| 1. |
- Ferrow, Embaie, et al.
(författare)
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Characterization of pyrrhotite in Cu-Ni-ore bodies from mines in Botswana by Mossbauer spectroscopy, X-ray diffraction, and thermomagnetometry
- 2006
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Ingår i: European Journal of Mineralogy. - : Schweizerbart. - 1617-4011 .- 0935-1221. ; 18:5, s. 653-664
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Tidskriftsartikel (refereegranskat)abstract
- Cu-Ni ore bodies from five Botswana mines were investigated, using a combination of X-ray diffraction, thermomagnetic measurements, Mossbauer spectroscopy, and Inductively Coupled Plasma-Atomic Absorption Electro-optic Spectroscopy (ICP-AES). The four dominant phases in the ores were found to be pyrrhotite, magnetite, pentlandite, and chalcopyrite. Mossbauer spectroscopy shows that pyrrhotite is the dominant Fe-bearing phase in the samples with 75 % in B-S1, 84 % in B-S2, and 80 % in B-S4 and was selected for detailed characterization. Our results show that the three first techniques provide an excellent approach to estimate the degree of polytype intermixing in pyrrhotite. Accordingly, the ores can be grouped into three types: A hexagonal (or peak-type), with a 5c stacking arrangement; a monoclinic (or Weiss-type), with a 4c stacking arrangement; and a mixture of monoclinic and hexagonal types. A low-temperature study of the monoclinic form shows that the low-temperature magnetic transition close to 35 K has features similar to the Verwey transition in magnetite induced by Jahn-Teller distortion due to orbital order-disorder transition.
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| 2. |
- Ferrow, Embaie
(författare)
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Non-integral hybrid ions in tourmaline: buffering and geo-thermometry
- 2009
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Ingår i: European Journal of Mineralogy. - : Schweizerbart. - 1617-4011 .- 0935-1221. ; 21:1, s. 241-250
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Tidskriftsartikel (refereegranskat)abstract
- The tourmaline group of minerals is indeed an enigma. Experimental data from optical spectroscopy, electron microscopy, and Mossbauer spectroscopy reveal a host of physical properties that lack a common structural clarification. For example, tourmaline samples change colour when irradiated with X-ray and gamma-ray radiations some reverting back when heated in air; exhibit simultaneous oxidation and reduction on annealing in an atmosphere of H-2; display different plane group symmetries under TEM; possess the most complicated Mossbauer spectra of all Fe-bearing silicates. In this study, four Brazilian samples were chosen for detailed study by Mossbauer spectroscopy to find out a common structural factor to the physical anomalies reported in the literature. It was found out that the tourmaline group of minerals contain multi-valence elements that are involved in electron exchange between the edge-sharing asymmetric Y and Z crystallographic sites. It is conceivable that the host of physical properties recorded in the literature could be due to the inherent structural misfit between the Y and Z sites and the mechanisms adopted to reduce the strain associated along the shared edges. The complexity of non-integral oxidation states possible - due to electron sharing among the different multi-valence elements present in the structure - further enhances the diverse physical properties observed. Moreover, on heating in air, no net oxidation or reduction takes place in the tourmaline group of minerals over a temperature range as long as there are electron donors and acceptors left in the structure, serving simultaneously as potential single-phase buffers and geo-thermometers.
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| 3. |
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| 4. |
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| 5. |
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| 6. |
- Ladenberger, Anna, et al.
(författare)
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CO2 fluid inclusions in mantle xenoliths from Lower Silesia (SW Poland) : formation conditions and decompression history
- 2009
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Ingår i: European journal of mineralogy. - : Schweizerbart. - 0935-1221 .- 1617-4011. ; 21:4, s. 751-761
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Tidskriftsartikel (refereegranskat)abstract
- Raman spectroscopy has been applied to determine the density and pressure of formation of CO2 fluid inclusions in mantle xenoliths, carried to the surface at Wilcza Gora in southwestern Poland by early Miocene alkaline magmas. The magmas were generated by partial melting in the transition zone between the spinel and garnet stability fields. Determination of the densities of CO2 inclusions allows calculation of the partial pressures and reconstruction of the depth of xenolith origin as well as their history en route to the surface. The density of CO2 inclusions ranges from 0.06 to 1.10 g/cm(3) in olivines and 0.17 to 1.11 g/cm(3) in orthopyroxenes. Only inclusions with a density above 0.8 g/cm(3) reflect lower crust (ca. 15-30 km) and upper mantle (ca. 30-38 km) conditions. Slight differences in density of the inclusions between olivines and orthopyroxenes can be attributed to their different theological properties during magma ascent. Modelling of pressure and depth within the temperature range in which the xenoliths equilibrated with the magmas yields information about the complex eruption history of basanitic volcanoes. Our estimates are consistent with seismic data that show a regional high-velocity layer at the Moho (ca. 30-35 km deep) with p-wave velocities up to 8 km/s which have been attributed to mafic and ultramafic lithologies.
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| 7. |
- Sundvall, Rickard, 1976-, et al.
(författare)
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Dehydration-hydration mechanisms in synthetic Fe-poor diopside
- 2009
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Ingår i: European journal of mineralogy. - Stuttgart : E. Schweizerbart'sche Verlagsbuchhandlung. - 0935-1221 .- 1617-4011. ; 21:1, s. 17-26
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Tidskriftsartikel (refereegranskat)abstract
- Small amounts (ppm) of OH in nominally anhydrous minerals (NAMs) can have a dramatic effect on the physical properties of the upper mantle. The pyroxenes of the upper mantle have been shown to incorporate substantial numbers of protons forming hydroxyl ions. Enstatite and diopside are the most important endmembers of the pyroxenes in terms of bulk volume in the upper mantle. To further constrain the behavior of hydroxyl ions in clinopyroxene, the dehydration-hydration mechanisms of synthetic 57Fe-doped diopside were investigated. Dehydration was carried out by stepwise heating in air of crystals synthesized at high pressure under water-saturated conditions. FTIR spectra were obtained after each step. Mössbauer spectra were recorded for three of the crystals when there had been a significant decrease in FTIR absorbance intensity. From the Mössbauer spectra we see an increase in the Fe3+ doublet with successive dehydration, although this increase is less than the decrease in OH in terms of atoms per formula unit. This means that the dehydration only partly follows the redox reaction OH− + Fe2+ = O2− + Fe3+ + ½H2, and that additional reactions occur. Hydration experiments were conducted on one crystal in the same manner as the dehydrations, with the exception that hydrogen gas was used during heating. Hydration experiments resulted in re-hydration of the sample to 73 % of the original amount of OH. The calculated Arrhenius equation derived from the diffusion rates during dehydration along [010] yields an activation energy (Ea) of −292 ± 50 kJ mol−1, and D0 = 10±1.9 + 2.3 m2 s−1. The result of the rehydration experiment agrees well with the established diffusion law. Diffusion rates determined for synthetic diopside are almost two orders of magnitude slower than for synthetic enstatite with comparable Fe contents. Compared to natural diopside, diffusion rates in these synthetic samples are slower, probably because of the low iron content. Ea is similar to that of dehydration of pure and low-Fe enstatite.
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| 8. |
- Sundvall, Rickard, 1976-, et al.
(författare)
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Hydrogen diffusion in synthetic Fe-free diopside
- 2009
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Ingår i: European journal of mineralogy. - Stuttgart : E. Schweizerbart'sche Verlagsbuchhandlung. - 0935-1221 .- 1617-4011. ; 21:5, s. 963-970
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Tidskriftsartikel (refereegranskat)abstract
- Hydrogen is a widespread trace element in many nominally anhydrous minerals (NAMs) in the Earth's crust and mantle and has profound influence on the physical properties of the host mineral. Of all NAMs from the upper mantle, clinopyroxenes have been shown to contain the highest amount of hydrogen. This study focuses on the dehydration kinetics of pure diopside along [010] and [100]* by thermal annealing under normal atmospheric pressure. The diopside crystals used were synthesized at high pressure under water-saturated conditions. FTIR spectra were obtained after each step, including untreated samples. The Arrhenius equation gives an activation energy (Ea) of -331 ± 50 kJ mol-1 and D0 = 100.9 ± 2.3 m2 s-1 for diffusion along [010]. Diffusion along [100]* gives an Ea-value of -312 ± 55 kJ mol-1 with D0 = 100.5 ± 2.4 m2 s-1. Therefore, our experimental results show no difference between diffusion along [010] and [100]* (within error limits). The diffusion rate in pure synthetic diopside is about one order of magnitude faster than for synthetic diopside with very low Fe contents. A suitable explanation for this behavior is that in the case of low Fe diopside, the rate-limiting process for the protons associated with Fe is probably Fe-diffusion. In contrast, in pure diopside all protons are associated to Mg-defects, which are more mobile than Fe. Nevertheless, compared to natural diopside with appreciable Fe contents, diffusion rates in these synthetic samples are several orders of magnitude slower.
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| 9. |
- Konrad-Schmolke, Matthias, 1970, et al.
(författare)
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Compositional re-equilibration of garnet: the importance of sub-grain boundaries
- 2007
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Ingår i: European journal of mineralogy. - 0935-1221 .- 1617-4011. ; 19:4, s. 431-438
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Tidskriftsartikel (refereegranskat)abstract
- Garnets from meta-granitoid high pressure rocks (Sesia Zone, Western Alps) show complex internal sub-grain textures in electron forescatter images. All investigated garnets consist of a large number of sub-grains with different shapes and sizes. Some garnets exhibit a sub-texture with very fine-grained (<20µm) sub-grains in their cores overgrown by palisade-like sub-grains in the rims. Sub-grain boundaries in these garnets have enabled diffusive element exchange between the garnet core and the surrounding matrix. Compositional mapping reveals zonation patterns of Mg that indicate modification of the garnet composition during prograde metamorphism. The extent of diffusional re-equilibration is dependent on sub-grain size and element diffusivities. Our samples show that XMg is strongly influenced by diffusion along the sub-grain boundaries, whereas apparently slow diffusing elements, such as Ca, Ti and Y preserve their original concentric zonation pattern. This differential re-equilibration leads to very complex chemical zonation that cannot be easily interpreted in terms of simple prograde growth zonation or of normally-applied spherical diffusion models. The observation that almost all garnets in the investigated samples exhibit a sub-grain pattern suggests this might be a common feature in high pressure/low temperature rocks.
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| 10. |
- Janak, M., et al.
(författare)
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Eclogite-hosting metapelites from the Pohorje Mountains (Eastern Alps): P-T evolution, zircon geochronology and tectonic implications
- 2009
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Ingår i: European Journal of Mineralogy. - : Schweizerbart. - 0935-1221.
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Tidskriftsartikel (refereegranskat)abstract
- Abstract: Phase-equilibrium modelling, geothermobarometry, ion-microprobe dating and mineral chemistry of zircon have been used to constrain the P–T–t evolution of metapelitic kyanite-bearing gneisses from the ultrahigh-pressure (UHP) metamorphic terrane of the Pohorje Mountains in the Eastern Alps. These eclogite-hosting rocks are part of the continental basement of the Austroalpine nappes. Based on calculated phase diagrams in the system Na2O-CaO-K2O-FeO-MgO-MnO-Al2O3-SiO2-H2O (NCKFMMnASH) and conventional geothermobarometry, the garnet-phengite-kyanite-quartz assemblages of gneisses record metamorphic conditions of 2.2–2.7 GPa at 700–800 C. These are considered as minima because of the potential for a diffusion-related modification and reequilibration of the garnet and phengite during early stages of decompression. It is therefore most likely that the gneisses experienced the same peak UHP metamorphism at 3 GPa as associated kyanite eclogites. Decompression and cooling to 0.5 GPa and 550 C led to the consumption of garnet and phengite, and the development of matrix consisting of biotite, plagioclase, K-feldspar sillimanite and staurolite. Textures and phase diagrams suggest a low extent of partial melting during decompression. Cathodoluminescence images as well as zircon chemistry reveal cores encompassed by two types of metamorphic zircon rims. Ion probe U-Pb dating of three zircon cores yielded Permian (286 10, 258 7 Ma) and Triassic (238 7 Ma) concordia ages. The zircon rims are Cretaceous with a mean concordia age of 92.0 0.5 Ma and some cores gave a similar age. The Cretaceous zircons all exhibit very low Th/U ratio (,0.02) typical of metamorphic origin. In these zircons, nearly flat HREE patterns, (Lu/Gd)N ¼ 1–4, and only small negative Eu anomalies indicate formation in the presence of garnet and absence of plagioclase, which is corroborated by occurrence of Mg- and Ca-rich garnet inclusions. Therefore, these zircons are interpreted to record the Cretaceous HP/UHP metamorphism. The 92.0 0.5 Ma age obtained in this study agrees with that (93–91 Ma) determined earlier in the Pohorje eclogites from U/Pb zircon, Sm-Nd and Lu-Hf garnet-whole-rock dating. This implies that the eclogites and their country rocks were subducted and exhumed together as a coherent piece of continental crust. There is no evidence for a me´lange-like assemblage of rocks, which followed different P–T–t paths, or several subduction and exhumation cycles as proposed for some other UHP metamorphic terranes.
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