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1.
N. N. Eremin R. A. Talis A. E. Grechanovskii V. S. Urusov 《Moscow University Geology Bulletin》2013,68(2):72-78
An original methodology for the atomistic computer modeling of solid solutions was applied for the study of the mixing properties and local structure of the grossular-uvarovite, i.e., Ca3Al2[SiO4]3 Ca3Cr2][SiO4]3, garnet series. The parameters of the interatomic potentials for end members of this series were optimized using experimental data on their structural, elastic, and thermodynamic characteristics. The optimized model of the potentials allowed us to describe the elastic, structural, and thermodynamic characteristics of grossular and uvarovite and estimate the energy of point defects in these crystal structures. Calculations of the mixing properties and local structure for seven different compositions of the solid solution were carried out on a “Chebyshev” supercomputer (Moscow State University) in a 2 × 2 × 4 supercell of the garnet-type structure containing 2560 atoms. Mixing properties, such as the enthalpy of mixing, parameters of interaction, excess mixing volume, deviation of bulk modulus from additivity, and the vibrational and configuration contribution to the entropy of mixing, were determined. This allowed us to estimate the stability field for the grossular-uvarovite solid solution. Histograms of the interatomic distances M-O (M = Ca, Al, Cr, Si) and O-O in supercells were plotted and the parameters of relaxation and changes of the CrO6 and AlO6 octahedron volumes were estimated. The data of the simulation are quite consistent with the experimental data on this system and supplement it significantly. 相似文献
2.
Rb-Sr isotopic investigation of forty rocks from three different granulite localities in the Strangways Ranges, central Australia indicated two distinct granulite metamorphisms, namely M1: 1860±80 m.y., and M2: 1470±60 m.y. and 1430±60 m.y., λ=1.39x10?11yr?1. M2 has been recorded in the felsic granulites of two of the three localities. Although the Sr isotopic systems in the felsic granulites seem to show a normal growth in a closed system during M2, the Rb-Sr systems for some calcareous and mafic granulites have not completely re-equilibrated during M2, as shown by the scatter with respect to the reference isochrons for M1 and M2. Ca-carbonate rocks display unusually low level of Sr (20–120 ppm. Sr, and high Ca/Sr ratios). Schistose zones of retrogression in the granulites, dated by K-Ar and 40Ar-39Ar methods at ≈350 m.y., yield anomalous Rb-Sr data. 相似文献
3.
The nucleation kinetics of binary solid solutions, with general formula BxC1−xA, crystallising from aqueous solution can be described using a generalised expression for the nucleation rate: the function, J(x), in which supersaturation, interfacial free energy and other parameters of the classical nucleation rate equation are considered as functions of the solid composition. As an example, we studied the behaviour of such J(x) functions for the case of the (Ba,Sr)SO4 and (Ba,Sr)CO3 solid solutions. J(x) functions are very sensitive to slight changes in the composition of the aqueous solution, which result in strong modifications of the nucleation kinetics. The implications of the relationship between supersaturation and nucleation rate functions for the general nucleation behaviour in solid solution-aqueous solution (SS-AS) systems are discussed. Finally, we present a method for constructing non-equilibrium Roozeboom diagrams based on the nucleation kinetics in SS-AS systems. Our Roozeboom diagrams calculated for different departures from equilibrium conditions are consistent with previous experimental work and they can be used to predict actual distribution coefficients. 相似文献
4.
Tremolite (CaxSr1–x)2Mg5[Si8O22/(OH)2] and diopside (CaxSr1–x)Mg[Si2O6] solid solutions have been synthesized hydrothermally in equilibrium with a 1 molar (Ca,Sr)Cl2 aqueous solution at 750°C and 200 MPa. The solid run products have been investigated by optical, electron scanning and high resolution transmission electron microscopy, electron microprobe, X-ray-powder diffraction and Fourier-transform infrared spectroscopy. The synthesized (Ca,Sr)-tremolites are up to 2000 µm long and 30 µm wide, the (Ca,Sr)-diopsides are up to 150 µm long and 20 µm wide. In most runs the tremolites and diopsides are well ordered and chain multiplicity faults are rare. Nearly pure Sr-tremolite (tr0.02Sr-tr0.98) and Sr-diopside (di0.01Sr-di0.99) have been synthesized. A continuous solid solution series, i.e. complete substitution of Sr2+ for Ca2+ on M4-sites exists for (Ca,Sr)-tremolite. Total substitution of Sr2+ for Ca2+ on M2-sites can be assumed for (Ca,Sr)-diopsides. For (Ca,Sr)-tremolites the lattice parameters a, b and β are linear functions of composition and increase with Sr-content whereas c is constant. For the diopside series all 4 lattice parameters are a linear function of composition; a, b, c increase and β decreases with rising Sr-content. The unit cell volume for tremolite increases 3.47% from 906.68 Å3 for tremolite to 938.21 Å3 for Sr-tremolite. For diopside the unit cell volume increases 4.87 % from 439.91 Å3 for diopside to 461.30 Å3 for Sr-diopside. The observed splitting of the OH stretching band in tremolite is caused by different configurations of the next nearest neighbors (multi mode behavior). Resolved single bands can be attributed to the following configurations on the M4-sites: SrSr, SrCa, CaCa and CaMg. The peak positions of these 4 absorption bands are a linear function of composition. They are shifted to lower wavenumbers with increasing Sr-content. No absorption band due to the SrMg configuration on the M4-site is observed. This indicates a very low or negligible cummingtonite component in Sr-rich tremolites, which is also supported by electron microprobe analysis. 相似文献
5.
X-ray absorption spectroscopy, including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques, have been used to determine the structure and speciation of complexes for Fe2+ and Fe3+ chloride solutions at a variety of pH's, ionic strengths, and chloride/iron ratios.Low intensity K-edge transition features and analysis of modified pair correlation functions, derived from Fourier transformation of EXAFS spectra, show a regular octahedral coordination of Fe(II) by water molecules with a first-shell Fe2+-O bond distance, closely matching octahedral Fe2+-O bonds obtained from solid oxide model compounds. Solution Fe2+-O bond distances decrease with chloride/iron ratio, pH, and total FeCl2 concentration. A slight intensification of the transition with increasing FeCl2 concentration suggests that chloride may begin to mix with water as a nearest-neighbor octahedral ligand. Fe3+ solutions show a pronounced increase in the transition intensities between 1.0 M FeCl3/7.8 M Cl? to 1.0 M FeCl3/ 15 M Cl?, indicating a coordination change from octahedral to tetrahedral complexes. EXAFS analyses of these solutions show an increase in first-shell Fe3+-ligand distances despite this apparent reduction in coordination number. This can be best explained by a change from regular octahedral complexes of ferric iron (either Fe(H2O)63+ or trans-Fe(H2O)4Cl2 or both; Fe3+-O bond distances of 2.10 Å) to tetra-chloro complexes [Fe3+-Cl bond distances of 2.25 Å]. 相似文献
6.
Olivine-related (Ni, Mg)3(PO4)2 solid solutions were prepared and equilibrated at 1070 K. Accurate monoclinic unit cell dimensions were determined from Guinier-Hägg photographic data. Structural refinements based on the X-ray profile-fitting technique after Rietveld were carried out for pure nickel (II) orthophosphate and for three Ni/Mg solid solutions. (Ni1-x Mg x )3(PO4)2 phases with 0.40≦x≦0.60 are probably isostructural with Ni3(PO4)2 (P21/a) while phases with low magnesium contents (<27 atom % Mg) deviate structurally from Ni3(PO4)2. The results also show that Ni2+ is partially ordered at the octahedralM(1) sites, withK D (Ni, Mg)=4.0±0.2 相似文献
7.
8.
Babingtonite, Ca2Fe2+Fe3+[Si5O14(OH)] (Z?=?2, space group $ P\overline{1} $ ) from Yakuki mine (Japan), Grönsjöberget (Sweden), Kandivali Quarry (India), Baveno Quarry (Italy), Bråstad Mine (Norway), and Kouragahana (Japan), and manganbabingtonite, Ca2(Mn2+, Fe2+)Fe3+[Si5O14(OH)], from Iron Cap mine (USA) were studied using electron-microprobe analysis (EMPA), 57Fe Mössbauer analysis and single-crystal X-ray diffraction methods to determine the cation distribution at M1 and M2 and to analyze its effect on the crystal structure of babingtonite. Although all studied babingtonite crystals are relatively homogeneous, chemical zonation due to mainly Fe ? Mn substitution is observed in manganbabingtonite. Mössbauer spectra consist of two doublets with isomer shift (I.S.)?=?1.16–1.22 mm/s and quadrupole splitting (Q.S.)?=?2.33–2.50 mm/s and with I.S.?=?0.38–0.42 mm/s and Q.S.?=?0.82–0.90 mm/s, assigned to Fe2+ and Fe3+ at the M1 and M2 octahedral sites, respectively. The determined ratio of Fe2+/total Fe in manganbabingtonite (0.26) was smaller than that in the others (0.35–0.44) because of high Mn2+ content instead of Fe2+. The unit-cell parameters of babingtonite are a?=?7.466–7.478, b?=?11.624–11.642, c?=?6.681–6.690 Å, α?=?91.53–91.59, β?=?93.86–93.94, γ?=?104.20–104.34º, and V?=?560.2–562.3 Å3, and those of manganbabingtonite are a?=?7.4967(3), b?=?11.6632(4), c?=?6.7014(2) Å, α?=?91.602(2), β?=?93.989(2), γ?=?104.574(3)º, and V =565.09(5) Å3. Structural refinements converged to R 1 values of 1.64–3.16 %. The <M1-O> distance was lengthened due to the substitution of large octahedral cations such as Mn2+ for Fe2+. The increase of the M1-O8, M1-O8’ and M1-O13 lengths with mean ionic radii is slightly more pronounced than of the other M1-Oi lengths. The lengthened M1-O13 distance leads the positive correlation between Si5-O15-Si1 angle and M1-O13 distance. The increase of Si2-O3-Si1 and Si5-O12-Si4 angles due to the increase of mean ionic radius of M2 is also observed. 相似文献
9.
David M. Singer Stephen B. Johnson Jeffrey G. Catalano François Farges Gordon E. Brown Jr. 《Geochimica et cosmochimica acta》2008,72(20):5055-5069
Calcium oxalate monohydrate (CaC2O4·H2O—abbreviated as CaOx) is produced by two-thirds of all plant families, comprising up to 80 wt.% of the plant tissue and found in many surface environments. It is unclear, however, how CaOx in plants and soils interacts with metal ions and possibly sequesters them. This study examines the speciation of Sr(II)aq following its reaction with CaOx. Batch uptake experiments were conducted over the pH range 4-10, with initial Sr solution concentrations, [Sr]aq, ranging from 1 × 10−4 to 1 × 10−3 M and ionic strengths ranging of 0.001-0.1 M, using NaCl as the background electrolyte. Experimental results indicate that Sr uptake is independent of pH and ionic strength over these ranges. After exposure of CaOx to Sraq for two days, the solution Ca concentration, [Ca]aq, increased for all samples relative to the control CaOx suspension (with no Sr added). The amount of Sraq removed from solution was nearly equal to the total [Ca]aq after exposure of CaOx to Sr. These results suggest that nearly 90% of the Sr is removed from solution to a solid phase as Ca is released into solution. We suggest that the other 10% is sequestered through surface adsorption on a solid phase, although we have no direct evidence for this. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the molecular-level speciation of Sr in the reaction products. Deconvolutions of the Sr K-edge EXAFS spectra were performed to identify multi-electron excitation (MEE) features. MEE effects were found to give rise to low-frequency peaks in the Fourier transform before the first shell of oxygen atoms and do not affect EXAFS fitting results. Because of potential problems caused by asymmetric distributions of Sr-O distances when fitting Sr K-edge EXAFS data using the standard harmonic model, we also employed a cumulant expansion model and an asymmetric analytical model to account for anharmonic effects in the EXAFS data. For Sr-bearing phases with low to moderate first-shell (Sr-O pair correlation) anharmonicity, the cumulant expansion model is sufficient for EXAFS fitting; however, for higher degrees of anharmonicity, an analytical model is required. Based on batch uptake results and EXAFS analyses of reaction products, we conclude that Sr is dominantly sequestered by a solid phase at the CaOx surface, likely the result of a dissolution-reprecipitation mechanism, to form SrC2O4 of mixed hydration state (i.e. SrOx·nH2O, where n = 0, 1, or 2). Surprisingly, no spectroscopic or XRD evidence was found for a (Sr,Ca)Ox solid solution or for a separate SrCO3 phase. In addition, we found no evidence for Sr(II) inner-sphere sorption complexes on CaOx surfaces based on lack of Sr-Ca second-neighbor pair correlations in the EXAFS spectra, although some type of Sr(II) surface complex (perhaps a type B Sr-oxalate ternary complex or an outer-sphere Sr(II) complex) or some as yet undetected Sr-bearing solid phases are needed to account for approximately 10% of Sr uptake by CaOx. The formation of a hydrated SrOx phase in environments under conditions similar to those of our experiments should retard Sr mobility and could be a significant factor in the biogeochemical cycling of Sr in soils and sediments or in plants and plant litter where CaOx is present. 相似文献
10.
Ronald K. Stoessell 《Geochimica et cosmochimica acta》1981,45(10):1733-1741
A quasi-chemical model for illites has been derived, and local electrostatic balance has been added to a random regular solution site-mixing model for illites (Stoessell, 1979). Each model assumes similar order-disorder conditions for both the end-members micas and the solid solution. Thermodynamic properties of illites predicted by the random, electrostatic, and quasi-chemical models are compared as a function of composition. For natural illite compositions, molar entropies of mixing in the electrostatic model are about 1 entropy unit less than those in the random model. Intermediate values are given by the quasi-chemical model. Each model predicts an increased entropy of mixing in dominantly trioctahedral illites as compared to dioctahedral illites. Each model also predicts destabilization of trioctahedral illites using absolute molar exchange energies greater than 2 RT/Zx, where Zx is the number of adjacent cation interactions per site in the Xth site class. The most negative free energies of mixing are predicted by the quasi-chemical model. Intermediate values predicted by the random model are apparently the result of error cancellation due to overestimation of both the entropy and enthalpy of mixing. 相似文献
11.
Tomas Paces 《Geochimica et cosmochimica acta》1973,37(12):2641-2663
A hypothesis is presented that the dissolution of albite includes the exchange of sodium for hydrogen ion in a surface layer of the mineral and the structural collapse of the residual anionic lattice of the layer. The ion exchange is described by the first law of diffusion (D25°C = 3 × 10?22 and 1.5 × 10?20 cm2sec?1 at PCO2 = 0 and 26.2 atm, respectively). The surface residual layer reaches a steady-state thickness ranging from n × 10?8 to n × 10?5 cm according to the temperature and PCO2. The increase in aqueous sodium with time in a continuous ground-water system is described by a simple exponential equation. The equation is used to estimate the percolation time of ground water from the data on the chemical composition of a water sample. The probable times range from 14 to 3840 days for various ground-water systems and are compared to the times of percolation calculated from the geothermal and hydraulic data. Both estimates are found to be in general agreement. The concentrations of Al and Si in cold water from granitic rocks are shown to be controlled by the chemical equilibrium with respect to an aged aluminosilicate. The aluminosilicate precipitates from ground water as an amorphous isoelectric solid. Its chemical composition is represented by a simplified stoichiometric formula [Al(OH)3](1?x)[SiO2]x and varies linearly with pH of the solution. The atoms of Al, O and H tend to occupy a fixed position in the solid given by the gibbsite structure upon aging in the field. The solubility product of the solid is estimated from the published data on experimental and field research into the dissolution of feldspars: logK = (1 ? x) × log [Al3+] + xlog [H4SiO4] ? (3 ? 3x) log [H+] = 8.56 ? 11.26x, where x is the molar fraction of silica in the aluminosilicate. 相似文献
12.
13.
Juraj Majzlan Hannes Schlicht Maria Wierzbicka-Wieczorek Gerald Giester Herbert Pöllmann Beatrix Brömme Stephen Doyle Gernot Buth Christian Bender Koch 《Mineralogy and Petrology》2013,107(2):221-233
Voltaite is a mineral of fumaroles, solfatares, coal-fire gas vents, and acid-mine drainage systems. The nominal composition is K2Fe5 2+Fe3 3+Al(SO4)12·18H2O and the nominal symmetry is cubic, $Fd\overline{3}c$ . The tetragonal (I41/acd) superstructure of voltaite is known as the mineral pertlikite. In this study, we investigated 22 synthetic voltaite samples in which Fe2+ was partially or completely replaced by Mg, Zn, Mn, or Cd, by single-crystal and powder X-ray diffraction (both in-house and synchrotron). Two samples contained NH4 + instead of K+. The structure of voltaite is based on a framework defined by kröhnkite-like heteropolyhedral chains which host both M3+ and M2+ in octahedral coordination. Unit cell dimensions of the end-members scale almost linearly with the size of M2+. In the Fe2+-Mg-Zn solid solutions, the Fe2+-Mg and Fe2+-Zn solutions are linear (ideal) in terms of their lattice-parameter variations. The Mg-Zn solid solution, however, is strongly non-ideal. A detailed analysis of the topology of the chains showed that this behavior originates in expansion and contraction of individual M2+-O bonds within the chains. In the Mg-Zn solid solution, some of the M2+-O bonds expand while none contract. In the other solid solutions, expansion of some M2+-O bonds is always compensated by contraction of the other ones. Parts of the nominally cubic crystals are optically anisotropic and their symmetry is found to be tetragonal by single crystal X-ray diffraction measurements. The coexistence of cubic and tetragonal sectors within a single crystal without any detectable difference in their chemical composition is difficult to explain in terms of growth of such composite crystals. Mössbauer and infrared spectra collected on our synthetic crystals conform with previously published data. 相似文献
14.
In a series of timed experiments, monazite inclusions are induced to form in the Durango fluorapatite using 1 and 2 N HCl
and H2SO4 solutions at temperatures of 300, 600, and 900°C and pressures of 500 and 1,000 MPa. The monazite inclusions form only in
reacted areas, i.e. depleted in (Y+REE)+Si+Na+S+Cl. In the HCl experiments, the reaction front between the reacted and unreacted
regions is sharp, whereas in the H2SO4 experiments it ranges from sharp to diffuse. In the 1 N HCl experiments, Ostwald ripening of the monazite inclusions took
place both as a function of increased reaction time as well as increased temperature and pressure. Monazite growth was more
sluggish in the H2SO4 experiments. Transmission electron microscopic (TEM) investigation of foils cut across the reaction boundary in a fluorapatite
from the 1 N HCl experiment (600°C and 500 MPa) indicate that the reacted region along the reaction front is characterized
by numerous, sub-parallel, 10–20 nm diameter nano-channels. TEM investigation of foils cut from a reacted region in a fluorapatite
from the 1 N H2SO4 experiment at 900°C and 1,000 MPa indicates a pervasive nano-porosity, with the monazite inclusions being in direct contact
with the surrounding fluorapatite. For either set of experiments, reacted areas in the fluorapatite are interpreted as replacement
reactions, which proceed via a moving interface or reaction front associated with what is essentially a simultaneous dissolution–reprecipitation
process. The formation of a micro- and nano-porosity in the metasomatised regions of the fluorapatite allows fluids to permeate
the reacted areas. This permits rapid mass transfer in the form of fluid-aided diffusion of cations to and from the growing
monazite inclusions. Nano-channels and nano-pores also serve as sites for nucleation and the subsequent growth of the monazite
inclusions. 相似文献
15.
A suite of 14 synthetic Ca-free P2 1 /c low-clinopyroxenes of compositions across the enstatite-ferrosilite (En-Fs) join have been studied by X-ray powder diffraction and Mössbauer spectroscopy. The crystal structure of one sample with composition X Fs=0.39 was determined by single-crystal X-ray diffraction. The powder diffraction data show that there is no significant (<0.25% at X Fs=0.5) excess volume of mixing on the enstatite-ferrosilite join. The molar volumes are described by V=31.261(17)+1.677(23) X Fs cm3.mol(MSiO3)–1. The linear behaviour of volume with composition arises from the compensating effects of non-linear changes in the unit-cell parameters with composition. Unweighted fits to the data yielded the relationships: a=9.6100(15)+0.105(2)X Fs, b=8.8156(28)+0.218(12)X Fs+0.0481(117)X 2 Fs , c?=5.1702(4)+0.0879(21)X Fs–0.0214(21)X 2 Fs , β=108.345(8)+0.947(30)X Fs–0.805(26)X 2 Fs . The strong positive deviation of β from linearity is directly correlated to the difference in site occupancies between M1 and M2 as determined by Mössbauer spectroscopy and arises directly from the expansion of M2-O1 and M2-O3 bonds upon initial substitution of Fe for Mg in enstatite. The hyperfine parameters from Mössbauer spectra are consistent with variations in the average local environment as recorded by the X-ray data. Asymmetric line broadening of the Mössbauer spectra provides evidence for next-nearest neighbour effects, and is consistent with no significant clustering of Fe or Mg within the samples. 相似文献
16.
Local structural relaxation around Co2+ along the hardystonite–Co-?kermanite melilite solid solution
Six pure compounds belonging to the hardystonite (Ca2ZnSi2O7)–Co-?kermanite (Ca2CoSi2O7) solid solution were investigated by the combined application of X-ray powder diffraction and electronic absorption spectroscopy. Structural refinements of the XRPD data revealed a negative excess volume of mixing due to the single isovalent substitution of Co for Zn in the tetrahedral site. In agreement with the diffraction data, deconvolution of the optical spectra showed a progressive decreasing of the crystal field strength parameter 10Dq moving toward the Co-?kermanite end-member, meaning that the local cobalt–oxygen bond distance, $ \langle {\text{Co}}{-}{\text{O}}\rangle^{\text{local}} $ , increased along the join with the amount of cobalt. The calculated structural relaxation coefficient around the fourfold coordinated Co2+ in the Ca2(Zn1?x Co x )Si2O7 join was ε?=?0.69, very far from the one predicted by the Vegard’s law (ε?=?0) and at variance with ε?=?0.47 previously found for tetrahedrally coordinated Co2+ in gahnite–Co-aluminate spinel solid solution. This difference is consistent with the largest constraints existing on the spinel structure, based on cubic closest packing, compared to the more flexible layered melilite structure. 相似文献
17.
In this study, we present ac susceptibility measurements for a synthetic and a natural hemo-ilmenite (HI) solid solution (x)FeTiO3–(1 − x)Fe2O3 with compositions x = 0.87(1) and 0.88(8), respectively. The focus of the investigation is the magnetic ordering at the Curie temperature T
C and the spin-glass-like freezing at the freezing temperature T
f. The sharpness of T
C for the synthetic solid solution with well-defined structure indicates the chemical homogeneity of the solution, whereas
the disperse magnetic ordering of the natural solid solution reveals inhomogeneities described as spin glass system ofvariations
in composition x. The frequency dispersion of T
f was determined between 10 Hz and 10 kHz and was found to obey a dynamic scaling power law. The relaxation rates deviate by
five orders of magnitude where the synthetic solid solution exhibits ω0 = 3(1) × 104 Hz and the natural one 5.5 × 109 Hz. The strong deviation is attributed to the difference in the ordered state above T
f. These findings provide an insight into the cooling-rate effects of natural solid solutions and how magnetic thermodynamics
can be used to probe the chemical homogeneity of such systems. 相似文献
18.
Jens Wenzel Andreasen Emil Makovicky Bente Lebech Sven Karup Møller 《Physics and Chemistry of Minerals》2008,35(8):447-454
Rietveld refinement of neutron powder diffraction data on four samples of synthetic, iron-bearing tetrahedrite (Cu12?xFexSb4S13) with x = 0.28, 0.69, 0.91, 2.19 and four samples of synthetic tennantite (Cu12?xFexAs4S13) with x = 0.33, 0.38, 0.86, 1.5 indicate unambiguously that iron is incorporated into tetrahedral M1 (12d) sites and not into triangular M2 (12e) sites in the cubic crystal structure (space group I $ \ifmmode\expandafter\bar\else\expandafter\=\fi{4} Rietveld refinement of neutron powder diffraction data on four samples of synthetic, iron-bearing tetrahedrite (Cu12−xFexSb4S13) with x = 0.28, 0.69, 0.91, 2.19 and four samples of synthetic tennantite (Cu12−xFexAs4S13) with x = 0.33, 0.38, 0.86, 1.5 indicate unambiguously that iron is incorporated into tetrahedral M1 (12d) sites and not into triangular M2 (12e) sites in the cubic crystal structure (space group I 3 m). The refinement results also confirm that M2 is a split (24g), flat-pyramidal site situated statistically on both sides of the S1−S1–S2 triangle. In tetrahedrite, this split is about
0.6 ?, in tennantite about 0.7 ?. Trends in bond lengths and magnitude of the M2 split were evaluated by means of linear regression
with Fe concentration as the independent variable. 相似文献
19.
C. Carbonin A. Dal Negro S. Ganeo E. M. Piccirillo 《Contributions to Mineralogy and Petrology》1991,108(1-2):34-42
The crystallochemical variations of clinopyroxene in response to changes in fO
2 and melt composition have been determined for a basalt-pantellerite suite (Boseti Complex, Main Ethiopian Rift) by crystal
structure refinement and microprobe analysis. The pyroxene evolutionary trend has both a “Ca-minimum” and late iron enrichment.
During crystallization from basalts to trachytes, clinopyroxene geometry depends mainly on the relationships between T and
M2 sites; for example, high SiO2 activity in the magma causes high Si occupancy in T site, which in turn requires low Ca occupancy in M2 site in order to
fulfill the local charge balance requirements. In contrast, clinopyroxene crystallized from acid melts is characterized by
high Fe2+ (M1) content and therefore by a very large M1 site. Longer 〈M1-O1〉 and M1-O2 bond lengths require shorter T-O1 and T-O2 bond
lengths and high Si occupancy in T site. It is concluded that the “Ca-minimum” in the clinopyroxene structure is regarded
as the lowest value at which the charge balance requirements are satisfied in a C2/c clinopyroxene structure. 相似文献
20.
Apparent partition coefficients of Sr and Ba between calcium phosphate and water were measured experimentally for temperature ranging from 5°C to 60°C. Calcium phosphates were precipitated from an aqueous mixture of Na2HPO4 · 2H2O (10−2 M) and CaCl2 · 2H2O (10−2 M). Spiked solutions of Sr or Ba were introduced into the CaCl2 · 2H2O solution at Sr/Ca and Ba/Ca ratios of 0.1. The experiment consisted in sampling the liquid and solid phases after 1, 6, 48, and 96 h of interaction. The amorphous calcium phosphate (ACP) precipitated early in the experiment was progressively replaced by hydroxylapatite (HAP), except at 5°C where brushite (di-calcium phosphate di-hydrate or DCPD) was formed. We observed that the crystallinity of the solid phase increased with time for a given temperature and increased with temperature for a given time of reaction. With the exception of the experiment at 5°C, yield (R%) and apparent partition coefficients (Ka-wSr/Ca and Ka-wBa/Ca) both decreased with increasing reaction time. After 96 h, R%, Ka-wSr/Ca and Ka-wBa/Ca were observed to be constant, suggesting that the solid phases were at steady-state with respect to the aqueous solutions. The thermodependence of Sr and Ba partitioning between apatite and water at low temperature could therefore be calculated: