, was investigated by optical methods, electron microprobe analysis, UV-VIS-IR microspectrometry, and luminescence spectroscopy. The crystal structure was refined using single-crystal X-ray CCD diffraction data. Synthetic uvarovite is optically isotropic and crystallizes in the “usual” cubic garnet space group Ia3¯d [a=11.9973 Å, Z=8; 21524 reflections, R1=2.31% for 454 unique data and 18 variables; Cr–O=1.9942(6), Si–O=1.6447(6), Ca–Oa=2.3504(6), Ca–Ob= 2.4971(6) Å]. The structure of Ca3Cr2[SiO4]3 complies with crystal-chemical expectations for ugrandite group garnets in general as well as with predictions drawn from “cubically averaged” data of non-cubic uvarovite–grossular solid solutions (Wildner and Andrut 2001). The electronic absorption spectra of Cr3+ in trigonally distorted octahedra of synthetic uvarovite were analyzed in terms of the superposition model (SM) of crystal fields. The resulting SM and interelectronic repulsion parameters are =9532 cm?1, =4650 cm?1, power law exponent t 4=6.7, Racah B35=703 cm?1 at 290 K (reference distance R 0=1.995 Å; fixed power law exponent t 2=3 and spin-orbit parameter ζ=135 cm?1). The interelectronic repulsion parameters Racah B 55=714 cm?1 and C=3165 cm?1 were extracted from spin-forbidden transitions. This set of SM parameters was subsequently applied to previously well-characterized natural uvarovite–grossular solid solutions (Andrut and Wildner 2001a; Wildner and Andrut 2001) using their extrapolated Cr–O bond lengths to calculate the energies of the spin-allowed bands. These results are in very good agreement with the experimentally determined band positions and indicate the applicability of the superposition model to natural 3d N prevailing systems in geosciences. Single-crystal IR absorption spectra of synthetic uvarovite in the region of the OH-stretching vibration exhibit one isotropic absorption band at 3508 cm?1 at ambient conditions, which shifts to 3510 cm?1 at 77 K. This band is caused by structurally incorporated hydroxyl groups via the (O4H4)-hydrogarnet substitution. The water content, calculated using an integral extinction coefficient ?=60417 cm?2 l mol?1, is c H2O=33 ppm.  相似文献   

20.
Hydrothermal scheelite was synthesized using Na2WO4 · 2 H2O mixed with CaCl2 · H2O, CaSO4 · 2 H2O or CaF2 at different temperatures (270–720° C) and 108 Pa. The morphology of the crystals depends on the starting products. The observed faces include the {112}, {114}, {011}, and {013} forms. Pure or REE doped scheelites were studied by thermoluminescence (TL), fluorescence and electron paramagnetic resonance (EPR). The main TL peaks are located near 88, 149, 216, 277, and 315 K. Results obtained with EPR or optical fluorescence have been correlated with TL measurements and show that the trivalent lanthanide elements substitute for calcium ions without site distortion. The differences in TL observed between Eu and the other doping elements are related to the greater stability of Eu2+ caused by X-irradiation.  相似文献   

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