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The concept of compositional bimodality in carbonatites has become widely accepted and has been used to impose restrictions on the composition of carbonatite magmas. We agree that mineralogical bimodality exists in carbonatites (most are either calcitic or dolomitic/ankeritic), but we argue that there is no compositional bimodality. The idea of bimodality is based on the interpretation of a variety of element distribution diagrams which were compiled only from chemical analyses in which SiO2 is < 10 wt.%. All others were rejected. Even with such a restricted data set the case for compositional bimodality is extremely weak, but the inclusion of analyses with higher SiO2 content destroys it completely. Yet these more siliceous compositions must be included, for many carbonatites contain substantial amounts of Fe–Mg silicates which are an essential part of the magmatic mineralogy of the rocks. They account for much of the Mg in carbonatites that are otherwise calcitic. Many such carbonatites contain well in excess of 10 wt.% SiO2. Supporters of the bimodality concept argue that liquids having compositions between calcite and dolomite can precipitate neither calcite nor dolomite because the minimum on the solid solution loops in the system calcite–dolomite permits only a carbonate of intermediate composition. Therefore, it is argued, liquids of such intermediate composition cannot be parental to calcitic and dolomitic carbonatites; their parent magmas must be calcitic and dolomitic. This deduction is incorrect. It is well established that dolomitic liquids have calcite as the liquidus phase over substantial temperature intervals, and that this is followed by dolomite precipitation. Mixed calcite–dolomite carbonatites are explicable in this way. Therefore, dolomitic liquids can be parental to calcitic carbonatites. However, dolomitic carbonatites cannot crystallize from a calcitic liquid. We suggest that intermediate composition carbonatite magmas are probably common. Bimodality in carbonatites is solely mineralogical, not compositional. 相似文献
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Robert G. Acres Sarah L. Harmer David A. Beattie 《International Journal of Mineral Processing》2010,94(1-2):43-51
Chalcopyrite, CuFeS2, is an important source of copper and is recovered from ore by the flotation process. Chalcopyrite is commonly associated with other metal sulfides, e.g. bornite, Cu5FeS4. In this study the effect of bornite on the oxidation and leaching of chalcopyrite has been investigated by probing the surface evolution of pure chalcopyrite, bornite, and heterogeneous samples containing both chalcopyrite and bornite using Synchrotron X-ray Photoelectron Spectroscopy (SXPS). Samples were freshly fractured in a N2 atmosphere and the resulting surfaces were oxidised in pH 9 KOH for 30 min or leached in pH 1 HCl for 2 h before being transferred into vacuum without leaving the N2 atmosphere. Analysis of the chalcopyrite region of each sample indicates that exposure to pH 9 for 30 min when bornite is present results in a decreased concentration of hydrophobic polysulfide species (from 43% to 36% of the total S 2p spectrum). In addition to this decrease in hydrophobic species, there is an increase in the amount of hydrophilic sulfate on the surface, from trace amounts to 3%. For those samples leached at pH 1 there was a small decrease in the amount of polysulfide species (43% to 39%), but also a slight increase in disulfide species (16% to 19%) indicating an alteration to the oxidation process at low pH in the presence of bornite. 相似文献
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Ian D. Howarth Nolan R. Walborn Danny J. Lennon Joachim Puls Yaël Nazé K. Annuk I. Antokhin D. Bohlender H. Bond J.-F. Donati L. Georgiev D. Gies D. Harmer A. Herrero I. Kolka D. McDavid T. Morel I. Negueruela G. Rauw P. Reig 《Monthly notices of the Royal Astronomical Society》2007,381(2):433-446
We present extensive optical spectroscopy of the early-type magnetic star HD 191612 (O6.5f?pe–O8fp). The Balmer and He i lines show strongly variable emission which is highly reproducible on a well-determined 538-d period. He ii absorptions and metal lines (including many selective emission lines but excluding He ii λ4686 Å emission) are essentially constant in line strength, but are variable in velocity, establishing a double-lined binary orbit with P orb = 1542 d, e = 0.45 . We conduct a model-atmosphere analysis of the spectrum, and find that the system is consistent with a ∼O8 giant with a ∼B1 main-sequence secondary. Since the periodic 538-d changes are unrelated to orbital motion, rotational modulation of a magnetically constrained plasma is strongly favoured as the most likely underlying 'clock'. An upper limit on the equatorial rotation is consistent with this hypothesis, but is too weak to provide a strong constraint. 相似文献
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An isotopic study on the volcanics of the Rooiberg Group: age implications and a potential exploration tool 总被引：1，自引：0，他引：1
Many geochronological studies on silicic magmatic rocks associated with the Bushveld Complex (rhyolitic lavas of the Rooiberg Group and granites of the Lebowa Granite Suite) have shown evidence of open-system behaviour of the Rb-Sr and Pb-Pb isotopic systems until 1600–1000 Ma, many hundreds of million years after crystallisation of these rocks. This pervasive open-system behaviour has been attributed to sustained hydrothermal circulation driven by the high heat productivity of the Bushveld granites. New Sr and Pb isotopic data are presented for basaltic to rhyolitic volcanics from the Rooiberg Group of the Transvaal Sequence in the Dullstroom-Loskop Dam area of the eastern Transvaal. These data show little evidence of open-system behaviour after about 1950 Ma and many sample suites retain ages which could reflect the formation of the Rooiberg Group i.e. older than 2070 Ma. It is argued that this preservation is due to the absence of fractionated, fluid/vapour-rich Bushveld granites in the immediate vicinity of the volcanic occurrences. Rooiberg Group volcanics with extensively perturbed Rb-Sr and particularly Pb-Pb isotopic systems reflect the action of granite-derived hydrothermal fluids. As a consequence, the isotope systematics in these volcanics could prove a useful exploration tool for sites of granite-derived metal deposits. 相似文献
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Chalcopyrite (CuFeS2) leaching in perchloric acid (HClO4) at an initial pH of one and a temperature of 85 °C has been examined. The rate of leaching of Cu and Fe increased progressively over the duration of the experiment. The Cu leach rate was initially greater (up to 24 h) but thereafter the leach rates for Cu and Fe were approximately equal. After 313 h 81% Cu release was achieved at which time the leach experiment was terminated. Only 25% of the available S was released into solution during the leaching process. Surface speciation over the duration of the leach was examined using X-ray photoelectron spectroscopy (XPS), time of flight secondary ion mass spectrometry (ToF-SIMS) and scanning electron microscopy (SEM). As a result, a three-step reaction pathway is proposed. The first oxidation step involves the release of Cu and Fe into solution and the polymerisation of monosulfide (S2−) to polysulfide . The subsequent reduction step does not result in the release of cations to solution but does result in the reformation of surface S2− and other short chain polysulfides, which then on further oxidation restructure to form crystalline elemental sulfur (S0). This final oxidation step is accompanied by further cation release. 相似文献
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The intracratonic Bushveld igneous province that formed 2.1–1.9 Ga ago contains three contrasting suites of siliceous rocks that are demonstrably of magmatic origin. The oldest of these are the high-Mg (HMF) and low-Mg (LMF) felsites, which form interstratified flows in the Rooiberg Group. Bushveld granites intrude the Rooiberg Group and constitute the youngest component of the province.Well-defined interelement variation trends illustrate that the granites do not share the same petrogenetic history as the Rooiberg magmas. Nd isotope measurements indicate that the two eruptive suites probably formed under similar differentiation conditions but from parental magmas that were derived from compositionally different sources. On trace-element discrimination diagrams, the Bushveld granites and LMF are usually correctly assigned to a within-plate setting but, conversely, the HMF are generally misclassified as subduction-related eruptives. It is argued that the trace-element signatures of the granites and felsites do not identify their tectonic setting per se, but rather point to the melting and crystallization histories of the source regions from which their parent magmas were extracted. As such, tectonic discrimination diagrams may provide valuable pointers to processes that have affected igneous source materials in much earlier magmatic cycles. 相似文献