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Crystallization conditions of micas in oxidized igneous systems
No full textThe Catalão II and Catalão I carbonatite complexes are Cretaceous intrusions in the northwestern part of the Alto Paranaíba Igneous Province, central Brazil, and contain various trioctahedral micas. Drill-hole sampling and mineralogical and geochemical data suggest the existence of different types of cumulate rocks as carbonatites (calcio- and ferrocarbonatites in Catalão II; magnesiocarbonatites in Catalão I), magnetitites, apatitites, and phlogopitites. In Catalão II complex the presence of ultra-mafic lamprophyres (phlogopite-picrites) and fenites (syenites and clinopyroxenites) are identified. Phlogopite, Fe3+-rich phlogopite, and tetraferriphlogopite are ubiquitous, with marked and variable pleochroism. In the Catalão II, micas from magnetitites, apatitites, and calciocarbonatites are close in composition to the tetraferriphlogopite end-member. The Mg/(Mg+[VI]Fetot) average value is 0.959 for micas from magnetitites, 0.878 for micas from apatitites, and 0.875 for micas from calciocarbonatites, suggesting an enrichment of octahedral iron in crystals from calciocarbonatites. The trioctahedral micas from the fenites are intermediate in composition between phlogopite and tetraferriphlogopite, with 0.227 ≤ [IV]Fe ≤ 0.291 apfu. Micas of phlogopitites, ferrocarbonatites, and phlogopite-picrites show a significant variation in [IV]Fe, [IV]Al, [VI]Mg, and [VI]Fe contents, suggesting the existence of different mica populations. In the Catalão I magnetitites micas show a quite constant Mg content but marked differences in [IV]Fe3+ and [IV]Al content, so there are [IV]Fe3+-bearing phlogopite and tetrafer-riphlogopite. The micas from Catalão I apatitites have variable [IV]Fe3+, [IV]Al, and Mg contents and are mainly tetraferriphlogopite with a minor phlogopite population. Trioctahedral micas from Catalão I magnesiocarbonatites contain crystals close to tetraferriphlogopite with [IV]Fe3+ and a limited variation in Mg content. In these complexes, heterovalent octahedral substitutions are mainly related to Ti4+ and, only in a few samples, to Fe3+. The primary mechanism regulating Ti uptake into the mica structure is the Ti-oxy [[VI]Ti4+[VI](Mg,Fe)2-+1(OH)--2O22-] substitution. Crystal structural analysis shows that all mica crystals are 1M polytypes with the expected space group C2/m. The nearly equal size and mean electron count between M1 and M2 octahedral sites suggest a disorder of octahedral cations between these two sites. The low Al and the high Fe3+ content in the tetrahedral site of these micas, as well as the high (OH)- content, reflects the general enrichment in FeO and H2O and the peralkaline nature of magma from which Catalão I and Catalão II micas crystallized. Micas strongly enriched in Fe3+ and poor in Ti and Al, are used, for the first time, as indicators of crystallization temperature using published geothermometers. Temperatures range between ∼800 and 558°C for the Catalão II cumulate rocks, in high oxygen fugacity conditions, mainly over the HM condition. In contrast, the variability in temperature (703-631 and 1050-952°C) for Catalão II phlogopite-picrites is consistent with mica crystallization in an HM-NiNiO environment. The calculated temperatures for Catalão I cumulate rocks (742-542°C for magnesiocarbonatites, 923-540°C for magnetitites, and 679-550°C for apatitites) are quite similar with those of Catalão II rocks, but they generally show higher oxygen fugacity conditions (over HM condition)
CRYSTALLOGRAPHIC FEATURES AND CLEAVAGE NANOMORPHOLOGY OF CHLINOCHLORE: SPECIFIC APPLICATIONS
No full textPhlogopites from the alkaline-carbonatite complex of Tapira (Brasil): Implications for the teir petrogenetical significance
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Angular dependence of potassium K-edge XANES spectra of trioctahedral micas: Significance for the determination of the local structure and electronic behavior of the interlayer site.
No full textThe X-ray absorption angle-dependent behavior at the potassium K-edge has been determined for two end-members and two intermediate trioctahedral micas, which are representative of the Fe2+Mg-1, Fe3+Al-1, SiAl-1FeLi-1, and F-1OH exchange vectors, using horizontally polarized synchrotron radiation. Experimental spectra are interpreted by the multiple scattering theory following decomposition according to the angular dependence approach of Brouder (1990). The experimental spectra sum up anisotropic effects deriving from in-plane and out-of-plane photoelectron interactions with the potassium near- and next-near neighbors up to the fifth/sixth coordination sphere. For the first time, the absorption edge of a low Z atom is decomposed so as to produce two partial patterns giving, respectively, the full in-plane absorption spectrum (σ
Potassium coordination in trioctahedral micas by K-edge XANES spectroscopy
No full textFine-grained homogeneous powder samples of thirteen trioctahedral micas, mostly intermediate members of the phlogopite – annite solid solution series, and samples close to the phlogopite, fluor-phlogopite and tetra-ferriphlogopite end members have been examined at the potassium K-edge by X-ray absorption fine structure spectroscopy. The interlayer K+ cation is in a coordination that is certainly lower than 12, in contrast to what is expected from the ideal hexagonal symmetry model of the mica structure, and approaches – but it does not reach – coordination 6, as it should be when the effective ligands are the three nearest outer bridging oxygens of two facing upper and lower tetrahedral sheets. The observed range of coordinations implies that only some of the three inner bridging oxygen atoms in each sheet are involved, thus leading to 6±(1...6) effective configurations depending on the composition of the individual mica terms. The effective coordination number was found to vary continuously with composition from 11 to 7 and to be related to the tetrahedral rotation angle (α) according to two different linear relationships for the phlogopite – annite series (Fe2+Mg−1 exchange vector, involving the octahedral sheet only) and the phlogopite – tetra-ferriphlogopite series (Fe3+Al−1 vector, involving the tetrahedral sheet), respectively. Substitutions affecting either the A cation in the interlayer or the X anion in the octahedral sheet also affect the observed trends. In particular, the latter substitution effect is best seen in two near end member phlogopites, where the fluorine to hydroxyl substitution (F− (OH)−−1 exchange vector), which greatly changes the α tetrahedral rotation angle is, reflected in the experimental K XANES spectra by modifying not only the energy but also the intensities of most multiple scattering features
Crystal chemistry of trioctahedral micas-1M from the Alto Paranaiba Igneous Province, Southeastern Brazil
No full textThe crystal structures of nine trioctahedral mica-1M crystals (phlogopite and ferroan phlogopite containing Fe3+ in tetrahedral position, ferroan tetra-ferriphlogopite, and titanian phlogopite), from ultramafic alkaline-silicate lavas and alkaline-silicate, silicate-carbonatite and carbonatite plutonic rocks occurring in the Alto Paranaiba Igneous Province (southeastern Brazil), were refined from single-crystal X-ray data in the space group C2/m to R values between 0.025 and 0.039. These mica crystals are characterized by low to very low At content; therefore, Fell may be essential to fill the tetrahedral site. The octahedral position of all the crystals studied is characterized by variable amounts of Fe2+, Fe3+ and Ti4+ in substitution for Mg. The two independent octahedral sites. M1 and M2, show equivalent mean electron-density and mean bond-distances in most cases. Order at the octahedral sites is enhanced in the Ti-rich crystal from Presidente Olegario owing to the preference of Ti for the M2 site. The lateral parameters a and b reflect variations in tetrahedron composition, whereas the periodicity along c depends on octahedron composition. The crystal-chemical features of these micas are controlled by the peculiar composition of the rocks, by the high f(O-2), a(H2O), and f(CO2), and by metasomatic events that have affected the Alto Paranaiba Igneous Province
Crystal chemistry of phlogopite -1M and tetraferriphlogopite 1M from the Alto Paranà igneous province, southeastern Brazil
No full textsupplemento italiano all’European Journal of Mineralog
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