Mg,Fe2+ site occupancies in coexisting pyroxenes

The separate distributions for MgSiO₃ and FeSiO₃ in coexisting pyroxenes from the Skaergaard and Bushveld intrusions and charnockites, which were introduced in an earlier communication, indicate directly that significant amounts of both Fe²⁺ and Mg were present in the M(2) site of the Ca-rich pyroxene at the temperature of final intercrystalline equilibration. The calculated Fe²⁺ M(2) site occupancy in the Ca-rich pyroxene increases markedly with decrease in total MgSiO₃ content but the corresponding Mg site occupancy appears largely independent of MgSiO₃. The mean value of the distribution constant for intracrystalline exchange in the Ca-rich pyroxene decreases, away from unity, with decreasing temperature of equilibration. Occupancy of Mg and Fe²⁺ in the M (2) site of the Ca-rich pyroxene effectively compensates for the expected variation in K _D with composition resulting from intracrystalline partition in Ca-poor pyroxene, and this largely accounts for the difference in K _D between igneous and metamorphic pyroxenes. The variation of the augite limb of the pyroxene solvus within the pyroxene quadrilateral is developed as a possible geothermometer.     

Structural change in lead fluorapatite at high pressure

The structure of lead fluorapatite [PbFAP; Pb₁₀(PO₄)₆F₂], crystallized from the melt in a platinum capsule at 1,000°C and 1 atm, has been investigated by single-crystal X-ray diffraction. Crystal data are a = 9.7638 (6), c = 7.2866 (4) Å, space group P6₃/m, R = 0.043, R _w = 0.034. We have also studied the compressional behaviour of the c-axis channel of PbFAP up to 9 GPa at 25°C, using a diamond-anvil cell, synchrotron X-radiation, and Rietveld powder structure refinement. Pressure–volume data for the channel polyhedron of PbFAP fitted to the third-order Birch–Murnaghan equation resulted in K _T  = 33.2 ± 1.2 GPa when K _T ′ is fixed at 4. The c-axis channel of PbFAP is about twice as compressible as the unit-cell volume of PbFAP and the channel of calcium apatites. This is attributed to the anomalous narrowing of the channel of PbFAP with increase in confining pressure. Flexibility of the apatite channel is a key factor in the scavenging of toxic heavy metals by calcium apatites.     

浅海爆炸声传播损失及简正波分离实验研究

文章简要介绍了浅海声传播损失的研究背景和基本理论方法。通过1/3倍频程滤波对浅海爆炸宽带信号的不同频段在浅海的传播损失情况进行了分析,描述了浅海环境对声传播损失的影响;同时在不知道先验信息的条件下,利用垂直阵声压数据成功地进行了简正波分离,并与理想条件下Kraken简正波模型计算出的结果进行了比较分析。     

Synthetic lead bromapatite: X-ray structure at ambient pressure and compressibility up to about 20 GPa

Lead bromapatite [Pb₁₀(PO₄)₆Br₂] has been synthesized via solid-state reaction at pressures up to 1.0 GPa, and its structure determined by single-crystal X-ray diffraction at ambient temperature and pressure. The large bromide anion is accommodated in the c-axis channel by lateral displacements of structural elements, particularly of Pb2 cations and PO₄ tetrahedra. The compressibility of bromapatite was also investigated up to about 20.7 GPa at ambient temperature, using a diamond-anvil cell and synchrotron X-ray radiation. The compressibility of lead bromapatite is significantly different from that of lead fluorapatite. The pressure–volume data of lead bromapatite (P < 10 GPa) fitted to the third-order Birch-Murnaghan equation yield an isothermal bulk modulus (K _T ) of 49.8(16) GPa and first pressure derivative (K_T^￠ K_{T}^{\prime } ) of 10.1(10). If K_T^￠ K_{T}^{\prime } is fixed at 4, the derived K _T is 60.8(11) GPa. The relative difference of the bulk moduli of these two lead apatites is thus about 12%, which is about two times the relative difference of the bulk moduli (~5%) of the calcium apatites fluorapatite [Ca₁₀(PO₄)₆F₂]_, chlorapatite [Ca₁₀(PO₄)₆Cl₂] and hydroxylapatite [Ca₁₀(PO₄)₆(OH)₂]. Another interesting feature apparently related to the replacement of F by Br in lead apatite is the switch in the principle axes of the strain ellipsoid: the c-axis is less compressible than the a-axis in lead bromapatite but more compressible in lead fluorapatite.     

The origins of rare earths in metalliferous sediments of the Troodos Massif. Cyprus

The ferromanganoan umbers which overlie the uppermost pillow lavas of the Troodos Massif, Cyprus, are strongly enriched in REE relative to normal pelagic clays. Together, the umbers, related ferruginous sediments, and the ochres which are associated with massive cupriferous sulphides located within the lavas, all show light REE enrichment. There is a marked negative Ce anomaly which is characteristic of seawater. Field and chemical data suggest that the REE were incorporated into ferruginous precipitates derived from deep leaching of tholeiitic lavas by seawater. All the metalliferous sediments can be interpreted as related events in the evolution of the Troodos ocean ridge. In contrast, REE patterns of bentonitic clays above the umbers indicate a predominantly continental derivation.     

A Ti L-edge X-ray absorption study of Ti-silicate glasses

 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: β_rBa₂TiO₄ (^[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     

Investigation of phase relations in the (Na,K)AlGeO4 system

The phase relations in the (Na,K)AlGeO₄ system have been investigated at atmospheric pressure over the temperature range 700–1100° C by X-ray powder diffraction and electron diffraction/microscopy. Four distinct structure-types occur in this system including the beryllonite, nepheline, kalsilite and KAlGeO₄ types in order of increasing KAlGeO₄ content. In contrast to the (Na,K)Al-SiO₄ system, the nepheline structure is only stable over a narrow composition range around 25 percent K at temperatures above approximately 800° C and transforms reconstructively into the beryllonite structure at lower temperatures. The formation of domain microstructures in some K-rich phases has been directly observed by high-resolution electron microscopy and can be associated with the presence of diffuse scattering in their diffraction patterns. The structural trend observed across the (Na,K)AlGeO₄ series as a function of composition can be rationalized to a large extent in terms of the dependence of the framework topology of these tridymite-derivative structures on the size of the alkali atoms.     

Unit-cell dimensions and tetrahedral-site ordering in synthetic gallium albite (NaGaSi3O8)

The tetrahedral-site order-disorder transformation in gallium albite (NaGaSi₃O₈) has been investigated using Rietveld structure refinement. Study of gallium-substituted albite (in contrast to pure albite [NaAlSi₃O₈]) is facilitated by a relatively rapid order-disorder transformation and the large difference in X-ray scattering efficiencies of gallium and silicon. High albite-structure NaGaSi₃O₈, grown in a Na₂WO₄ flux, was ordered by hydrothermal annealing below 820° C and dry annealing above 820° C, to avoid melting, using a load pressure of approximately 1 kbar. Equilibration of the order-disorder reaction has been verified by three independent reversals of ordering. The transformation between low gallium albite and high gallium albite occurs over the temperature range 890° C 970° C. The gallium content of the T ₁o site increases continuously with decreasing temperature. The gallium contents of the T ₁m and T ₂m sites decrease smoothly with increasing ordering while the gallium content of the T ₂o site decreases, then increases and then decreases again with decreasing temperature. Unit-cell parameters and the triclinic obliquity vary throughout the order-disorder transformation and undergo abrupt changes at 913±3° C and 937±3° C. These abrupt changes correlate with changes in the gallium content of the T ₂o site, the X and Z ordering parameters and the configurational entropy. The order-disorder transformation in gallium-aluminum albite (NaGa_0.5Al_0.5Si₃O₈) occurs in the temperature range 765° C-850° C, at a temperature intermediate to the transformation in albite (50% order at about 680±20° C) and gallium albite.     

Partition of Ni between olivine and sulfide: the effect of temperature,f_{{\text{O}}_{\text{2}} } and f_{{\text{S}}_{\text{2}} }

The experimental distribution coefficient for Ni/ Fe exchange between olivine and monosulfide (K_D3) is 35.6±1.1 at 1385° C, \(f_{{\text{O}}_{\text{2}} } = 10^{ - 8.87} ,f_{{\text{S}}_{\text{2}} } = 10^{ - 1.02} \) , and olivine of composition Fo96 to Fo92. These are the physicochemical conditions appropriate to hypothesized sulfur-saturated komatiite magma. The present experiments equilibrated natural olivine grains with sulfide-oxide liquid in the presence of a (Mg, Fe)-alumino-silicate melt. By a variety of different experimental procedures, K _D3 is shown to be essentially constant at about 30 to 35 in the temperature range 900 to 1400° C, for olivine of composition Fo97 to FoO, monosulfide composition with up to 70 mol. % NiS, and a wide range of \(f_{{\text{O}}_{\text{2}} } \) and \(f_{{\text{S}}_{\text{2}} } \) .     

Coordination of Fe,Ga and Ge in high pressure glasses by Mössbauer,Raman and X-ray absorption spectroscopy,and geological implications

Mössbauer spectra of glasses of NaFeSi₃O₈ and 3NaAlSi₂O₆ · NaFeSiO₄ starting compositions consist of a dominant Fe³⁺ and subordinate Fe²⁺ quadrupole-split doublet, in agreement with previous work. Fe³⁺ is assigned to tetrahedral coordination. Pressure-induced coordination changes are not observed in the pressure range 1 bar to 30 kbar. A gradual increase in isomer shift of the Fe³⁺ doublet with increase in pressure is attributed to steric effects. Raman spectra of GeO₂, NaGaSi₃O₈ and NaGaSiO₄ glasses are dominated by network structure vibrations. There is no detectable change in the nearest-neighbor coordination of Ge⁴⁺ in GeO₂ from 1 bar to 14 kbar, of Ga³⁺ in NaGaSi₃O₈ from 1 bar to 28 kbar and of Ga³⁺ in NaGaSiO₄ from 1 bar to 25 kbar. However, some structural reorganization outside of the first coordination sphere occurs in the high pressure glasses.XANES and EXAFS spectra on powdered samples of 1 bar and 25 kbar NaGaSiO₄ glasses and crystalline NaGaSiO₄ were obtained from K edge absorption spectra at the Stanford Synchrotron Radiation Laboratory using a double crystal monochromator equipped with Si(220) crystals. The XANES spectra indicate that Ga³⁺ has a similar extended coordination geometry in both glasses. The EXAFS spectra reveal that Ga³⁺ is four-coordinated with oxygen in all three samples with a Ga³⁺-O distance of about 1.83 Å. The radial distribution functions of the two glasses are virtually identical. However, both XANES and EXAFS spectra indicate significant structural differences between crystalline NaGaSiO₄ (nepheline-type structure) and vitreous NaGaSiO₄ beyond the first coordination shell of Ga³⁺. Thus, X-ray absorption spectroscopy independently confirms the Raman results on the unchanged coordination of Ga³⁺ in NaGaSiO₄ glasses with pressures up to 25 kbar.Glass compositions were selected in anticipation that larger and/or lower charged cations would exhibit pressure-induced coordination changes at lower pressures than Al³⁺ and Si⁴⁺. The present null result suggests that the stabilizing features of open network structures in the liquid state (large entropy and minimized cation-cation repulsion) more than compensate for large molar volume in the pressure range accessible to experimentation. It appears that network structures in natural magmas should remain stable throughout the upper mantle. Consequently, the densities of magmas at high pressures which are calculated from compressibility data and the appropriate equation of state will be only slightly underestimated, due to the effect of minor structural changes beyond the first coordination sphere.