77 research outputs found
Crystal structure refinement of 13 olivines in the Forsterite-Fayalite series from volcanic rocks and ultramafic nodules.
No full textThe results of the present paper confirm the overall dependence of the geometric parameters of olivines on the Mg/(Fe+Mn) ratio. M1 and M2 volumes linearly decrease with increasing forsterite (Fo) content but at different rates. The difference between the volumes of the octahedral sites decreases gradually fromFo 92% toFo 50% and then remains virtually constant down toFo 0.2%.
Fe(M1)/Fe(M2) ratios indicate that Fe2+ prefers the M1 site in fayalite (Fa)-rich olivines (Fa=39% to 92%; average Fe(M1)/Fe(M2)=1.04±0.02). Instead, in Mg-olivines (Fo=80% to 92%) a Mg-M1 preference (average Mg(M1)/Mg(M2)=0.96±0.05) may exist. Finally, Mn(M1)/Mn(M2) ratios do not indicate a definite Mn preference for M2 in the studied olivines (MnO=0.44% to 4.5 wt.%; average Mn(M1)/Mn(M2)=0.99±0.13).
No relevant influence on geometric parameters was observed between Ca-free and Cabearing (maximum CaO=0.66 wt.%) olivines
Contratto di rete e professionisti: pubblicità e dubbi applicativi sulla nuova previsione normativa
No full textIl contratto di rete è un istituto giuridico nato per gli imprenditori, strettamente legato alla sua origine economica ed appare, di per sé e così come è stato strutturato, un modello poco adattabile al mondo delle professioni. L’art. 12 della L. 22 maggio 2017, n. 81, il c.d. “Jobs Act autonomi”, prevede ora che anche i professionisti possano far parte di contratti di rete di imprese; tuttavia, il legislatore ha inserito questa nuova possibilità di aggregazione fra imprenditori e professionisti senza, però, modificare la normativa di cui alla L. n. 33/2009. Uno dei punti maggiormente critici e di dubbia applicazione, riguarda il tema della pubblicità nel Registro delle imprese di un contratto di rete a cui partecipino dei professionisti
Ordering kinetics in synthetic Mg(Al,Fe3+)2O4 spinels: Quantitative elucidation of the whole Al-Mg-Fe partitioning, rate constants, activation energies
No full textIn this study, we report results of the application of a novel procedure for modeling cation ordering in two synthetic Mg(Al2–yFey3+)O4 spinels (y ~ 0.39 and 0.54, samples F39 and F54, respectively). The kinetic profiles suggest a two-stage mechanism, with rapid intersite exchange of Fe3+ with Mg followed by slow exchange of Al with Mg. The trial to apply classical approaches, based on explicit solutions of the differential equations corresponding to single-cation (Sha-Chappel model) or two-cation (Müller model) exchange reactions, proved not feasible in the whole time range, thus implying a lack of information about exchange processes involving the Fe3+ cation. Therefore we decided to adopt an alternative methodology where a multistep pathway is simulated by Gepasi modeling. The suggested set of reactions and the simultaneous solving of the related rate laws allowed us to calculate rate constants and corresponding activation energies not only for the Al/Mg cationic interchange (257 and 264 kJ/mol for the F39 and F54 samples, respectively), but also, for the first time in such kind of samples (three cations with two sites), for the Fe/Mg cationic interchange (204 and 234 kJ/mol for low- and high-Fe3+ samples, respectively).
The evaluation of the significant effect of the iron content, which actually inhibits Mg-Al exchange between the T and M sites, provided new insights useful for the construction of geothermometers based upon iron-bearing spinel
Studio mineralogico e palinologico su alcuni sedimenti della Grotta dell'Edera (Carso Triestino): un tentativo di ricostruzione paleoclimatica.
No full textCrystal chemistry of a clinopyroxene series in ultrafemic xenoliths from North-Eastern Brazil
No full textA series of clinopyroxenes (Cpx) in peridotitic spinel nodules from Rio Grande do Norte, North-Eastern Brazil, was investigated by X-ray diffraction and electron microprobe analyses and compared with an analogous series from spinel peridotite nodules from Mt. Leura, Victoria, Australia. The examined Cpx series is mainly characterized by increasing Mg/(Mg+Fe2+) ratio (i.e. increase of refractory character of the host nodules), by the substitutions of Alvi vs Mg in the M1 site and Na vs (Ca+Mg) in the M2 site, whereas Aliv in the T site remains substantially constant. The strong Na depletion in the Brazilian Cpx causes a different charge balance from that in the Australian Cpx, in which Na remains quite constant. This feature, associated with the differences in polyhedral and cell volumes which are significantly higher in the Brazilian Cpx than in those of the Australian Cpx, suggests that the Brazilian Cpx crystal chemistry may be related to a lower pressure regime, compared with that of the Australian Cpx series
Relationships between tetrahedral kink angles and site occupations in C2/c pyroxene from volcanic rocks
No full textThe relationships between kinking of the T tetrahedral chain and the site configuration of C2/c pyroxenes from volcanic rocks were investigated, and about two hundred samples were examinated. Kink angle (KA) is defined by the equation KA = arccos [1-c 2/(2d 2)], in which c is the cell edge andd the O3A1–O3A2 distance; in the pyroxenes studied here, it varies from about 164° to about 167°.The ratio betweenc cell edge and O3A1–O3A2 distance depends on the various M1, M2 and T site configurations which, in turn, are related to their different mechanisms of cation substitution. Thec/d ratio varies if isomorphic substitutions involve cations with different charge, while it is quite constant if the substitutions involve cations with the same net charge.Thec/d ratio variation is related to the amount of cation substitution in each crystallographic site, and reaches its maximum range in synthetic and natural end-member pyroxenes
Whithin plate continental magmatism and its mantle sources.
No full textAn informal meeting on within-plate continental magmatism was
held at Padova University on April 2012. This meeting was organized
to honor the memory of our friend and colleague Prof. Enzo Michele
Piccirillo (1943–2012) (Fig. 1). The contribution of Prof. Piccirillo towards our understanding of the petrogenesis of tholeiitic and alkaline
within-plate continental magmatism and of its mantle source rocks
was crucial for both the international and Italian scientific communities.
In particular, the international research team lead by Prof. Piccirillo on
the Paraná and CAMP continental flood basalts in South America was
among the first to unravel the origin of these major large igneous provinces (LIPs) of the Phanerozoic (and of their Proterozoic and Archean counterparts) and representmilestones in our knowledge of the geology and petrology of the SouthAmerican platform. Prof. Piccirillo worked also on alkaline magmatism (e.g., from Kenya, Ethiopia, Cameroon, Brazil, Paraguay) and investigated the mineral chemistry of
clinopyroxene in rocks of various tectonic settings. The results are reported in some 140 peer reviewed papers. Those who knew him as a
teacher and as a colleaguewill always remember his enthusiasm, imagination, the ability to work and the rigorous approach with which he faced every new research topic and, in general, Enzo's extraordinary
human qualities. Therefore we would like to thank this special issue as
a tribute to Prof. Piccirillo and a way of thanking himfor all he taught us
The pressure-induced ringwoodite to Mg-perovskite and periclase post-spinel phase transition: a Bader’s topological analysis of the ab initio electron densities
No full textIn order to characterize the pressure-induced
decomposition of ringwoodite (c-Mg2SiO4), the topological
analysis of the electron density q(r), based upon the theory of
atoms in molecules (AIM) developed by Bader in the framework
of the catastrophe theory, has been performed. Calculations
have been carried out by means of the ab initio
CRYSTAL09 code at the HF/DFT level, using Hamiltonians
based on the Becke- LYP scheme containing hybrid Hartree–
Fock/density functional exchange–correlation terms. The
equation of state at 0 K has been constructed for the three
phases involved in the post-spinel phase transition (ringwoodite
-> Mg-perovskite + periclase) occurring at the transition
zone–lower mantel boundary. The topological results show
that the decomposition of the ringwoodite at high pressures is
caused by a conflict catastrophe. Furthermore, topological
evidences of the central role played by the oxygen atoms to
facilitate the pressure-induced ringwoodite decomposition and
the subsequent phase transition have been noticed
Structural parameters of chromites from H6 Kernouvè chondrite and detrital extraterrestrial L-chondrites
No full textCongresso Associazione Internazionale di Mineralogia, Budapest 21-27 agosto 201
Petrogenesis of the paleozoic magmatic rocks of the Carnian Alps
No full textOn the basis of whole-rock and mineral chemistry two main magmatic groups can be established for the Paleozoic Volcanism of the Carnian Alps. High-Ti Volcanics (HTV) and Low-Ti Volcanics (LTV). Spilitization processes have not masked the primary magmatic character. The two groups are characterized by very different trace element patterns and cannot be correlated by a fractionation process. The HTV require an enriched mantle source whereas the geochemistry of the LTV indicates strong crustal contamination. Both magmatic groups are consistent with a major rifting episode. The contamination via assimilation of crustal rocks of the LTV group implies that rifting developed in a continental area
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