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161.
Hongwu Xu Alexandra Navrotsky M. Lou. Balmer Yali Su 《Physics and Chemistry of Minerals》2005,32(5-6):426-435
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
162.
Matthias Heuer Alexandra L. Huber Geoffrey D. Bromiley Karl Thomas Fehr Klaus Bente 《Physics and Chemistry of Minerals》2005,32(8-9):552-563
Clinopyroxenes of the solid solution series hedenbergite (CaFeSi2O6)–petedunnite (CaZnSi2O6) were synthesized at temperatures of 825–1200°C and pressures of 0.5–2.5 GPa. Compositions were determined by electron microprobe analysis. Selected crystals were investigated by means of single crystal diffraction and structure refinement and the structural distortion was studied depending on the substitution of iron by zinc on the octahedral M1 site. It is shown that the coordination of the M1 site has the most significant effect on M–O bond lengths, with changes on the other sites accommodating this distortion. The mean quadratic elongation and the octahedral angle variance as quantitative measures of the distortion of the coordination polyhedron were correlated with former results of 57Fe Mössbauer spectroscopy at 298 K. The results presented now complete an earlier work on synthetic, crystalline powders of the same material and deliver exact structural data that were not possible to obtain by Rietveld refinements on powder data. 相似文献
163.
Distribution and Autecology of Chrysophyte Cysts from High Arctic Svalbard Lakes: Preliminary Evidence of Recent Environmental Change 总被引:1,自引:1,他引:1
Chrysophycean stomatocyst assemblages were analysed from the sediments of 17 lakes and ponds from Svalbard as one component of a multi-proxy investigation of recent environmental change in the high Arctic. Sediment cores and water chemistry were collected from each of the study lakes, and chrysophyte stomatocysts were investigated from the top 0.25 cm of sediment (present-day) and bottom (i.e., bottom of short sediment core, pre-industrial) sediment samples. This study represents the first undertaking of chrysophyte cyst morphology and distribution on Svalbard. A total of 153 cyst morphotypes were described with light microscopy and/or scanning electron microscopy, of which 21 are new forms. Canonical correspondence analysis indicates that the present-day distribution of cysts is significantly related to pH (p= 0.02), altitude (p= 0.02), and Na+ (p= 0.04). Marked shifts in chrysophyte cyst assemblages were recorded between the top and bottom sediment samples of most lakes. A recent study has demonstrated that Svalbard lakes receive atmospheric contaminants from both local and remote sources. The observed assemblage shifts may be the result of the combined effects of these point sources and long-range pollutants, or the effects of recent climate change, or both. 相似文献
164.
Brian L. Phillips Pamela C. Burnley Karen Worminghaus Alexandra Navrotsky 《Physics and Chemistry of Minerals》1997,24(3):179-190
We present NMR spectroscopic data, obtained by 1H MAS, 1H static spin-echo, and 29Si{1H} CP-MAS techniques, for a series of hydrous magnesium silicate samples synthesized at high pressure. This series includes chondrodite, β-Mg2SiO4, and phases A, B, superhydrous B, and E. Phases B and superhydrous B give very narrow 29Si NMR peaks and display the most de-shielded SiVI chemical shifts yet reported: ?170.4?ppm for B and ?166.6 for superhydrous B. The 1H NMR spectra of B and superhydrous B confirm the presence of paired hydroxyls, as determined from refinement of the H positions from X-ray diffraction data. The 1H MAS NMR spectra of phase B contain peaks for the two distinct hydrogen positions, with chemical shifts of +4.7 and +3.3?ppm. The static 1H spectrum contains a powder pattern characteristic of a strongly coupled hydrogen pair, from which a dipolar coupling constant of 18.6(4)?kHz and inter-hydrogen distance of d(H–H)=1.86(2)?Å were obtained. Superhydrous B appears to give two poorly resolved 1H MAS peaks, consistent with the presence of two distinct hydrogen pairs in the P21 mn crystal structure. Analysis of its spin-echo spectrum gives d(H–H)=1.83(3)?Å, slightly shorter than for phase B. β-Mg2SiO4, coexisting with phases B and superhydrous B, appears to give 29Si{1H} CP-MAS signal, indicating that it contains significant H concentration. The 29Si chemical shifts for phases B, superhydrous B, and chondrodite, together with those reported previously for other Mg-silicates, show a good correlation with structural parameters. 相似文献
165.
Klaus-Dieter Grevel Mirko Schoenitz Volker Skrok Alexandra Navrotsky Werner Schreyer 《Contributions to Mineralogy and Petrology》2001,142(3):298-308
The enthalpy of drop-solution in molten 2PbO·B2O3 of synthetic and natural lawsonite, CaAl2(Si2O7)(OH)2·H2O, was measured by high-temperature oxide melt calorimetry. The enthalpy of formation determined for the synthetic material is (fHOxides=-168.7Dž.4 kJ mol-1, or (fH0298=-4,872.5dž.0 kJ mol-1. These values are in reasonable agreement with previously published data, although previous calorimetric work yielded slightly more exothermic data and optimisation methods resulted in slightly less exothermic values. The equilibrium conditions for the dehydration of lawsonite to zoisite, kyanite and quartz/coesite at pressures and temperatures up to 5 GPa and 850 °C were determined by piston cylinder experiments. These results, other recent phase equilibrium data, and new calorimetric and thermophysical data for lawsonite and zoisite, Ca2Al3(SiO4)(Si2O7)O(OH), were used to constrain a mathematical programming analysis of the thermodynamic data for these two minerals in the chemical system CaO-Al2O3-SiO2-H2O (CASH). The following data for lawsonite and zoisite were obtained: (fH0298 (lawsonite)=-4,865.68 kJ mol-1 , S0298 (lawsonite)=229.27 J K-1 mol-1 , (fH0298 (zoisite)=-6,888.99 kJ mol-1 , S0298 (zoisite)=297.71 J K-1 mol-1 . Additionally, a recalculation of the bulk modulus of lawsonite yielded K=120.7 GPa, which is in good agreement with recent experimental work. 相似文献
166.
Short and high frequency internal waves, propagating through horizontal gradients of temperature formed by long and low-frequency
waves, induce vertical transferof heat with effective coefficients 10−4 – 10−3 m2/s and lead to a fine structure formation. Horizontal turbulence with seals of order 100 m produce the same effect in the
presence of internal waves. The same is true for other substances, such as salt, oxygen, organic matter. It is suggested that
this augmenting of vertical transports due to internal waves is an essential factor for high biological productivity in shelf
zones. 相似文献
167.
A new determination, using high temperature drop-solution calorimetry, of the enthalpy of transformation of MgSiO3 pyroxene to ilmenite gives H
298 = 59.03 ±4.26 kJ/mol. The heat capacity of the ilmenite and orthopyroxene phases has been measured by differential scanning calorimetry at 170–700 K; Cp of MgSiO3 ilmenite is 4–10 percent less than that of MgSiO3 pyroxene throughout the range studied. The heat capacity differences are consistent with lattice vibrational models proposed by McMillan and Ross (1987) and suggest an entropy change of -18 ± 3 J-K-1 ·mol-1, approximately independent of temperature, for the pyroxene-ilmenite transition. The unit cell parameters of MgSiO3 ilmenite were measured at 298–876 K and yield an average volume thermal expansion coefficient of 2.44 × 10-5 K-1. The thermochemical data are used to calculate phase relations involving pyroxene, -Mg2SiO4 plus stishovite, Mg2SiO4 spinel plus stishovite, and ilmenite in good agreement with the results of high pressure studies. 相似文献
168.
With increasing pressure, MnSiO3 rhodonite stable at atmospheric pressure transforms to pyroxmangite, then to clinopyroxene and further to tetragonal garnet, which finally decomposes into MnO (rocksalt) plus SiO2 (stishovite). High temperature solution calorimetry of synthetic rhodonite, clinopyroxene and garnet forms of MnSiO3 was used to measure the enthalpies of these transitions. ΔH 974 0 for the rhodonite-clinopyroxene and ΔH 298 0 for the clinopyroxene-garnet transition are 520±490 and 8,270±590 cal/mol, respectively. The published data on the enthalpy of the rhodonite-pyroxmangite transition, phase equilibrium boundaries, compressibility and thermal expansion data are used to calculate entropy changes for the transitions. The enthalpy, entropy and volume changes are very small for all the transitions among rhodonite, pyroxmangite and clinopyroxene. The calculated boundary for the clinopyroxene-garnet transition is consistent with the published experimental results. The pyroxene-garnet transition in several materials, including MnSiO3, is characterized by a relatively small negative entropy change and large volume decrease, resulting in a small positiveP – T slope. The disproportionation of MnSiO3 garnet to MnO plus stishovite and of Mn2SiO4 olivine to garnet plus MnO are calculated to occur at about 17–18 and 14–15 GPa, respectively, at 1,000–1,500 K. 相似文献
169.
Michael A Carpenter Andrew Putnis Alexandra Navrotsky J.Desmond C McConnell 《Geochimica et cosmochimica acta》1983,47(5):899-906
Measurements of the heats of solution (ΔHsoln) in molten Pb2B2O5 at 708°C of anhydrous magnesian cordierites, prepared with a range of structural states, show that the enthalpy effect associated with Al/Si ordering is substantial (? 9.76 ± 1.56 kcal mole?1). Differences in the state of order between synthetic cordierites used in phase equilibrium studies and cordierites in the natural environment could lead to significant errors in the estimation of palaeo-pressures and temperatures. A continuous change of ΔHsoln with annealing time supports the suggestion of putnis (1980) that the hexagonal → orthorhombic transformation in cordierite, which can occur via a modulated structure, is truly continuous under metastable conditions. In addition, a linear relation between ΔHsoln and the logarithm of annealing time has been found, which provides some insight into the nature of the ordering mechanisms at an atomic level. Al and Si exchanges occur continuously between neighbouring tetrahedral sites with a net drift towards increasing order. No kinetic or thermochemical distinction can be made between the development of long range and short range order.The enthalpy of vitrification (~ 12 kcal mole?1) for a metastable stuffed β-quartz polymorph of cordierite composition is similar to that for pure quartz (on a per two oxygen basis), while the heat of vitrification for even the most disordered cordierite seen in this study is more than a factor of three greater (~40 kcal mole?1). This is consistent with the view that cordierite glass resembles the quartz structure more closely than the crystalline cordierite structure, and that crystallisation of the glass below ~900°C is controlled by a tetrahedral framework. 相似文献
170.
An Upper Maastrichtian horizon rich in Loftusia is for the first time described in situ in Greece in the province of Boeotia. It is found in a continuous undisturbed carbonate sequence of the eastern Greece platform (Subpelagonian zone) of Maastrichtian–Paleocene age followed by flysch sedimentation. Loftusia is found in a facies reflecting an outer shelf environment, associated with debris of rudists, Orbitoides spp., Siderolites calcitrapoides, Omphalocyclus macroporus, Hellenocyclina beotica, Sulcoperculina sp., and echinoderms. Similar fauna, but without Loftusia, is found in the surrounding levels of Late Maastrichtian age as well. This recovery of the genus in the western part of its distribution area, where it is not as abundant as in the eastern part, is considered significant for the palaeobiogeography of the genus in Tethys Ocean during Late Cretaceous. To cite this article: A. Zambetakis-Lekkas, A. Kemeridou, C. R. Geoscience 338 (2006). 相似文献