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1.
 The results of a rigid unit mode (RUM) analysis of the inherent displacive structural flexibility of the tetrahedral framework of the ideal hexacelsian structure type are presented. One of the three types of RUM found to exist is characterized by modulation wave vectors perpendicular to <110> and atomic displacement patterns involving tetrahedral rotation around the parent c axis while a second type of RUM is found to be soft at any modulation wave vector and to involve tetrahedral rotation about in-plane rotation axes. It is shown how a combination of these two types of RUM motion associated with the same q=1/2<1ˉ101>* modulation wave vector enables the outstanding crystal chemical problems and apparently mutually contradictory results as regards polymorphism in Ba-hexacelsian to be resolved. Received: 30 December 1999 / Accepted: 16 May 2000  相似文献   

2.
Two new isostructural compounds with fresnoite, Sr2TiSi2O8 and Ba2TiGe2O8, have been synthesized and investigated by X-ray diffraction and vibrational spectroscopy. The infrared (IR) and Raman spectra of fresnoite and fresnoite-like pyrosilicates are discussed on the basis of a factor group analysis and with the help of 28–30Si and 46–50Ti isotopic shifts. The short Ti-O bond participates to the 900–800 cm?1 region of the spectrum. However, because of an important mixing with some Si-O stretching modes having the same symmetry properties, no frequency is specifically related to the stretching of this bond. An assignment is also proposed for the remaining Si-O stretching frequencies. The existence or absence of a correlation between short Ti-O distances and high vibrational frequencies is briefly discussed.  相似文献   

3.
 We have studied the plastic deformation of Mg2SiO4 wadsleyite polycrystals. Wadsleyite was synthesized from a forsterite powder in a multianvil apparatus. It was then recovered and placed in a second multianvil assembly designed to induce plastic deformation by compression between two hard alumina pistons. After the deformation experiment, the microstructures are characterized by transmission electron microscopy (TEM) and large-angle convergent beam electron diffraction (LACBED). Deformation experiments have been carried out at 15–19 GPa and at temperatures ranging from room temperature to 1800–2000 °C. Five different dislocation types have been identified by LACBED: [100], 1/2〈111〉, [010], 〈101〉 and [001]. The [001] dislocations result from dislocation reactions and not from activation of a slip system. The [010] dislocations are activated under high stresses at the beginning of the experiments and further relax by decomposition into 1/2〈111〉 dislocations or by dissociation into four 1/4[010] partial dislocations. The following slip systems have been identified: 1/2〈111〉{101}, [100](010), [100](001), [100]{011}, [100]{021}, [010](001), [010]{101} and 〈101〉(010). Received: 15 July 2002 / Accepted: 14 February 2003 Acknowledgements High-pressure experiments were performed at the Bayerisches Geoinstitut under the EU IHP – Access to Research Infrastructures Programme (Contract no. HPRI-1999-CT-00004 to D.C. Rubie). P.C. has benefited from a Congé thématique pour recherche from the University of Lille, and would like to thank warmly all the people in Bayreuth who contributed to this work by daily assistance and discussions: Nathalie Bolfan-Casanova, Daniel Frost, Jed L. Mosenfelder and Brent Poe. The quality of the preparation of the TEM specimens by H. Schultze is greatly appreciated.  相似文献   

4.
We report here a single-crystal polarized-light study of stoppaniite, ideally (Fe,Al,Mg)4(Be6Si12O36)(H2O)2(Na,□), from Capranica (Viterbo). Polarized-light FTIR spectra were collected on an oriented (hk0) section, doubly polished to 15 μm. The spectrum shows two main bands at 3,660 and 3,595 cm−1; the former is strongly polarized for E ⊥c, while the latter is polarized for E //c. A sharp and very intense band at 1,620 cm−1, plus minor features at 4,000 and 3,228 cm−1 are also polarized for E //c. On the basis of literature data and considering the pleochroic behavior of the absorptions, the 3,660 cm−1 band is assigned to the ν3 stretching mode and the 1,620 cm−1 (associated with an overtone 2*ν2 at 3,230 cm−1) band to the ν2 bending mode of “type II” water molecules within the structural channels of the studied beryl. The sharp band at 3,595 cm−1 is not associated with a corresponding ν2 bending mode; thus it is assigned to the stretching vibration of O–H groups in the sample. The minor 4,000 cm−1 feature can be assigned to the combination of the O–H bond parallel to c with a low-frequency metal-oxygen mode such as the Na–O stretching mode. The present results suggest that the interpretation of the FTIR spectrum of Na-rich beryl needs to be carefully reconsidered.  相似文献   

5.
The effects of pressure and oxygen fugacity (fO2) on trace element partitioning between pargasitic amphibole and alkali-basaltic melts have been determined at pressures from 1.5 to 2.5 GPa and oxygen fugacities at 2 log units above and below the nickel–nickel oxide buffer. Amphibole crystallization experiments were performed in a piston cylinder apparatus and partition coefficients between amphibole and quenched melt of large-ion-lithophile elements (LILE: Rb, Sr, Ba), high-field-strength elements (HFSE: Zr, Nb, Ta, Hf, U, Th) and rare-earth elements (REE: La to Lu; +Y) were measured with a LASER ablation inductively coupled plasma – mass spectrometer. Increasing pressure from 1.5 to 2.5 GPa at similar temperatures and approximately constant fO2 increases D Rb but decreases D Zr and D Hf and D REE (D La, D Ce, D Pr). An empirical relationship was observed between D Zr and (Ti/Al)M2 in the amphibole, which can be described by:
Increasing the fO2 by ∼4 log units (∼NNO–2.0 to ∼NNO+2.2) at similar temperatures and constant pressure increases D Ba and D Nd but decreases D Ti. An increase in pressure or fO2 decreases the maximum partition coefficient (D o ), the Young's modulus (E) and the optimum ionic radius (r o ) of the A-, M2- and M4-lattice sites. The calculated r o values from the monovalent cations (Na, K, Rb) in the A site and the quadrivalent cations (Ti, Hf, Zr) in the M2 lattice sites suggests that amphiboles crystallized from alkaline basalt material have smaller 〈A-O〉 and 〈M2-O〉, mean bond-lengths than those formed from pargasitic materials at identical pressures and fO2's. The measured partition coefficients were used to calculate trace element concentrations in melts formed by partial melting of amphibole-bearing peridotite. This modeling demonstrates those changes in either the pressure or fO2 of melting can exert a significant effect on Rb/HFSE ratios in the melts and thus help explain the wide variations of these ratios sometimes observed in basaltic rock suites. Received: 7 August 1998 / Accepted: 7 June 2000  相似文献   

6.
Wadeite K2ZrSi3O9 and its analogues K2TiSi3O9 and Cs2ZrSi3O9, synthesized by high-temperature solid-state sintering, have been investigated using powder X-ray diffraction coupled with Rietveld analysis and high-temperature oxide melt solution calorimetry. The crystal chemistry and energetics of these phases, together with K2SiVISi3 IVO9, a high-pressure wadeite analogue containing both tetrahedral and octahedral Si, are discussed in term of ionic substitutions. As the size of the octahedral framework cation increases, Si4+ → Ti4+ → Zr4+, the cell parameter c increases at a much higher rate than a. In contrast, increasing the interstitial alkali cation size (K+ → Cs+) results in a higher rate of increase in a compared with c. This behavior can be attributed to framework distortion around the interstitial cation. The enthalpies of formation from the constituent oxides (ΔHf,ox0) and from the elements (ΔHf,el0) have been determined from drop-solution calorimetry into 2PbO·B2O3 solvent at 975 K. The obtained values (in kJ/mol) are as follows: ΔHf,ox0 (K2TiSi3O9) = −355.8 ± 3.0, ΔHf,el0 (K2TiSi3O9) = −4395.1 ± 4.8, ΔHf,ox0 (K2ZrSi3O9) = −374.3 ± 3.3, ΔHf,el0 (K2ZrSi3O9) = −4569.9 ± 5.0, ΔHf,ox0 (Cs2ZrSi3O9) = −396.6 ± 4.4, and ΔHf,el0 (Cs2ZrSi3O9) = −4575.0 ± 5.5. The enthalpies of formation for K2SiVISi3 IVO9 were calculated from its drop-solution enthalpy of an earlier study (Akaogi et al. 2004), and the obtained ΔHf,ox0 (K2SiSi3O9) = −319.7 ± 3.4 and ΔHf,el0 (K2SiSi3O9) = −4288.7 ± 5.1 kJ/mol. With increasing the size of the octahedral framework cation or of the interstitial alkali cation, the formation enthalpies become more exothermic. This trend is consistent with the general behavior of increasing energetic stability with decreasing ionic potential (z/r) seen in many oxide and silicate systems. Further, increasing the size of the octahedral framework cation appears to induce more rapid increase in stability than increasing the interstitial alkali cation size, suggesting that framework cations play a more dominant role in wadeite stability.  相似文献   

7.
 The crystal structure of intermediate incommensurate tridymite was refined at 150 °C from powder data. Upon cooling from above 220 °C, the basic structure with space group symmetry C2221 is gradually distorted from orthorhombic to monoclinic symmetry. With decreasing temperature, the monoclinic angle γ smoothly opens up to 90.3°, while a displacive modulation with temperature-dependent wavelength develops. The 3 + 1 dimensional superspace group of the incommensurate phase is C1121(αβ0). The modulation mainly consists of two sinusoidal transverse displacement waves for the silicon atoms coupled to rotations of the rigid SiO4/2 tetrahedra. The wave vector is r=0.1192(1)a* − 0.0043(1)b* at 150 °C. Below 150 °C tridymite discontinuously transforms to another orthorhombic phase and the modulation partially locks in at the wave vector r 1=1/3a*. Simultaneously, an additional incommensurate modulation with r 2= 0.0395(1)b* − 0.3882(1)c* is formed. The two-dimensional modulation does not vary significantly with the temperature. Received: 13 September 2000 / Accepted: 29 January 2001  相似文献   

8.
 Synthetic aegirine LiFeSi2O6 and NaFeSi2O6 were characterized using infrared spectroscopy in the frequency range 50–2000 cm−1, and at temperatures between 20 and 300 K. For the C2/c phase of LiFeSi2O6, 25 of the 27 predicted infrared bands and 26 of 30 predicted Raman bands are recorded at room temperature. NaFeSi2O6 (with symmetry C2/c) shows 25 infrared and 26 Raman bands. On cooling, the C2/c–P21/c structural phase transition of LiFeSi2O6 is characterized by the appearance of 13 additional recorded peaks. This observation indicates the enlargement of the unit cell at the transition point. The appearance of an extra band near 688 cm−1 in the monoclinic P21/c phase, which is due to the Si–O–Si vibration in the Si2O6 chains, indicates that there are two non-equivalent Si sites with different Si–O bond lengths. Most significant spectral changes appear in the far-infrared region, where Li–O and Fe–O vibrations are mainly located. Infrared bands between 300 and 330 cm−1 show unusually dramatic changes at temperatures far below the transition. Compared with the infrared data of NaFeSi2O6 measured at low temperatures, the change in LiFeSi2O6 is interpreted as the consequence of mode crossing in the frequency region. A generalized Landau theory was used to analyze the order parameter of the C2/c–P21/c phase transition, and the results suggest that the transition is close to tricritical. Received: 21 January 2002 / Accepted: 22 July 2002  相似文献   

9.
Experiments ranging from 2 to 3 GPa and 800 to 1300 °C and at 0.15 GPa and 770 °C were performed to investigate the stability and mutual solubility of the K2ZrSi3O9 (wadeite) and K2TiSi3O9 cyclosilicates under upper mantle conditions. The K2ZrSi3O9–K2TiSi3O9 join exhibits complete miscibility in the P–T interval investigated. With increasing degree of melting the solid solution becomes progressively enriched in Zr, indicating that K2ZrSi3O9 is the more refractory end member. At 2 GPa, in the more complex K2ZrSi3O9–K2TiSi3O9–K2Mg6Al2Si6O20(OH)4 system, the presence of phlogopite clearly limits the extent of solid solution of the cyclosilicate to more Zr-rich compositions [Zr/(Zr + Ti) > 0.85], comparable to wadeite found in nature, with TiO2 partitioning strongly into the coexisting mica and/or liquid. However, at 1200 °C, with increasing pressure from 2 to 3 GPa, the partitioning behaviour of TiO2 changes in favour of the cyclosilicate, with Zr/(Zr + Ti) of the K2(Zr,Ti)Si3O9 phase decreasing from ∼0.9 to ∼0.6. The variation in the Ti content of the coexisting phlogopite is related to its degree of melting to forsterite and liquid, following the major substitution VITi+VI□=2VIMg. Received: 26 January 1999 / Accepted: 10 January 2000  相似文献   

10.
Raman spectra and lattice dynamics calculations are presented for the silicate mineral gillespite, BaFeSi4O10, which contains sheets of linked four-membered silicate rings. The results are analyzed in relation to earlier work done on the isolated four-membered ring silicate BaCuSi2O6 and to published claims that vibrational modes of four-membered rings are responsible for the sharp D 1“defect line” observed in the Raman spectra of SiO2 glass. The crystal structure of gillespite (space group P4/ncc or D ) consists of puckered Si4O12 rings, where each SiO4 tetrahedron is linked to two neighboring tetrahedra within a ring and to a third tetrahedron within a different ring. The rings are linked to each other in a staggered configuration to form sheets, which are also bonded together by Ba2+ and Fe2+. The calculation adjusts the bond bending and bond stretching force constants so that calculated fundamental mode frequencies best fit observed fundamental frequencies in the Raman spectra; eigenmodes associated with each calculated fundamental mode are then generated. Some eigenmodes calculated are unique to the gillespite structure, but many of the more localized four-membered ring modes are similar to those calculated for the four-membered rings in BaCuSi2O6 and for the three- and six-membered rings in the cyclosilicates studied earlier. The results for gillespite indicate that the Raman-active A 1g mode at 450 cm−1 is a four-membered ring bridging oxygen breathing mode that is mixed with other minor ring deformation displacements; because of this mixing, the calculated frequency of this mode is 45 to 61 cm−1 lower than that calculated for bridging oxygen binding breathing modes of puckered four-membered rings in BaCuSi2O6 and in simulated glass structures. Received: 9 April 1997 / Revised, accepted: 19 August 1997  相似文献   

11.
 A series of titanium silicate glasses along the composition joins SiO2–TiO2, Na2SiO3–TiO2, K2SiO3–TiO2, and CaSiO3–TiO2 have been examined using titanium (Ti) L-edge X-ray absorption near-edge spectroscopy (XANES). XANES spectra were collected at the Canadian Synchrotron Radiation Facility (CSRF), University of Wisconsin, Madison, using the Spherical Grating Monochromator (SGM) beamline. Glass spectra were compared with spectra obtained from crystalline analogues with differing Ti coordination environments: βrBa2TiO4 ([4]Ti); fresnoite ([5]Ti); and rutile and anatase ([6]Ti). The Ti L-edge data indicate that for homogeneous TiO2–SiO2 glasses Ti coordination is [5]Ti at TiO2 contents below 3.6 wt% but predominantly [4]Ti with some [5]Ti present above this content. There is no evidence for [6]Ti. For the Na2 O-containing glasses the L-edge data indicate that Ti is [4]Ti with some [5]Ti at low TiO2 contents, becomes a mix of [4]Ti and [5]Ti with increasing TiO2 content, but is exclusively [5]Ti by 14.3 wt% TiO2. The K2O composition glasses exhibit similar behavior but contain a greater proportion of [4]Ti and less [5]Ti than the equivalent Na2O-bearing glasses. These findings are consistent with previous Raman and XANES pre-edge studies. Alkaline-earth-containing glasses behave somewhat differently, with [5]Ti occurring in low TiO2 content glasses, becoming a mix of [4]Ti and [5]Ti, and then gradually changing to predominantly [4]Ti at higher TiO2 compositions. Finally, we have obtained data for a fresnoite composition (Ba2TiSi2O8) glass. Previous pre-edge and Raman data had suggested that this composition glass contained [5]Ti; however, our Ti L-edge data indicate that Ti is almost exclusively [4]Ti, although some [5]Ti may also be present. Received: 20 December 2000 / Accepted: 10 July 2001  相似文献   

12.
The kinetics of monodomain order-disorder processes in monoclinic (I2/c) BaAl2Ge2O8-feldspar have been investigated by X-ray powder diffraction, Hard Mode IR Spectroscopy, and TEM darkfield imaging on quenched samples. Compared to the behaviour predicted by the TDGL approximation ordering kinetics observed at low temperatures slow down significantly when equilibrium is approached. Such a delay is not observed in disordering experiments starting from essentially ordered cation distributions. The deviation from TDGL behaviour is interpreted in terms of partial order parameter conservation in a non-uniformly ordered phase. Modifications to the uniform TDGL rate equation are tested against the available data. An activation energy of 352 ± 28 kJ/mol is obtained for Al,Ge ordering. The mixing coefficient ξc 2/ξ2, which describes the degree of order parameter conservation, is obtained as a function of temperature. While this coefficient vanishes in the vicinity of the transformation temperature T tr, it saturates towards a level of ξc 2/ξ2≈0.4 for T→0. ξ c 2/ξ2 determines the kinetic stability of ordered clusters quenched from T≈T tr. Received: 21 April 1999 / Revised, accepted: 19 July 1999  相似文献   

13.
Bond critical point properties calculated for the MN bonds in a number of geometry optimized nitride molecules containing first- and second-row M cations are compared with those calculated for a number of oxide molecules. As reported for the oxides, the value of the electron density, ρ(r c ), at the bond critical points, r c , increases with decreasing bond length while for the more electronegative cations, the local energy density, H(r c ) decreases nonlinearly in value as the relative electronegativities of the M-cations, χ M , tend to increase. In the majority of cases, χM, |λ1|/λ3 and ∇2ρ(r c ) increase with decreasing minimum energy bond lengths. The bond lengths adopted by the molecules are indicated to be an important determinant of the critical point properties of the electron density distributions. The relative electronegativities derived from the electron density distributions of the nitrides agree with those derived for the oxides and Pauling’s electronegativities to within ∼5%, on average. Received: 3 February 1997 / Revised, accepted: 11 July 1997  相似文献   

14.
The structural variations along the solid solution Sr2−x Ba x MgSi2O7 (0 ≤ x ≤ 2), combined to the high-pressure characterization of the two end-members, have been studied. A topological change from the tetragonal (melilite-type) to the monoclinic (melilite-related) structure along the join Sr2MgSi2O7 (e.g., P[`4]21 m P\bar{4}2_{1} m )–Ba2MgSi2O7 (e.g., C2/c) occurs with a Ba content higher than 1.6 apfu. Favored in the crystallization from a melt, the tetragonal form has a tetrahedral sheet topology exclusively based on five-membered rings, which provide a regular “4 up + 4 down” ligand arrangement. In contrast, the melilite-related structure, favored by solid-state reaction synthesis, is made by alternating six- and four-membered tetrahedral rings, which give an asymmetric arrangement of alternated “5 up + 3 down” and “3 up + 5 down” ligands around Sr or Ba. This latter configuration is characterized by an additional degree of freedom with Ba polyhedra hosted in the interlayer with a more irregular and compact coordination and longer Ba–O bond distances. Further insights into the relationships between the two melilite typologies were achieved by investigating the in situ high-pressure behavior of these systems. The synchrotron high-pressure experiments allowed to calculate the elastic moduli for the Sr melilite-type end-member and for the Ba monoclinic polymorph (Sr2MgSi2O7: K T0 = 107, K a=b  = 121, and K c  = 84 GPa; m-Ba2MgSi2O7: K T0 = 85, K a  = 96, K b  = 72, and K c  = 117 GPa) and compare them with those reported in the literature for ?kermanite (Ca2MgSi2O7). The results show that, although the volume of Ba polyhedron in tetragonal polymorphs is larger than in the monoclinic forms, the interlayer compressibility is significantly lower in the former structures due to the occurrence of very short Ba–O distances. This unfavored Ba environment also makes tetragonal Ba2MgSi2O7 a metastable phase at room conditions, possibly favored by high pressure. However, no phase transition occurs from monoclinic to tetragonal form due to kinetic hindrance in reconstructing the sheet topology.  相似文献   

15.
The high temperature volume and axial parameters for six C2/c clinopyroxenes along the NaAlSi2O6–NaFe3+Si2O6 and NaAlSi2O6–CaFe2+Si2O6 joins were determined from room T up to 800°C, using integrated diffraction profiles from in situ high temperature single crystal data collections. The thermal expansion coefficient was determined by fitting the experimental data according to the relation: ln(V/V 0) = α(T − T 0). The thermal expansion coefficient increases by about 15% along the jadeite–hedenbergite join, whereas it is almost constant between jadeite and aegirine. The increase is related to the Ca for Na substitution into the M2 site; the same behaviour was observed along the jadeite–diopside solid solution, which presents the same substitution at the M2 site. Strain tensor analysis shows that the major deformation with temperature occurs in all samples along the b axis; on the (010) plane the higher deformation occurs in jadeite and aegirine at a direction almost normal to the tetrahedral–octahedral planes, and in hedenbergite along the projection of the longer M2–O bonds. The orientation of the strain ellipsoid with temperature in hedenbergite is close to that observed with pressure in pyroxenes. Along the jadeite–aegirine join instead the high-temperature and high-pressure strain are differently oriented.  相似文献   

16.
We have studied the polarized optical absorption and the EPR spectra of Ni-doped beryls grown by hydrothermal, flux and gas-transport methods, and chrysoberyl grown by the Czochralski and flux methods. In beryls, three groups of bands belonging to three various Ni centres were distinguished by analysis of the absorption band intensities. The first group, bands with maximums at 21740 (E ⊥ c), 17240 (E || c) and 9260 (E ⊥ + || c), 7140 (E || + ⊥ c) cm−1, are due to Ni3+ in octahedral Al3+ site. The second group is bands at 25640 (E ⊥ c), 22220 (E || c) and 13520 (E || + ⊥ c), 13160 (E ⊥+ || c) cm−1 and 8930 (E ⊥ + || c), 7460 (E || c) cm−1, which are caused by Ni2+ in octahedral Al3+ site. Weak wide bands at 17540 (E ⊥ c), 15500 (E || c) cm−1 and 6580 (E || + ⊥ c), 5950 (E || c) cm−1 are related to Ni2+ in tetrahedral Be2+ site. The occurrence of Ni ions in Be2+ site is proved by the EPR spectra of 1VNi+ in γ-irradiated samples. According to the spectra of optical absorption of Ni-doped chrysoberyl, two types of Ni centres have been established: Ni3+ and Ni2+ ions in octahedral Al3+ sites. From the EPR spectra of the X-ray irradiated crystals BeAl2O4: Ni, it follows that 68% of Ni+ ions occupy octahedral Al3+ sites with mirror symmetry and 32% are in Al3+ sites with inversion symmetry. In the approximation of trigonal field with regard to Trees correction, the energy levels of Ni3+ and Ni2+ have been calculated in octahedral and tetrahedral coordination. There is good agreement between the obtained experimental and calculated data. The polarization dependence of the optical absorption bands is well explained in terms of the spin–orbit interaction.  相似文献   

17.
Summary The structure type of joaquinite has been derived from precession photographs (Mo radiation) of a polysynthetic twin. Space group: C 2/m; lattice constants:a=10.50Å,b=9.58Å;c=11.78Å;=109°32;cell content: Ba4Fe2RE4Ti4O4[Si4O12]4.The atomic arrangement was determined by Patterson-, Fourier-, and (F0–Fc)-syntheses. The least squares refinement (with isotropic temperature factors) led toR=0.16. The authors are of the opinion that this relativeley highR-value is due to the poor quality of the crystal.The main structural feature are double layers of [Si4O12]-rings parallel to (001) which are connected by 10-coordinated Ba and octahedrally coordinated Ti. These double layers have pseudo-symmetry C mmm, with mirror planes perpendicular toa, b, andc *. They are stacked alongc * and translated –3/8a. The orthorhombic pseudo-symmetry allows also the translation +3/8a which is responsible for both the formation of twins and the frequent stacking disorder. The described double layers are connected by 7-coordinated RE and 4-coordinated Fe.The rôles of Fe, which has an occupancy of only 1/2, and of Na and OH. which are reported in published analyses, are discussed.
Der Strukturtyp von Joaquinit
Zusammenfassung Der Strukturtyp von Joaquinit wurde aus Präzessionsaufnahmen (MoStrlg.) eines polysynthetischen Zwillings abgeleitet. Raumgruppe: C 2/m; Gitterkonstanten:a=10,50Å;b=9,58Å,c=11,78Å; =109°32, Zellinhalt: Ba4Fe2SE4Ti4O4[Si4O12]4.Die Atomanordnung wurde aus Patterson-, Fourier- und (F0–Fc)-Synthesen abgeleitet. Die Verfeinerung mit der Methode der Kleinsten Quadrate führte (mit isotropen Temperaturfaktoren) aufR=0,16. Die Autoren sind der Ansicht, daß dieser relativ hoheR-Wert von der schlechten Qualität des Kristalles herrührt.Wesentliches Strukturmotiv sind Doppelschichten von [Si4O12]-Ringen parallel (001), die durch 10-koordiniertes Ba und oktaedrisch koordiniertes Ti verknüpft werden. Diese Doppelschichten haben die Pseudosymmetrie C mmm mit Symmetrieebenen senkrechta, b undc *. Sie werden nachc * gestapelt und um –3/8a translatiert. Die rhombische Pseudosymmetrie läßt andererseits die Translation +3a zu, was für die Zwilligsbildung und die häufige Stapelungsunordnung verantwortlich ist. Diese Doppelschichten werden weiter durch 7-koordinierte SE und 4-koordinertes Fe verknüpft.Die Rollen des Fe, welches eine Punktlage nur zur Hälfte besetzt, und des Na und OH, welche in publizierten Analysen angegeben werden, werden diskutiert.

Mit 10 Abbildungen  相似文献   

18.
Daily and annual integrated rates of primary productivity and community respiration were calculated using physiological parameters measured in oxygen-based photosynthesis-irradiance (P-I) incubations at 8 stations throughout central and western Long Island Sound (cwLIS) during the summer and autumn of 2002 and 2003 and the late spring of 2003. Each calculation takes into account actual variations in incident irradiance over the day and underwater irradiance and standing stock with depth. Annual peak rates, ±95% confidence interval of propagated uncertainty in each measurement, of gross primary production (GPP, 1,730±610 mmol O2 m−2 d−1), community respiration (Rc, 1,660±270 mmol O2 m−2 d−1), and net community production (NCP, 1,160±1,100 mmol O2 m−2 d−1) occurred during summer at the western end of the Sound. Lowest rates of GPP (4±11 mmol O2 m−2 d−1), Rc (−50±300 mmol O2 m−2 d−1), and NCP (−1,250±270 mmol O2 m−2 d−1) occurred during late autumn-early winter at the outer sampled stations. These large ranges in rates of GPP, Rc, and NCP throughout the photic zone of cwLIS are attributed to seasonal and spatial variability. Algal respiration (Ra) was estimated to consume an average of 5% to 52% of GPP, using a literature-based ratio of Ra:Rc. From this range, we established that the estimated Ra accounts for approximately half of GPP, and was used to estimate daily net primary production (NPP), which ranged from 2 to 870 mmol O2 m−2 d−1 throughout cwLIS during the study. Annual NPP averaged 40±8 mol O2 m−2 yr−1 for all sampled stations, which more than doubled along the main axis of the Sound, from 32±14 mol O2 m−2 yr−1 at an eastern station to 82±25 mol O2 m−2 yr−1 at the western-most station. These spatial gradients in productivity parallel nitrogen loads along the main axis of the Sound. Daily integrals of productivity were used to test and formulate a simple, robust biomass-light model for the prediction of phytoplankton production in Long Island Sound, and the slope of the relationship was consistent with reports for other systems.  相似文献   

19.
A single crystal X-ray diffraction study on lithium tetraborate Li2B4O7 (diomignite, space group I41 cd) has been performed under pressure up to 8.3 GPa. No phase transitions were found in the pressure range investigated, and hence the pressure evolution of the unit-cell volume of the I41 cd structure has been described using a third-order Birch–Murnaghan equation of state (BM-EoS) with the following parameters: V 0  = 923.21(6) Å3, K 0  = 45.6(6) GPa, and K′ = 7.3(3). A linearized BM-EoS was fitted to the axial compressibilities resulting in the following parameters a 0  = 9.4747(3) Å, K 0a  = 73.3(9) GPa, K′ a  = 5.1(3) and c 0  = 10.2838(4) Å, K 0c  = 24.6(3) GPa, K′ c  = 7.5(2) for the a and c axes, respectively. The elastic anisotropy of Li2B4O7 is very large with the zero-pressure compressibility ratio β 0c /β 0a  = 3.0(1). The large elastic anisotropy is consistent with the crystal structure: A three-dimensional arrangement of relatively rigid tetraborate groups [B4O7]2− forms channels occupied by lithium along the polar c–axis, and hence compression along the c axis requires the shrinkage of the lithium channels, whereas compression in the a direction depends mainly on the contraction of the most rigid [B4O7]2− units. Finally, the isothermal bulk modulus obtained in this work is in general agreement with that derived from ultrasonic (Adachi et al. in Proceedings-IEEE Ultrasonic Symposium, 228–232, 1985; Shorrocks et al. in Proceedings-IEEE Ultrasonic Symposium, 337–340, 1981) and Brillouin scattering measurements (Takagi et al. in Ferroelectrics, 137:337–342, 1992).  相似文献   

20.
 Ferrian magnesian spodumene was synthesized in the MLFSH system at P=0.4 GPa, T=700 °C, fO2=NNO+2.3. The space group at room T is P21/c [a=9.638(3) ?, b=8.709(2) ?, c=5.258(2) ?, β=109.83(3)∘, V=415.2 ?3]. The structure is topologically equivalent to that of ferrian spodumene, LiFeSi2O6, and has two symmetrically independent tetrahedral chains, A and B, and two independent octahedral sites, M1 and M2. The crystal-chemical composition was determined combining EMP, SIMS and single-crystal XRD analysis, yielding M2(Li0.85Mg0.09Fe2+ 0.06) M1(Fe3+ 0.85Mg0.15)Si2O6. Li is ordered at the M2 site and Fe3+ is ordered at the M1 site, whereas Mg (and Fe2+) distribute over both octahedral sites. Structure refinements done at different temperatures (25, 70, 95, 125, 150 and 200 °C) allowed characterization of a reversible displacive P21/c→C2/c transition at 106 °C. Previous HT-XRD studies of Li-clinopyroxenes had shown that the transition temperature is inversely related to the size of the M1 cation. For the crystal of this work, the aggregate ionic radius at M1 is longer than that of ferrian spodumene, for which the transition temperature is −44 °C. The higher transition temperature observed can only be explained on the basis of the shorter aggregate radius at the M2 site (due to the presence of Mg substituting after Li), in keeping with the results obtained for ferromagnesian P21/c pyroxenes. The effects of all the chemical substitutions must be considered when modelling transition temperatures and thermodynamic behaviour in clinopyroxenes. Received: 7 May 2002 / Accepted: 23 October 2002  相似文献   

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