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| 34. |
- Karlsson, Andreas, 1986-, et al.
(författare)
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Adding complexity to the garnet supergroup: monteneveite, Ca3Sb5+2(Fe3+2Fe2+)O12, a new mineral from the Monteneve mine, Bolzano Province, Italy
- 2020
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Ingår i: European Journal of Mineralogy. - : Copernicus GmbH. - 0935-1221 .- 1617-4011. ; 32:1, s. 77-87
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Tidskriftsartikel (refereegranskat)abstract
- Monteneveite, ideally Ca3Sb5+2(Fe3+2Fe2+)O12, is a new member of the garnet supergroup (IMA 2018-060). The mineral was discovered in a small specimen belonging to the Swedish Museum of Natural History coming from the now abandoned Monteneve Pb-Zn mine in Passiria Valley, Bolzano Province, Alto Adige (South Tyrol), Italy. The specimen consists of mainly magnetite, sphalerite, tetrahedrite-(Fe) and oxycalcioroméite. Monteneveite occurs as black, subhedral crystals with adamantine lustre. They are equidimensional and up to 400 μm in size, with a subconchoidal fracture. Monteneveite is opaque, grey in reflected light, and isotropic under crossed polars. Measured reflectance values (%) at the four COM wavelengths are 12.6 (470 nm), 12.0 (546 nm), 11.6 (589 nm) and 11.4 (650 nm). The Vickers hardness (VHN100/ is 1141 kgmm-2, corresponding to H D 6:5-7, and the calculated density is 4.72(1) g cm-3. A mean of 10 electron microprobe analyses gave (wt %) CaO 23.67, FeO 3.75, Fe2O3 29.54, Sb2O5 39.81, SnO2 2.22, ZnO 2.29, MgO 0.15, MnO 0.03 and CoO 0.03. The crystal chemical formula calculated on the basis of a total of eight cations and 12 anions, and taking into account the available structural and spectroscopic data, is (Ca2:97Mg0:03)6D3:00 (Sb5+1.73Sn4+0.10Fe3+0.17)6D2.00(Fe3+2.43Fe2+0.37Zn0:20)6D3:00O12. The most significant chemical variations encountered in the sample are related to a substitution of the type Y Sn4CCZFe3CY Sb5CCZFe2C. Mössbauer data obtained at RT and 77K indicate the presence of tetrahedrally coordinated Fe2C. Raman spectroscopy demonstrates that there is no measurable hydrogarnet component in monteneveite. The six strongest Bragg peaks in the powder X-ray diffraction pattern are [d (Å), I (%), (hkl)]: 4.45, 100, (220); 3.147, 60, (400); 2.814, 40, (420); 2.571, 80, (422); 1.993, 40, (620); 1.683, 60, (642). Monteneveite is cubic, space group Ia-3d, with a D 12:6093(2) Å, V D 2004:8(1)Å3, and Z D 8. The crystal structure was refined up to R1 D 0:0197 for 305 reflections with Fo 4.Fo/ and 19 parameters. Monteneveite is related to the other Ca-, Sb- and Fe-bearing, nominally Si-free members of the bitikleite group, but it differs in that it is the only known garnet species with mixed trivalent and divalent cations (2 V 1) at the tetrahedral Z site. Textural and mineralogical evidence suggests that monteneveite formed during peak metamorphism (at ca. 600 C) during partial breakdown of tetrahedrite-(Fe) by reactions with carbonate, under relatively oxidizing conditions. The mineral is named after the type locality, the Monteneve (Schneeberg) mine. © Author(s) 2020.
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| 35. |
- 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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| 36. |
- Majka, Jaroslaw, et al.
(författare)
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Pressure-temperature estimates of the blueschists from the Kopina Mt., northern Bohemian Massif, Poland - constraints on subduction of the Saxothuringian continental margin
- 2016
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Ingår i: European journal of mineralogy. - : E SCHWEIZERBARTSCHE VERLAGSBUCHHANDLUNG. - 0935-1221 .- 1617-4011. ; 28:6, s. 1047-1057
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Tidskriftsartikel (refereegranskat)abstract
- The blueschist occurrence at the Kopina Mt. is situated at the eastern margin of the Karkonosze Izera Massif in the West Sudetes of the northern Bohemian Massif. Metabasic rocks with continental crust affinity occur as lenses within the prevailing metasediments. The metabasites consist mainly of garnet, blue amphibole, epidote, chlorite-I, titanite, hematite and quartz forming the high-pressure assemblage. Synkinematic garnet exhibits prograde zoning, a feature allowing for assessment of the prograde course of the pressure temperature (P-T) path. Phase equilibrium modelling has been used to estimate prograde and peak metamorphic conditions. Based on the P-T pseudosections, calculated in the system Na2O-CaO-K2O-FeO-MgO-MnO- Al2O3-SiO2-H2O-TiO2-O-2 (NCKFMMnASHTO), the garnet + glaucophane + epidote + chlorite + white mica(?) + Fe oxide + quartz titanite assemblage was formed between 12-15 kbar and 480-520 degrees C. These results are based on garnet and Na-amphibole compositional isopleths. Garnet shows a prograde zoning expressed by the rapid rim-ward decrease of spessartine, moderate increase of almandine and as light increase of other components. It is inferred here that white mica must have decomposed to secondary albite chlorite-II K-feldspar. The obtained results point to the formation of the Kopina blueschists along a low-temperature P-T gradient of 8-10 degrees C/km, typical of rocks from a subduction exhumation channel. We postulate that the Kopina blueschists were formed in a subduction system developed along the eastern termination of the Saxothuringian Ocean and operating beneath the Tepla Barrandian upper plate. When combined with previous data on continental crust affinity for the protolith, our results demonstrate a derivation of the Kopina blueschists from subducted part of the Saxothuringian margin.
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