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1.
The pressure-dependent elastic properties of the Fe–S system are important to understand the dynamic properties of the Earth’s interior. We have therefore undertaken a first-principles study of the structural and elastic properties of FeS2 polymorphs under high pressure using a method based on plane-wave pseudopotential density function theory. The lattice constants, elastic constants, zero-pressure bulk modulus, and its pressure derivative of pyrite are in good agreement with the previous experiments and theoretical approaches; the lattice constants of marcasite are also consistent with the available experimental data. Calculations of the elastic constants of pyrite and marcasite have been determined from 0 to 200 GPa. Based on the relationship between the calculated elastic constants and the pressure, which can provide the stability of mineral, it would appear that pyrite is stable, whereas marcasite is unstable when the pressure rises above 130 GPa. Static lattice energy calculations predict the marcasite-to-pyrite phase transition to occur at 5.4 GPa at 0 K.  相似文献   

2.
《Chemical Geology》2006,225(3-4):278-290
The thermodynamic mixing properties of As into pyrite and marcasite have been investigated using first-principles and Monte Carlo calculations in order to understand the incorporation of this important metalloid into solid solution. Using quantum-mechanical methods to account for spin and electron transfer processes typical of sulfide minerals, the total energies of different As–S configurations were calculated at the atomic scale, and the resulting As–S interactions were incorporated into Monte Carlo simulations. Enthalpies, configurational entropies and Gibbs free energies of mixing show that two-phase mixtures of FeS2 (pyrite or marcasite) and FeAsS (arsenopyrite) are energetically more favorable than the solid solution Fe(S,As)2 (arsenian pyrite or marcasite) for a wide range of geologically relevant temperatures. Although miscibility gaps dominate both solid solution series, the solubility of As is favored for XAs < 0.05 in iron disulfides. Consequently, pyrite and marcasite can host up to ∼6 wt.% of As in solid solution before unmixing into (pyrite or marcasite) + arsenopyrite. This finding is in agreement with previously published HRTEM observations of As-rich pyrites (> 6 wt.% As) that document the presence of randomly distributed domains of pyrite + arsenopyrite at the nanoscale. According to the calculations, stable and metastable varieties of arsenian pyrite and marcasite are predicted to occur at low (XAs < 0.05) and high (XAs > 0.05) As bulk compositions, respectively.  相似文献   

3.
A thin film of marcasite, FeS2, was synthesized under vacuum and its structure and reactivity under oxidizing conditions was investigated by means of diffraction and surface analytical techniques, respectively. Synthesis of the film was carried out by codepositing Fe and S2 onto a Ta support. The thickness of the film could be varied from approximately 10 Å to 1 μm. High-resolution S 2p synchrotron-based photoemission showed S22−, with undetectable amounts of S2− impurity that is typically present on natural sample surfaces. X-ray diffraction of the micron-thick films showed that the film crystallized in the marcasite phase of FeS2. Atomic force microscopy indicated that the thin film had a nanometer-scale roughness suggesting the film contained defects such as steps and kinks. X-ray photoelectron spectroscopy studies found the thin marcasite film to be more reactive than natural pyrite (the most ubiquitous FeS2 dimorph) after exposure to a gaseous O2/H2O environment on the basis of the amount of sulfate formation. Likely the oxidation of marcasite was dominated by its short-range order (e.g., presence of steps), because the density of nonstoichiometric defect sites (e.g., S2−) was low as assessed by photoelectron spectroscopy.  相似文献   

4.
The polarized far-infrared reflection spectra of single crystals of FeS2-marcasite are presented in the range from 40–700 cm?1. The spectra show 7 reststrahlen bands, as predicted by group theory. The oscillator parameters ?α ∞, ωα f, ?α f, γα f, and the transversal and longitudinal optical phonon frequencies ωTO and ωLO as well as effective ionic charges and oscillator strength weighted mean phonon frequencies were calculated. The anisotropic behaviour of these quantities is discussed in relation to the data for FeS2-pyrite. It is shown that the ionicity of marcasite is considerably smaller than that of pyrite, especially in the a and c direction. The directional dependence of the phonon frequencies is given and discussed with regard to the spectra of polycrystalline samples.  相似文献   

5.
Single-crystal Raman spectra of marcasite-type FeS2, FeSe2, and FeTe2 and loellingite-type FeP2, FeAs2, and FeSb2 are presented and discussed with reference to the energies of the two X-X stretching modes v x-x (A g, B 1g) and the four X2 librations Rx2 (A g, B 1g, B 2g, B 3g). The main results obtained are that (i) the intraionic X-X bonds of FeS2 marcasite and FeS2 pyrite are nearly equal in strengths (mean values of the S-S stretching modes 418 and 420 cm-1, respectively) and (ii) the interactions of the metal ions and the dumbbell-like X2 units increase on going from the chalcogenides to the respective pnictides and from FeS2 marcasite to pyrite (as shown from the frequencies of the X2 librations).  相似文献   

6.
Eighteen pyrite and twelve marcasite samples which have different provenances have been investigated to determine the systematics of the influence of mineralogical and geological factors on the 57Fe Mössbauer spectra at 298 K. The following results have been obtained: there is no ambiguity in distinguishing single phase pyrite from single phase marcasite by means of 57Fe Mössbauer spectroscopy at 298 K. At 298 K the average electric quadrupole splitting, 〈ΔEQ〉, and average isomer shift, 〈δ〉, with respect to Fe metal, are 0.6110 ± 0.0030 mm s?1 and 0.313 ± 0.008 mm s?1, respectively, for the 18 pyrites; 〈ΔEQ〉 = 0.5030 ± 0.0070 mm s?1 and 〈δ〉 = 0.2770 ± 0.0020 mm s?1 for the 12 marcasites. At 77 K, ΔEQ is 0.624 mm s?1 for pyrite and 0.508 mm s?1 for marcasite. In distinguishing pyrites from marcasites, spectra obtained at 77 K are not warranted.The Mössbauer parameters of pyrite and marcasite exhibit appreciable variations, which bear no simple relationship to the geological environment in which they occur but appear to be selectively influenced by impurities, especially arsenic, in the pyrite lattice. Quantitative and qualitative determinations of pyrite/marcasite mechanical mixtures are straightforward at 298 K and 77 K but do require least-squares computer fittings and are limited to accuracies ranging from ±5 to ±15 per cent by uncertainties in the parameter values of the pure phases. The methodology and results of this investigation are directly applicable to coals for which the presence and relative amounts of pyrite and marcasite could be of considerable genetic significance.  相似文献   

7.
Compounds with mixed and intermediate sulfur valences form cloudy small inclusions in banded pyrite and 2–4 mm large rounded aggregates in pyrite stalactites. Such areas under high magnification of SEM appear to be a mixture of FeS2 with FeS2O3. Some of the microareas of 40–50 m are homogeneous and can be identified by reflected light microscopy, microprobe and soft X-ray spectroscopy as Fe-thiosulphate, FeS2O3 or (Fe, Pb)2S3O7 with this compound having one sulfur –2 and two sulfurs +6. The intermediate sulfur compounds are enriched in Pb, Ni, Co and As.  相似文献   

8.
《Chemical Geology》2007,236(3-4):228-246
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9.
Electronic and optical properties of Fe, Zn and Pb sulfides   总被引：1，自引：0，他引：1  
Ab initio quantum-chemical calculations of the spatial and electronic structures of sphalerite (ZnS), pyrite (FeS2) and galena (PbS), using the density functional theory (DFT) local density approximation (LDA) and generalized gradient approximation (GGA), the Hartree–Fock (HF) method and the hybrid functional B3LYP, have been carried out. For galena, the DFT LDA and GGA functionals provided the best estimate of the band gap, from within –0.1 eV to +0.4 eV of the measured value. B3LYP and RHF gave rise to errors of +1.3 and +5.4 eV, respectively. The unit cell parameter error varied from between –1.1% and +2.3% for all the functionals examined. For sphalerite the B3LYP functional provided the best estimate of the band gap (error +0.3 eV). The unit cell parameter error varied between –2.1% and +2.0% for the various DFT functionals and B3LYP. RHF gave rise to an error of +3.8%. For FeS2, the DFT-GGA approach provides the best results for both the unit cell and the band gap. This may be due to mutual cancellation of the crystal field splitting and band separation force, which are of equal but opposite magnitudes. The calculated density of states (DOS) for the conduction band is used to interpret the experimental features of the S 1s XANES (X-ray absorption near-edge structure) spectra obtained using synchrotron radiation. Because of the l = ±1 selection rule for electron excitation, the S K-edge XANES spectra represent a transition of the S 1s electron to conduction band S p-like orbitals. The near-edge region, up to 15 eV past the edge is approximated well by the DOS. Individual peaks in the DOS correlate with peaks in the XANES spectra. In addition, the imaginary part of the dielectric function, which reflects the transitions from occupied to unoccupied levels, is used to model the near-edge region of the XANES, using the DFT-GGA formalism. Individual peaks in the XANES spectrum are moderately well resolved using the dielectric function, especially for ZnS and FeS2, while the DOS for the conduction band is more successful in predicting the shape of the XANES spectra for all three minerals.  相似文献   

10.
We present the results of an experimental study into the sulfidation of magnetite to form pyrite/marcasite under hydrothermal conditions (90-300 °C, vapor saturated pressures), a process associated with gold deposition in a number of ore deposits. The formation of pyrite/marcasite was studied as a function of reaction time, temperature, pH, sulfide concentration, solid-weight-to-fluid-volume ratio, and geometric surface area of magnetite in polytetrafluoroethylene-lined autoclaves (PTFE) and a titanium and stainless steel flow-through cell. Marcasite was formed only at pH21°C <4 and was the dominant Fe disulfide at pH21°C 1.11, while pyrite predominated at pH21°C >2 and formed even under basic conditions (up to pH21°C 12-13). Marcasite formation was favored at higher temperatures. Fine-grained pyrrhotite formed in the initial stage of the reaction together with pyrite in some experiments with large surface area of magnetite (grain size <125 μm). This pyrrhotite eventually gave way to pyrite. The transformation rate of magnetite to Fe disulfide increased with decreasing pH (at 120 °C; pH120°C 0.96-4.42), and that rate of the transformation increased from 120 to 190 °C.Scanning electron microscope (SEM) imaging revealed that micro-pores (0.1-5 μm scale) existed at the reaction front between the parent magnetite and the product pyrite, and that the pyrite and/or marcasite were euhedral at pH21°C <4 and anhedral at higher pH. The newly formed pyrite was micro-porous (0.1-5 μm); this micro-porosity facilitates fluid transport to the reaction interface between magnetite and pyrite, thus promoting the replacement reaction. The pyrite precipitated onto the parent magnetite was polycrystalline and did not preserve the crystallographic orientation of the magnetite. The pyrite precipitation was also observed on the PTFE liner, which is consistent with pyrite crystallizing from solution. The mechanism of the reaction is that of a dissolution-reprecipitation reaction with the precipitation of pyrite being the rate-limiting step relative to magnetite dissolution under mildly acidic conditions (e.g., pH155°C 4.42).The experimental results are in good agreement with sulfide phase assemblage and textures reported from sulfidized Banded Iron Formations: pyrite, marcasite and pyrrhotite have been found to exist or co-exist in different sulfidized Banded Iron Formations, and the microtextures show no evidence of sub-μm-scale pseudomorphism of magnetite by pyrite.  相似文献   

11.
In the mining complex of Kherzet Youcef, including the Chaabet-el-Hamra mine and processing plant, chemical analyzes were carried out on soil and sediment samples and exceeded the normative guidelines for zinc, lead, and cadmium except a small area in the foothills. X-ray diffractometer analyzes confirmed the presence of a considerable inclusion of sulfides (up to 20%), such as pyrite, marcasite (FeS2) and sphalerite (ZnS) especially at the liquid waste disposal site and technogenic sediments. To predict changes in the mineral composition of soils and waste under the condition of their contact with rain water and to assess the potential contamination of ground waters, a thermodynamic model SELECTOR was applied on selected samples. The results of this study conclude that the calculated equilibrium mineral composition is very close to the real phase composition only for the least polluted soils. For technogenic sediments or waste, the equilibrium mineral composition differs significantly from the experimental one. The modeling of potential transition of the main pollutants in water soluble forms allows concluding that in the study area should be feared of exceeding of normative guidelines of cadmium, zinc, and lead.  相似文献   

12.
Auriferous quartz pebble conglomerates (QPC) formed during Tertiary sedimentary recycling in the Waimumu district, Southland, New Zealand. These sediments contain fine-grained gold of detrital origin with abundant surface textures and gold-forms associated with authigenic gold remobilisation. Most authigenic gold contains no detectable silver and occurs as overgrowths on detrital Au–Ag and Au–Ag–Hg alloys that contain up to 13 wt.% Ag, and 9 wt.% Hg. Fine-grained Au–Ag and Au–Ag–Hg alloys are compositionally heterogeneous, exhibiting both well-defined silver-depleted and silver-enriched rims. Rare coarse Au–Ag alloy is intergrown with quartz and is homogenous. Discrete grains of authigenic, porous, sheet-like gold occur in carbonaceous mudstone within a QPC sequence. Some QPC contain abundant sulphide minerals. Some of these sulphides (pyrite and arsenopyrite) are of long-distance detrital origin, presumably from the Otago Schist, whereas the bulk of the sulphide suite is marcasite of variably transported diagenetic origin, derived from the erosion of QPC and underlying Tertiary sediments. There has also been authigenic deposition of sulphide minerals in the QPC themselves. These diagenetic sulphides include framboidal and anhedral marcasite, and framboidal and euhedral pyrite. Sulphur isotope data for the sulphide minerals range from − 45‰ to + 18‰ (relative to VCDT). Sulphur isotope data for euhedral detrital pyrite and arsenopyrite range from − 9‰ to − 1‰ and are most likely derived from the Otago Schist to the north. Both framboidal and anhedral marcasite have lower values (< − 20‰) reflecting microbial sulphate reduction as a source for the precursor hydrogen sulphide. Anhedral marcasite contains elevated concentrations of Ni, Co, As and Cr, commonly with compositional banding of these metals.Both the gold and diagenetic sulphides from the Belle-Brook QPC are compositionally similar to gold and sulphides from Archaean QPC. Porous, sheet-like authigenic gold is morphologically similar to gold associated with carbonaceous material in the Witwatersrand. In addition, Southland marcasite textures resemble the rounded and banded pyrite in Witwatersrand QPC placers. There is abundant evidence from these Tertiary QPC in southern New Zealand for sedimentary transport of sulphide minerals and post-depositional sulphide mineralisation in the surficial environment despite an oxygen-rich atmosphere. These young deposits thus provide an example of authigenic gold and sulphide textures formed during diagenesis in unmetamorphosed placers. Many of these textures are similar to those commonly ascribed to metamorphic processes in Archaean auriferous QPC.  相似文献   

13.
To enhance the computer simulation of hydrothermal processes using the HCh program package, an external ZnS_FeS module has been created on the basis of a nonideal asymmetric model of sphalerite solid solution. FeS and ZnS activity coefficients computed in line with this model within a temperature range 200?C350°C lead to the decrease in FeS mole fraction (X FeS) in sphalerite by 3.0?C1.5 times as compared with the ideal model. The calculated data on composition of sphalerite at the pyrite-pyrrhotite buffer with allowance for pyrrhotite nonideality are consistent with experimental results within the limits of 2% X FeS of its value (0.215). A nonlinear relationship logX FeS versus $\left( {\log f_{S_2 } } \right)$ . has been established, involving additional calculated data on equilibria of sphalerite with pyrite and magnetite, as well as pyrite and barite. With transition from pyrrhotite to magnetite and barite, a FeS mole fraction in sphalerite decreases to 0.1 and 0.006, respectively, because of increase in sulfur fugacity. The feasibility of using the calculation results based on the nonideal model of sphalerite for interpretation of natural data is exemplified in the Rainbow ore occurrence at the Mid-Atlantic Ridge (MAR). The computed pyrite-pyrrhotite and pyrite-cubanite-chalcopyrite buffer equilibria (X FeS = 0.215 and 0.10?C0.12, respectively) are consistent with compositions of sphalerite in the pyrrhotite-cubanite-sphalerite and sphalerite ores (X FeS = 0.20?C0.33 and 0.05?C0.14, respectively).  相似文献   

14.
Gold mineralization related to a sulphide-bearing quartz vein within Triassic phyllites has been identified in a former study of an abandoned Cu–Zn–Pb mine in southern Tuscany–northern Latium. The vein-hosted mineral assemblage includes pyrite, sphalerite, chalcopyrite, galena, arsenopyrite, and tetrahedrite; the gold content reaches up to 20 ppm. The mineralization was related to fluids characterised by moderate temperatures and moderate salinities (5–15% NaCl equivalent) and represents the highest grade of Au in the ore deposits of southern Tuscany. The difference in the estimated values of Te of primary, L+V, pseudo-secondary L+V, secondary L+V fluids inclusions reflects different components present in the solutions.Based on these results one can conclude that euhedral quartz and pyrite deposition was related to fluids having variable salinity and chemical components.The origin of gold in Ponte San Pietro (PSP) deposit may be related to leaching of metapelites/phyllites derived from pelitic rocks containing some amount of sulphur-bearing organic matter; it is worthy noting that quartz–marcasite bearing lenses, interstratified in Triassic phyllites, contain significant amount of visible gold. The Co, Ni, Mn, as well as the δ34S data, suggest a different origin for the phyllite-hosted marcasite, the PSP quartz-vein hosted pyrite, and for gold occurrences.  相似文献   

15.
Metal K- and L3-, sulfur K- and arsenic K- and L3-edge X-ray absorption near-edge spectra of a series of metal disulfides, FeS2 (both pyrite and marcasite), CoS2, NiS2, and CuS2, and their isomorphs, FeAsS and CoAsS, are presented. The features in this region of these spectra are interpreted using band structure and molecular orbital calculations in terms of the transitions from the 1s or 2p3/2 state to unoccupied states. The 3d transition metal L3-edge spectra of these materials show dependence on the degree of multiplet splitting in the final state, and thus offer less information on the electronic ground state. There are substantial differences in the spectra of the isostructural materials, whereas the spectra of the isotopes pyrite and marcasite show several similarities, illustrating the dependence of near-edge region on electronic structure.  相似文献   

16.
The nature of the surface oxidation phase on pyrite, FeS2, reacted in aqueous electrolytes at pH = 2 to 10 and with air under ambient atmospheric conditions was studied using synchrotron-based oxygen K edge, sulfur LIII edge, and iron LII,III edge X-ray absorption spectroscopy. We demonstrate that O K edge X-ray absorption spectra provide a sensitive probe of sulfide surface oxidation that is complementary to X-ray photoelectron spectroscopy. Using total electron yield detection, the top 20 to 50 Å of the pyrite surface is characterized. In air, pyrite oxidizes to form predominantly ferric sulfate. In aqueous air-saturated solutions, the surface oxidation products of pyrite vary with pH, with a marked transition occurring around pH 4. Below pH = 4, a ferric (hydroxy)sulfate is the main oxidation product on the pyrite surface. At higher pH, we find iron(III) oxyhydroxide in addition to ferric (hydroxy)sulfate on the surface. Under the most alkaline conditions, the O K edge spectrum closely resembles that of goethite, FeOOH, and the surface is oxidized to the extent that no FeS2 can be detected in the X-ray absorption spectra. In a 1.667 × 10−3 mol/L Fe3+ solution with ferric iron present as FeCl3 in NaCl, the oxidation of pyrite is autocatalyzed, and formation of the surface iron(III) oxyhydroxide phase is promoted at low pH.  相似文献   

17.
Pyritization in late Pleistocene sediments of the Black Sea is driven by sulfide formed during anaerobic methane oxidation. A sulfidization front is formed by the opposing gradients of sulfide and dissolved iron. The sulfidization processes are controlled by the diffusion flux of sulfide from above and by the solid reactive iron content. Two processes of diffusion-limited pyrite formation were identified. The first process includes pyrite precipitation with the accumulation of iron sulfide precursors with the average chemical composition of FeSn (n = 1.10-1.29), including greigite. Elemental sulfur and polysulfides, formed from H2S by a reductive dissolution of Fe(III)-containing minerals, serve as intermediates to convert iron sulfides into pyrite. In the second process, a “direct” pyrite precipitation occurs through prolonged exposure of iron-containing minerals to dissolved sulfide. Methane-driven sulfate reduction at depth causes a progressive formation of pyrite with a δ34S of up to +15.0‰. The S-isotopic composition of FeS2 evolves due to contributions of different sulfur pools formed at different times. Steady-state model calculations for the advancement of the sulfidization front showed that the process started at the Pleistocene/Holocene transition between 6360 and 11 600 yr BP. Our study highlights the importance of anaerobic methane oxidation in generating and maintaining S-enriched layers in marine sediments and has paleoenvironmental implications.  相似文献   

18.
胶东地区-1000 m以下深部找矿的重大突破,使得探明储量已达5000多t,成为探讨深部金的赋存状态及成矿作用的天然实验室。招贤金矿为焦家成矿带近年深部找矿重大突破之一,矿体主要产于-1260 m以深的晚侏罗世二长花岗岩中,受控于焦家断裂。金属矿物主要为黄铁矿、黄铜矿和银金矿等,脉石矿物包括石英、绢云母、方解石、钾长石等。围岩蚀变以钾长石化、硅化、黄铁绢英岩化、碳酸盐化为主。金矿物以自然金和银金矿为主,呈裂隙金或包体金分布于黄铁矿中,少数不可见金呈晶隙金分布于黄铁矿等矿物中。其中,黄铁矿w(S)=52.227%～54.915%、w(Fe)=44.749%～47.134%,原子个数比S/Fe=1.99～2.11,化学式FeS1.99～FeS2.11;黄铜矿w(S)=34.282%～35.140%、w(Fe)=29.263%～30.268%,w(Cu)=33.130%～34.114%,化学式Cu0.96FeS2.01～Cu1.01FeS2.10,平均化学式为C...  相似文献   

19.
Phase relations in the Ag-Fe-S system were determined from 700 to 150 °C by quench experiments with the use of evacuated, sealed, silica tubes as reaction vessels; these data were then used to interpret various aspects of natural occurrences of Ag-Fe-S minerals (e.g. argentiferous pyrite). The assemblages Ag2S+Fe1–x S and Ag2S+FeS2 become stable, with decreasing temperature, at 622±2 ° and 607±2 °C, respectively; their establishments involve ternary invariant conditions. The three condensed phases Ag2S+Fe1–x S+FeS2 become stable together at 532±2 °C through a ternary eutectic reaction near Ag2S in composition. An invariant reaction at 248±8 °C results in the formation of the Ag+FeS2 pair from the Ag2S+Fe7S8 assemblage, which is stable at higher temperatures. The associations of native silver and pyrite are found in certain massive sulfide deposits, whereas natural coexistence of argentite and pyrrhotite has not been documented. Experiments demonstrate the feasibility of retrograde reequilibration in ores to produce the silver+pyrite pair from argentite+pyrrhotite. Less than 0.05 and 0.1 at. % Ag are soluble in FeS2 and Fe1–x S, respectively, at 600 °C and less than 0.8 at. % Fe in Ag2S at 500 °C. Silver does not measurably affect the d 10.2 values of Fe1–x S or the cell dimension of FeS2 (a 25 °C=5.4175±0.0001 Å). This study also demonstrates that at low temperatures the binary fugacity data are applicable to ternary assemblages of the Ag-Fe-S system because of these very limited solubilities. The presence of Fe lowers the fcc bcc inversion temperature of Ag2S more than 50 °C; the exact amount of lowering is dependent on the associated Ag-Fe-S phases. The bcc mono. inversion temperature, however, is not measurably affected. No ternary solid phases were encountered above 150 °C. Heating of sternbergite and argentopyrite (both AgFe2S3) mineral samples shows instability at 152 °C (e.g. partial breakdown of sternbergite in 405 days); rate studies show that a 10 °C temperature increase results in approximately a 5-fold increase in breakdown rate.
Zusammenfassung Für die Interpretation von natürlichen Ag-Fe-S-Mineralen (z. B. silberhaltiger Pyrit) werden die Phasenbeziehungen im System Ag-Fe-S durch quenching Experimente bei Temperaturen von 700 ° bis 150 °C untersucht. Evakuierte und zugeschmolzene Quarzglasröhren dienen dabei als Reaktionsgefäße. Die Phasenassoziationen Ag2S+Fe1–x S (Argentit+Magnetkies) und Ag2S+FeS2 (Argentit+Pyrit) werden mit fallender Temperatur bei 622±2 °C und 607±2 °C stabil. Ihre Bildung ist nur unter ternären, invarianten Bedingungen möglich. Bei 532±2 °C bilden sich durch eutektische Reaktion (nahe der Ag2S-Zusammensetzung) als stabile Phasen Ag2S+Fe1–x S+FeS2. Bei 248±8 °C reagieren Ag2S+Fe7S8, die zwischen Temperaturen von 292 °C bis 248 °C stabil sind, zu Ag+FeS2. Paragenesen von gediegen Silber+Pyrit kommen in einigen massiven Sulfidlagerstätten vor; die Paragenese Argentit+Magnetkies ist dagegen noch nicht beobachtet worden. Die bisherigen experimentellen Ergebnisse machen eine retrograde Wiedereinstellung des Gleichgewichtes von gediegen Silber+Pyrit aus Argentit+Magnetkies wahrscheinlich. Bei 600 °C sind <0,05% bzw. 0,1% Ag in FeS2 und Fe1–x S löslich. Bei 500 °C lösen sich <0,8% Fe in Ag2S. Die Zellkonstanten von Magnetkies (gemessen als d 10,2) und von Pyrit (a 25 °C=5,4175±0,0001 Å) werden durch die Aufnahme von Ag nicht meßbar beeinflußt. Die vorliegenden Ergebnisse zeigen, daß die Fugazitäten bei niederen Temperaturen entlang den binären Schnitten Fe-S und Ag-S auch auf das ternäre System Ag-Fe-S angewendet werden können, weil nur sehr beschränkte Mischbarkeiten existieren. Die Gegenwart von Fe erniedrigt die Inversions-temperatur fcc bcc für Ag2S um mehr als 50 °C. Die genaue Inversions-temperatur wird durch die assozierten Ag-Fe-S Phasen festgelegt. Die bcc mono. Inversionstemperatur wird dagegen nicht meßbar beeinflußt. Oberhalb 150 °C werden keine ternären Phasen beobachtet. Sternbergit und Argentopyrit (beides AgFe2S3-Minerale) werden oberhalb 152 °C instabil (z. B. bricht Sternbergit teilweise nach 405 Tagen zusammen). Eine Temperaturerhöhung um ca. 10 °C erhöht die Zerfallsrate um ein Fünffaches.
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20.
高温高压下黄铁矿热力学性质的第一性原理研究   总被引：1，自引：0，他引：1  
刘善琪  李永兵  石耀霖 《岩石学报》2018,34(6):1813-1818
黄铁矿是自然界中分布最为广泛的硫化物矿物,同时也是重要的造矿矿物,在金属矿床、沉积岩、变质岩、花岗岩、基性-超基性岩浆岩、以及地幔岩中都有大量出现。因此,研究黄铁矿在不同温度压力下的热力学性质可以为深入探讨与黄铁矿有关的成岩、成矿、成藏问题提供有用的矿物学依据。本文利用基于密度泛函微扰理论的第一性原理方法,采用准谐近似计算了黄铁矿在高温高压下的热力学性质。我们计算的黄铁矿的晶格常数、零压下的体积模量及其对压力的导数与前人的实验及理论计算结果吻合得很好,零压下等压热容和熵随温度的变化与实验结果有很好的一致性。尤其是,本文计算了直至2500K、100GPa的高温高压下黄铁矿的等温体积模量、热膨胀系数、热容和熵等热力学性质。这为在有硫参与的情况下,人们开展下地壳-岩石圈地幔深度的地球动力学模拟和建立地球物理模型提供了有用的信息。  相似文献   

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