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1.
Synthetic pyrope crystals up to 0.5 mm in diameter, substituted by titanium or by titanium plus iron, were grown under defined conditions of P, T, $f_{O_2 }$ in the presence of water using a piston-cylinder device. The crystals were characterized by X-ray and microprobe techniques. Their single-crystal optical absorption spectra were measured by means of a microscope-spectrometer. Two absorption bands at 16100 and 22300 cm{cm-1} in the spectra of pale-blue Fe-free Ti-bearing pyropes, grown under reduced conditions, were identified as originating from spin-allowed transitions, derived from 2 T 2g → 2 E g of octahedral Ti3+ ions. The splitting value of the excited 2E g state, 6200 cm-1, and the crystal field parameter of Ti3+ in pyrope Δ 0 = 19 200 cm-1 are both in agreement with literature data. In spectra of brown Fe, Ti-bearing garnets, a broad band at 23000 cm-1 was interpreted as a Fe2+[8] → Ti4+[6] charge-transfer band. The spectral position and width of this band agree with those observed for a FeTi charge transfer band in natural garnets. Fe, Ti-containing garnets synthesized at relatively high oxygen fugacity (10-11,0 atm), which permits a fraction of Fe3+ to enter the garnet, show an additional Fe2+[8] → Fe3+[6] charge transfer band at 19800 cm-1.  相似文献   

2.
 Nuclear reaction analyses on boron in flux-grown Me3+- and Na+-doped diopside crystals utilising the 11B(p,2α)4He reaction in conjunction with EMP analyses for major elements and optical absorption spectroscopy for determination of Me-valence state distributions show that appreciable amounts of B may enter the diopside lattice in crystals produced in Na2B4O7 fluxes. The results indicate that the substitution [6]Me3+[4]B3+[6]Mg2+ −1 [4]Si4+ −1 operates in the present diopside samples in addition to the coupled substitution [6]Me3+[8]Na+[6]Mg2+ −1 [8]Ca2+ −1. The specific NRA technique applied allows for quantitative, high resolution (ca. 5 μm) B analyses at ppm level as well as high-resolution and high-contrast imaging of B-concentration patterns in minerals. The estimated detection limit and relative error of the B analyses are 10 ppm and 5–10%, respectively. Received: 20 September 1999 / Accepted: 6 June 2000  相似文献   

3.
High-pressure electronic absorption spectra at room temperature and at pressures 10?4<P[GPa]<8 were measured in the spectral range 380<λ[nm] <780(26218>ν?[cm?1]>12820) on analysed single crystal slabs, about 20?μm thick, of Cr3+-bearing spinel (I), kyanite (II), corundum (III), pyrope (IV) and uvarovite (V) using DAC-cell techniques in combination with single-beam microscopespectrometry. Ligand field theoretical evaluation of the spectra yielded following results: (i)?the octahedral crystal field parameter, 10DqCr3+[6], linearly shifts on increasing pressure to higher energies with slopes, (δ10DqCr3+[6]/δP), of 103.1 (I), 99.5 (II), 104.0 (III), 111.7 (IV) and 110.3?[cm?1/GPa] (V) (reliability parameters r≥0.92), (ii)?The Racah-parameter BCr3+[6], reflecting the covalency of the Cr–O bonds, does not significantly change with pressure up to 8?GPa, in those cases where it could be evaluated from the spectra (III, IV, V). This result is contrary to the behaviour of BCr3+[6] with increasing temperature (Taran et?al. 1994) and shows that P and T are not inversely correlated parameters with respect to BCr3+[6], a decrease of which reflects an increase in covalency. (iii)?This result enabled to extract octahedral compression moduli, kCr3+[6], from the pressure slopes of 10DqCr3+[6]: 312+48 (I), 297+70 (II), 298+44 (III), 275+35 (IV), 257+32?GPa (V). Quotients kcr3+[6]/kbulk,phase are nearly the same (ca. 1.6) for I, II, IV and V but significantly lower (ca. 1.1) for III. Deviations between spectroscopically determined kCr3+[6] and published kAl[6], obtained by HP-XRD on ruby and pyrope, are interpreted by lattice strain induced by [Cr3+,?Al3+?1][6] substitution.  相似文献   

4.
Examination of schorlomite from ijolite at Magnet Cove (USA) and silicocarbonatite at Afrikanda (Russia), using electron-microprobe and hydrogen analyses, X-ray diffraction and Mössbauer spectroscopy, shows the complexity of substitution mechanisms operating in Ti-rich garnets. These substitutions involve incorporation of Na in the eightfold-coordinated X site, Fe2+ and Mg in the octahedrally coordinated Y site, and Fe3+, Al and Fe2+ in the tetrahedrally coordinated Z site. Substitutions Ti4+Fe3+Fe3+–1Si–1 and Ti4+Al3+Fe3+–1Si–1 are of major significance to the crystal chemistry of schorlomite, whereas Fe2+ enters the Z site in relatively minor quantities (<3% Fe). There is no evidence (either structural or indirect, such as discrepancies between the measured and calculated Fe2+ contents) for the presence of [6]Ti3+ or [4]Ti4+ in schorlomite. The simplified general formula of schorlomite can be written as Ca3Ti4+2[Si3-x(Fe3+,Al,Fe2+)xO12], keeping in mind that the notion of end-member composition is inapplicable to this mineral. In the published analyses of schorlomite with low to moderate Zr contents, x ranges from 0.6 to 1.0, i.e. Ti4+ in the Y site is <2 and accompanied by appreciable amounts of lower-charged cations (in particular, Fe3+, Fe2+ and Mg). For classification purposes, the mole percentage of schorlomite can be determined as the amount of [6]Ti4+, balanced by substitutions in the Z site, relative to the total occupancy in the Y site: ([6]Ti4+–[6]Fe2+–[6]Mg2+– [8]Na+)/2. In addition to the predominant schorlomite component, the crystals examined in this work contain significant (>15 mol.%) proportions of andradite (Ca3Fe3+2Si3O12), morimotoite (Ca3Fe2+TiSi3O12), and Ca3MgTiSi3O12. The importance of accurate quantitative determination and assignment of Fe, Ti and other cations to the crystallographic sites for petrogenetic studies is discussed.
A. R. ChakhmouradianEmail: Phone: +1-204-4747278Fax: +1-204-4747623
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5.
The high-temperature enthalpies of liquid and glassy CaAl2Si2O8 were measured by drop calorimetry using a diphenyl ether drop calorimeter. These data are combined with published values of the high-temperature enthalpy of crystalline anorthite and the enthalpy of vitrification of anorthite to obtain the enthalpy of fusion of anorthite. Analysis of the data yields the following preferred values (enthalpy in kcal/mol, uncertainty limits correspond to two standard deviations):enthalpy of vitrification at 985 K, v H v 985=18.6±0.6; enthalpy of the liquid at 1,830 K, H 1830 l 300 g =130.4±1.2; enthalpy of the glass at 985 K, H 985 g -H 300 g =46.7±0.4; enthalpy of crystalline anorthite between 985 and 1,830 K, H 1830 c -H 985 c =69.9±1.4; calculated enthalpy of fusion of anorthite at 1,830 K, f H 1830= 32.4±2.1.The average heat capacity of supercooled liquid CaAl2Si2O8 between the glass transition (T g 1,086 K) and the melting point (T f7=1,830 K) is 102 ± 2 cal/mol/K. The large difference between the enthalpy of fusion and the enthalpy of vitrification for the minerals anorthite and diopside is emphasized. The practice of assuming fH vH should be discontinued for silicate compounds for which T f T g.  相似文献   

6.
NaBe-cordierites (Fe/(Fe+Mn+Mg)=0.49–0.57) with BeO contents up to 1.16 weight % and additional Li2O contents up to 0.21 wt.% occur in cordierite-apatite-uraninite-muscovite-biotite-chlorite-feldspar-quartz nodules within pegmatites penetrating gneissic roof pendants of a lower Palaeozoic granite batholith. Occasional small crystals of beryl are interpreted to coexist stably with unaltered cordierite. Be and Li are incorporated in cordieriteaccording to the substitutions Na[Channel] + Be[4] Al[4] and Na[Ch]+Li[6]R2+[6], respectively. The coexisting phyllosilicates do apparently not contain appreciable amounts of Li. According to powder IR-data, the analyzed water contents of the cordierites are dominantly of type II, and there is also little CO2. Their distortion indices are rather low (0.121–0.145) and so are their optic angles (2V=50-51°). Considering all eleven NaBeLi-cordierites known thus far there is a strong positive correlation between Na and (Be+Li) with a slope close to 1.0. However, there is virtually no correlation between Be and Li, their incorporation into cordierite depending on the local geochemical environment. A strong negative correlation exists between the distortion indices of the NaBeLi-cordierites and their Be contents. Li has a disturbing influence on this relationship, and the versus Na correlation is also statistically worse than versus Be.Deceased December 20, 1984  相似文献   

7.
The solubility of Gd2Ti2O7 ceramic in acidic solutions (HCl and HClO4) was studied at 250°C and saturation vapor pressure within pH 2.5–5.2. The dissolution process occurs mainly via two reactions: 0.5 Gd2Ti2O7(cr) + 3H+ = Gd3+ + TiO2(cr) + 1.5 H2O at pH < 3 and 0.5Gd2Ti2O7(cr) + H+ + 0.5H2O = Gd(OH) 2 + TiO2(cr) at pH 3–5. The thermodynamic equilibrium constants were calculated at the 0.95 confidence level as log K (1) o = 4.12 ± 0.47; = ?0.97 ± 0.16 at 250°C. It was shown that Gd3+ undergoes hydrolysis in solutions with pH > 3, and the species Gd(OH) 2 + dominates up to at least pH 5. At pH < 3, Gd occurs in solutions as Gd3+. The second constant of Gd3+ hydrolysis was determined at 250°C as K o = ?5.09 ± 0.5, and the thermodynamic characteristics of the initial Gd2Ti2O7 solid phase were determined: S 298.15 o = 251.4 J/(mol K) and ΔfG 298.15 o = ?3630 ± 10 kJ/mol.  相似文献   

8.
This paper describes the distribution of Fe and Ni between the octahedral and tetrahedral sites in pentlandite (Fe,Ni)9S8. The dependence of the distribution on pressure and temperature and the activation energy of the cation exchange reaction were determined through annealing experiments. Synthetic crystals were annealed at 433–723 K and pressures up to 4 GPa, and natural crystals were annealed at 423, 448 and 473 K in evacuated silica capillary tubes for various durations. The cation distributions in the synthetic crystals were determined with an X-ray powder method employing the anomalous dispersion effect of CuK. and FeK radiations, while those of natural crystals were calculated from the cell dimensions. The values of U, S and V for the Fe/Ni exchange reaction are –6818 J mol–1, 20.52 J K–1 mol–1, and 6.99 × 10–6 m3 mol–1, respectively. The dependence of the Fe/Ni distribution on pressure (Pa) and temperature (Kelvin) was determined as lnK = 2.47+8.20 × 102 T –1+8.41 x 10–7 T –1 P, where K = (Fe/Ni)octahedral /(Fe/Ni)tetrahedral. The activation energy of the cation exchange reaction was 185 kJ mol–1.  相似文献   

9.
Brillouin spectroscopy was used to measure the single crystal elastic properties of a pure synthetic pyrope and a natural garnet containing 89.9 mol% of the pyrope end member (Mg3Al2Si3O12). The elastic moduli, c ij , of the two samples are entirely consistent and agree with previous estimates of the elastic properties of pyrope based upon the moduli of solid solutions. Our results indicate that the elastic moduli of pyrope end-member are c 11=296.2±0.5, c 12=111.1±0.6, c 44=91.6±0.3, Ks=172.8±0.3, =92.0±0.2, all in units of GPa. These results differ by several percent from those reported previously for synthetic pyrope, but are based upon a much larger data set. Although the hydrous components of the two samples from the present study are substantially different, representing both dry and saturated samples, we find no discernable effect of structurally bound water on the elastic properties. This is due to the small absolute solubility of water in pyrope, as compared with other garnets such as grossular.  相似文献   

10.
Idiomorphic crystals of roedderite occur in melt-coated cavities of xenoliths of contact-altered quartz-sillimanite and quartz-feldspar gneisses which were ejected with the tephritic lava of the Bellerberg volcano. Physical and chemical properties of three different sets of crystals agree generally with those of roedderites from meteorites, in which so far the mineral had been found exclusively. In detail, however, there are characteristic chemical differences amongst the Eifel roedderites with one set of crystals matching closely the ideal formula (Na,K)2Mg5 Si12O30, a second set containing excess alkalies according to the substitution Na+0.5 Mg2+, and a third set richer in iron having an alkali deficiency following Fe3+Fe2++Na+.The terrestrial roedderites are considered to be precipitates from highly alkaline, MgSi-rich, but Aldeficient gas phases that evolved through contact heating of the gneisses by the tephrite magma.  相似文献   

11.
Low-temperature single-crystal Raman spectrum of pyrope   总被引：1，自引：1，他引：0  
 The single-crystal polarized Raman spectra of synthetic pyrope, Mg3Al2Si3O12, were measured at room temperature and 5 K, as were the room-temperature unpolarized spectra of two natural pyrope-rich crystals. No major differences in the spectra between room temperature and 5 K are observed or are present between the synthetic and the natural crystals. The spectra are consistent with the proposal that the Mg cation is dynamically disordered and not statically distributed over subsites in the large triangular-dodecahedral E-site in pyrope. A low-energy band at about 135 cm−1 softens and shows a large decrease in its line width with decreasing temperature. The presence of a weak, broad band at about 280 cm−1 may be due to anharmonic effects, as could the one at 135 cm−1. The latter is assigned to the rattling motion of Mg in pyrope in the plane of the longer Mg-O(4) bonds (Kolesov and Geiger 1998). The successful modeling of the anisotropic motion of the Mg cation in pyrope, which has an anharmonic character, provides a valuable test of the validity of empirical or semi-empirical lattice-dynamic calculations for silicates. Received: 10 May 1999 / Accepted: 10 April 2000  相似文献   

12.
The reaction 2 epidote+2 calcite+3 wollastonite3 grossular-andraditess+ 2 CO2+1 H2O has been explored by hydrothermal experiments at a total fluid pressure of 1000 bars. For a grossular-andraditess of andradite 25 composition, the isobaric univariant curve passes through the points 458°C: XCO2=0.00; 521°C: XCO2=0.026; 523°C: XCO2=0.052; 526°C: 0.088; 528°C: XCO2=0.104. This curve intersects the isobaric univariant curve of the reaction calcite+quartz+[H2O] wollastonite+CO2+[H2O] at the isobaric invariant point around 528°C and XCO2=0.12. At higher values of XCO2, this reaction is replaced by another one, namely: 2 epidote+5 calcite+3 quartz3 grossular-andraditess+5 CO2+ 1 H2O. It is demonstrated that both the reactions do actually take place during the metamorphism of calcareous rocks. The petrologic significance of contrasted sequence of reactions within this system observed by various workers is also discussed.  相似文献   

13.
Violet, non-pleochroic and greenish-blue, pleochroic chromium-substituted sapphirines were found in corundum-bearing spinel-websterite xenolites from the Yakutian kimberlite pipes Noyabrskaya (N) and Sludyanka (Sl), respectively. The crystallochemical formulae of sapphirine crystals from such xenolites were determined by EMP to be (Mg3.40Fe0.23Al3.25Cr0.16)[6] Al 1.00 [6] [O2/Al4.53Si1.47O18] (N) and (Mg2.53Fe0.55 Mn0.04Ti 0.03 4+ Al3.55Cr 0.08 3+ )[6]Al 1.00 [16] [O2/Al4.28Si1.73O18] (Sl). Single crystal spectra in the range 35000–6000 cm1- showed a slightly polarization dependent absorption edge near 3200 cm1- (N) or 30000 cm1- (Sl) and unpolarized bands at 25300 and 17300 cm1-, typical of spin-allowed transitions, derived from 4A2g→4T1g and 4A2g→4T2g, of Cr3+ in octahedral sites, with point symmetry C1, of the structure. Another weak band at 23000 cm?1 in the sapphirine-N spectra is attributed to low symmetry splitting of the excited 4T1 (F)-State of Cr3+. These assignments lead to crystal field parameters Dq=1730cm?1 and B= 685cm?1 of Cr3+ in sapphirine. Crystallochemical and spectroscopic arguments suggest that Cr3+ subsitutes for Al in the M(1) or M(8) sites of the sapphirine structure. In addition to Cr3+-transitions, spectra of Sl exhibit weak dd-bands of Fe2+ at 10000 and 7700 cm1-, which are unpolarized in consistency with the C1 site symmetry of the octahedra in the structure. Spectra of Sl show also prominent, broad bands (Δv1/2～-5000 cm1-) at 15000 and 11000 cm1-, which occur in E//Y(//b) and E//Z(//c=12°) only and exhibit an intensity ratio αY∶αz close to 1∶3. This result, the large half width, as well as band energy — MM distance considerations suggest that these bands originate from Fe2+[6]-Fe3+[6] charge-transfer transitions in wall octahedra M(1)M(2), M(6)M(7) etc., forming MM vectors of 30° with the c-axis. The lack of Fe2+-Fe3+ charge-transfer bands in sapphirine N might indicate a lower oxygen fugacity during the formation of the websterite from the Noyabrskaya pipe compared to that from the Sludyanka pipe.  相似文献   

14.
The effect of raising temperature on spin-allowed dd-transitions of octahedral Cr3+ was studied for various point symmetries of the Cr3+-bearing structural sites, i.e. 3 m and 3 with inversion center in spinel and garnets, respectively, or 32, 3, 2 and 1, lacking the inversion centre, in beryl, corundum, diopside and topaz, respectively. For this purpose, crystals of Cr3+-bearing spinel, pyrope, andradite, grossular, uvarovite, emerald, ruby, diopside and topaz were analyzed by microprobe, oriented, and measured in polarized radiation (except for the cubic minerals) in the spectral range 30 000 to 11 100 cm-1 and at temperatures between 77 and 797 K. The evaluation of the intensities, half widths, and energy positions of bands due to Cr3+-transitions derived from 4 A 2g 4 T 2g (F) and 4 T 1g (F) as well as of Dq- and B-values derived, had the following results:In all cases, red shift of the above bands and, hence, independent on the site symmetry of Cr3+, decreases in the Dq-values were obtained. The dependcies of Dq on T are nearly linear above room temperature and amount between -1.6% in topaz and -5.1% in pyrope in the temperature range studied. From this, values for the local thermal expansion of the Cr3+-centered octahedra, loc, were derived on the basis of the R M-0 -5 -proportionality of 10Dq. Such values are consistently higher than those obtained from X-ray refinements, a method averaging rm-o for all the respective octahedral positions.  相似文献   

15.
The mineral chemistry of cordierites from three different sanidinite facies localities-1) volcanic xenoliths from the Eifel, Germany; 1) buchites of the Blaue Kuppe, Germany; 3) paralavas from the Bokaro coalfield, India-is characterized by unusually high potassium contents up to 1.71 wt%, equivalent to 0.22 K atoms per formula unit (p.f.u.) based on 18 oxygens. Significantly, these cordierites are either hexagonal highcordierites (indialites) with =0 or exhibit intermediate -values 0<<0.20 relative to well Al,Si-ordered orthorhombic low-cordierite. Based on microprobe analyses, the predominant substitutional mechanism for alkali incorporation is Alk[Channel]+Al[4] for +Si[4], thus leading to Al/Si-ratios deviating considerably from the value 4:5 in ideal cordierite M2[Al4Si5O18]. The most highly substituted cordierite from Blaue Kuppe is about (K0.22Na0.07)[Ch](Mg1.33Fe 0.66 2+ )[6][Al4.16Si4.79O18]. Bokaro cordierites are further characterized by obvious (Al+Si)-deficiencies against the ideal value of 9.0 p.f.u., a tendency of which is apparent in most Blaue Kuppe analyses as well. As the tetrahedral deficiencies are often equivalent to excess cations in the octahedra, we assume that ferric iron fills up the remaining tetrahedral sites, again linked with the introduction of potassium according to K+Fe3+ for +Si. In comparison with the available experimental data, these natural potassic cordierites are considered stable high-temperature phases regarding their compositions, but not their structural states. Although the substitution KAl for Si in Mg-cordierite is known to lower the maximum -value to be attained, the hexagonal nature of the cordierites must be due to very rapid crystallization and subsequent quenching. The higher -values of the Blaue Kuppe cordierites might be caused by their topotactic origin from preexisting biotite. The complicated twin and domain patterns of the hexagonal Eifel and Bokaro cordierites as observed in thin section could perhaps be attributed to structural modulations as postulated recently for hexagonal cordierite shortly after its growth.  相似文献   

16.
Some of the most vanadium-rich silicate minerals known are present in green mica schist from the Hemlo gold deposit, Ontario, Canada. Vanadium-rich silicate minerals include green mica (up to 17.6 wt. % V2O3), phlogopite (10.1 wt. % V2O3), pumpellyite (25.7 wt. % V2O3), garnet (18.5 wt. % V2O3), epidote-group minerals (9.1 wt. % V2O3), antimonian vesuvianite (4.3 wt. % V2O3), and titanite (18.5 wt. % V2O5). In addition, minor amounts of V (<2 wt. % V2O3) are present in tourmaline, chlorite, talc and tremolite in other lithologies of the Hemlo deposit. The principal substitution that incorporates V into most of these silicate minerals is Al3+=V3+ in octahedral positions. Vanadium is incorporated into phlogopite mainly by the two substitutions: 3Mg2+ =2V3++ and VIMg2++IVSi4+=VIV3+ +IVAl3+, and all of the three substitutions Ti4++O2- =V3++(OH,F)-, Ti4+=V4+, and 5Ti4+=4V5+ + may have operated in titanite.Vanadium-enriched green mica schist from the Hemlo gold deposit is characterized by uniform Ti/Zr ratios, systematically low Ti, Ni, Co and Sc abundances, and low levels of incompatible elements Th, U, Hf and Zr and is distinct in these respects from its Cr-enriched counterpart. These geochemical features, along with textural evidence (relict quartz and oligoclase phenocrysts), indicate that the V-enriched green mica schist from Hemlo was most likely derived mainly from quartz-oligoclase porphyry. However, its anomalously high V and Cr contents were probably introduced metasomatically from local maficultramafic sources and were fixed in green mica and oxides during the waning of a second regional metamorphism. Vanadium was further remobilized, and its concentration probably enhanced, during the late hydrothermal alteration, which resulted in the formation of the characteristic V-rich calc-silicate minerals.  相似文献   

17.
An experimental study of the particulars of the solubility and crystallization of brushite Ca(HPO4) · 2H2O from aqueous solution in conditions of a variable pH (6.0–3.0) and the contents of impurity ions (K+, Na+, NH 4 + , Mg2+, SO 4 2? , CO 3 2? ) has been conducted. It is established that brushite solubility markedly rises with a decrease in pH from 6 to 3 and slightly rises with an increase in Mg2+ and SO 4 2? concentrations. The enrichment in K+, Na+, and NH 4 + does not affect brushite solubility. The changeable chemistry of the medium results in variation of the synthetic crystal habit, from rhombic tabular to thickened prismatic crystals.  相似文献   

18.
In pelitic rocks, under conditions of low f O 2 and low f H 2 O, the stability of the mineral pair cordierite-garnet is limited by five univariant reactions. In sequence from high pressure and low temperature to high temperature and low pressure these are: cordierite+garnet hypersthene+sillimanite+quartz, cordierite+garnet hypersthene+sapphirine+quartz, cordierite+garnet hypersthene+spinel+quartz and cordierite+garnet olivine+spinel +quartz. In this sequence of reactions the Mg/Mg+Fe2+ ratio of all ferro-magnesian minerals involved decreases continuously from the first reaction to the fifth. The five univariant boundaries delimit a wide P-T range over which cordierite and garnet may coexist.Two divariant equilibria in which the Mg/Mg+ Fe2+ ratio of the coexisting phases are uniquely determined by pressure and temperature have been studied in detail. P-T-X grids for the reactions cordierite garnet+sillimanite+quartz and cordierite+hypersthene garnet+quartz are used to obtain pressure-temperature estimates for several high grade metamorphic areas. The results suggest temperatures of formation of 700–850° C and load pressures of 5–10 kb. In rare occasions temperatures of 950–1000° C appear to have been reached during granulite metamorphism.On the basis of melting experiments in pelitic compositions it is suggested that Ca-poor garnet xenocrysts found in calc-alkaline magmas derive from admixed pelitic rocks and did not equilibrate with the calc-alkaline magma.  相似文献   

19.
Summary Baotite occurs in the Garaoulére orebody, at Pierreftte, France, as an accessory mineral, included in alstonite and celsian, and associated with sphalerite, galena, pyrite, siderite and calcite in hydrothermal veins crosscutting calcareous, rutile-bearing, siltstones. Microprobe analyses revealed high W03 concentrations (up to 6 wt.%) in baotite. The empirical formula of W-rich baotite is Ba3.959Ti4(Ti3.169W0.393Fe0.116Al0.073 Cr0.048Nb0.024)3.823 Si4.05O28Cl1.166. The excess of charges due to the presence of W6+ and Nb5+ is compensated by the introduction of M3+ (Fe, Al, Cr) into Ti-octahedra, by the appearance of Al in Si-tetrahedra (for W-poor baotite) and by the appearance of vacancies in Ti-octahedra (3Ti4 -> 2W6+ + and in Ba-sites (Ti4+ Ba2+ W6+, ). The unit-cell parameters of W-rich baotite are: a = 19.92(2), c = 5.930(8) Å. Niobium-rich baotites (Baiyun-Obo,Semenov et al., 1961; Karlstein,Nmec, 1987) are characterized by substitutions: Ti4+(VI), Si4+(IV)Nb5+(VI), Al3+(IV) and 2Ti4+, Ba2+ 2Nb5+, .
Wolfram führender Baotit von Pierrefitte, Pyrenäen, Frankreich
Zusammenfassung Baotit kommt in dem Garaoulére Erzkórper in Pierrefitte, Frankreich als ein akzessorisches Mineral in Einschlÿussen in Alstonit und Celsian vor. Er ist mit Zinkblende, Bleiglanz, Pyrit, Siderit und Calcit assoziiert. Diese Paragenese ist an hydrothermale Gänge gebunden, die kalkige rutil-führende Siltsteine durchsetzen. Mikrosondenanalysen zeigen hohe W03 Gehalte (bis zu 6 Gew.%) in Baotit. Die empirische Formel von wolfram-reichem Baotit ist: Ba3.959Ti4(Ti3.169W0.393Fe0.116Al0.073Cr0.048Nb0.024)3.823 Si4.05O28Cl1.66. Der durch die Anwesenheit von W6+ und Nb5+ erforderliche Ladungsausgleich ergibt sich durch das Eintreten von M3+ (Fe, Al; Cr) in Ti-Oktaeder, und von Al in Si-Tetraeder (für W-armen Baotit) und schließlich durch das Erscheinen von Leerstellen in Ti-Oktaedern (3Ti4+ 2W6+ + und in Ba-Stellen (Ti4+, Ba` W6+, Die Zellparameter von ldW-reichem Baotit sind: a = 19.92(2), c = 5.930(8) Å. Niob-reiche Baotite (Baiyun-Obo, Semenov et al., 1961; Karlstein, Nmec, 1987) sind durch Substitutionen charakterisiert: Ti4+(VI), Si4+(IV)Nb5+(VI), Al3+(IV) und 2Ti4+, Ba2+ 2Nb5+, .
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20.
 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   

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