1,721,200 research outputs found
Crystal-chemical study of cation disordering in Al-rich and Al-poor orthopyroxenes from spinel lherzolite xenoliths
No full textStructural refinements of natural and heated orthopyroxenes from two spinel lherzolite xenoliths were used to investigate the extent of disordering of Mg and Fe2+, and of the trivalent cations, in the orthopyroxene crystal structures. The orthopyroxenes were studied both before and after heating. Although disordering of Mg and Fe2+ between M1 and M2 sites was increased at high temperatures (1050-1150°C), trivalent cations continued to be restricted to M1 sites. Disordering of Mg and Fe2+ was more easily achieved when contents of trivalent cations in the orthopyroxene are low. -Author
Contributo della cristallochimica dei pirosseni all'indagine petrogenetica: una sintesi del lavoro svolto in un ventennio di ricerca italiana
No full textUna dettagliata analisi dei risultati ottenuti dalle indagini cristallochimiche sui pirosseni. condotte nell'ultimo ventennio presso alcune sedi italiane di ricerca, non è argomento che possa essere esaustivamente trattato in questi brevi appunti di lezione; l'obiettivo qui prefisso è pertanto quello di porre in risalto le potenzialità applicative che tali indagini hanno assunto nel campo delle Scienze della Terra, e in particolare delle possibili applicazioni petrologiche, secondo un ordine espositivo che ricalca in parte la seccessione temporale deli principali argomenti affrontati. Verrà fatto un costante riferimento alla estesa produzione bibliografica mettendo in luce gli aspetti che sono ritenuti più significativi per le finalità della scuola.Al ine di acquisire una maggiore semplicità espositiva non vengono qui citati i lavoti cristallochimico-strutturali fondamentali che generalmente procedono il periodo qui considerato, tali opere sono comunque estesamente citate negli articoli riportati nell'allegata bibliografia, a essi si rimanda per una più esaustiva trattazione. Certo di non essere riuscito a raccogliere in questo breve excursus tutta la relativa produzione scientifica sull'argomento dei numerosi Colleghi italiani mi scuso in anticpo per le omissioni
Intracrystalline cation distribution in natural clinopyroxenes of tholeiitic, transitional, and alkaline basaltic rocks.
No full textIt is possible to estimate Fe-Mg site occupancies on M1 and M2 sites in clinopyroxenes of intermediate composition through crystal structure refinements. There are distinct correlations between crystal-structure parameters and concentrations of trivalent and tetravalent cations in the M1 site and Si in the T site; thus it is possible to determine completely the M1 and M2 site occupancy by Al3+, Fe3+, Ti4+, Cr3+, Fe2+, Mg, Ca and Na. The composition-structural parametric relationships make it possible to predict the bulk composition and site occupancies in crystals that have not been chemically analysed. A new type of M2 site in intermediate clinopyroxenes (the M2' site) is a function of pyroxene chemistry and may have a bearing on the process of unmixing in calcic pyroxenes. Basaltic rocks may be distinguished by the crystallographic parameters of their pyroxenes.-R.A.H
Indagini sulle reliquie attribuite a "San Luca Evangelista", Basilica di Santa Giustina in Padova: indagini isotopiche e mineralogiche dell'epigrafe bilingue
No full textIl lavoro espone i risultati degli studi sui materiali costituenti
un'epigrafe greco-latina rinvenuta nella cassa, e secondo la tradizione ritenuta prova dell 'autenticità della reliquia. E' stato condotto uno studio mineralogico e isotopico sul piombo costituente l'epigraƒe e sul pigmento rosso di riempimento delle lettere. La composizione isotopica del piombo è chiaramente differente sia da quella della bara che dagli altri manufatti provenienti dalla basilica, e non riconducibile a specifici giacimenti delle area Alpina o Mediterranea. Le lettere dell 'epigrafe sono riempite con una miscela di cinabro e cera. L'analisi dei risultati della datazione con il metodo del 14C suggerisce che la cera sia databile alla seconda metà del XVI secolo. Questo dato conferma l'ipotesi sostenuta dall 'epigrafista, che l'iscrizione sia stata incisa in epoca rinascimentale in un contesto non padovano. Il pigmento rosso, inserito nelle lettere secondo la tecnica dell 'encausto, è stato probabilmente aggiunto durante l 'ultima ricognizione della reliquia (1562) al fine di rendere le lettere più leggibili
Crystal-chemical considerations on eight metamorphic epidotes
No full textThe crystal structures of eight natural samples belonging to the clino-zoisite-epidote series were refined. Close correlations were found between either the b cell dimension or the M 3-O bond length or the intensities of some reflections and the chemical composition of the samples (Fe3+ contents increasing in
the M3 site). Some brief considerations have been made upon the possible relations between the physical and chemical conditions of formation of the host-rocks and the geometrical variations occurring in the crystal structures of the epidotes
INTRACRYSTALLINE FE-2+-MG ORDERING IN AUGITE - EXPERIMENTAL-STUDY AND GEOTHERMOMETRIC APPLICATIONS
No full textAn augite crystal from a basaltic dyke on Vulcano Island (Aeolian Archipelago) was equilibrated at temperatures in the range 670-1180-degrees-C and then quenched to room temperature. Following each experiment, the partitioning of Fe2+-Mg between the M1 and M2 sites of the clinopyroxene was determined by crystal structure refinement. These Fe2+-Mg partitioning data were then used to calibrate a new clinopyroxene geothermometer which estimates closure temperatures for the exchange reaction.
The new geothermometer was then applied to six clinopyroxenes from a basaltic andesite lava flow on Salina Island (Aeolian Archipelago). Calculated closure temperature are : 600-degrees-C for samples near the base of the flow, 793-degrees-C for the sample from the top of the flow, and 513-degrees-C (the lowest temperature found) for the sample nearest the middle of the flow.
A simplified mathematical cooling model for lava flows (Jaeger, 1968) was then used in conjunction with these clinopyroxene equilibration temperatures to calculate cooling rates at different heights in the Salina Island lava
Crystal-chemistry of Clinopyroxene From Sunda Volcanic Arc
No full textThe crystal-chemistry of clinopyroxene from a suite of Sunda are volcanic rocks was investigated by X-ray structure refinement and microprobe analysis. Relationships with clinopyroxene from volcanic rocks of the intraplate environment were highlighted. Sunda clinopyroxenes were distinguished into two groups. The first consists of clinopyroxene from SiO2-saturated rock types and crystal rim from SiO2-undersaturated rock types, the second of crystal core from SiO2-undersaturated rock types. The crystal chemical behaviour of the two groups differs significantly, mainly in the geometrical relationships between M2 and T sites. While in the first group and bond lengths shorten and lengthen respectively due to Ca and Al-IV increase, in the second group in which M2 site is almost fully coordinated by Ca, bond lengths are forced to lengthen in spite of low Al-IV content, in order to mantain the local charge balance around the O3 oxygens. It is noteworthy that Sunda clinopyroxene shows strong analogies with that from basaltic rocks from the Ethiopian plateau and K-rich lavas from the Roman Province respectively, reflecting similarities between the composition and mineralogical assemblages of the host rocks, although they all came from different tectonic environments. A relationship between intracrystalline Mg-Fe2+ disorder and the explosive character of the volcanism is demonstrated
Crystallographic procedures in the study of experimental rocks: x-ray single-crystal structure refinement of C2/c clinopyroxene from lunar 74275 high-pressure experimental basalt
No full textThe possibility of applying X-ray single crystal techniques to minerals pertaining to rocks experimentally crystallized in laboratory has been investigated as a new approach to precisely relate crystal-chemistry to pressure, temperature, fO2, and composition of the magma from which the crystal formed.
A clinopyroxene from the experimental lunar basalt (74275 composition) ofGreen et al. (1975) crystallized at 12 Kb and 1320 °C, was successfully refined to an R factor of 0.041. Its structure configuration turned out to be exceptional, expecially in relation to the record low Ca(M2) occupancy (0.57 atoms per formula unit henceforth a.f.u.), but comparable with the other lunar C2/c clinopyroxene previously studied with X-ray single crystal methodology: augite 12052 (Takeda 1972 a). Comparative analysis of the two lunar clinopyroxene structures revealed their common high temperature origin and the high-pressure imprint of the experimental specimen relative to the natural one.
Both lunar samples differ radically from the studied terrestrial clinopyroxenes of C2/c diopside-like structure: the main difference of lunar clinopyroxenes with respect to terrestrial analogues is the high Ally (T) (AlIV > 0.20 a.f.u.) in spite of low Ca(M2). Relative to the ideal diopside structure, Si4+ AlIV and Ca Mg + Fe2+ substitutions in T and M2 sites, respectively, turned out to be not compatible with terrestrial clinopyroxenes.
Die Anwendung von Einkristallverfahren auf Minerale aus experimentell hergestellten Gesteinen wurde untersucht; sie wird als ein neuer Weg zur quantitativen Korrelation der Kristallchemie mit Druck, Temperatur, fO2 und der Zusammensetzung des Magmas, aus dem der Kristall gebildet wurde, gesehen.
Die Struktur eines Klinopyroxens aus dem experimentellen Mond-Basalt (Zusammensetzung 74275) vonGreen et al. (1975), der bei 1320 °C kristallisierte, wurde mit Erfolg bis auf einen R-Faktor von 0.041 verfeinert. Es zeigte sich, daß seine Struktur-Konfiguration außergewöhnlich war, besonders im Hinblick auf die besonders niedrige Ca(M2) Besetzungsdichte (0.57 Atome pro Formeleinheit). Sie ist jedoch vergleichbar mit dem anderen lunaren C2/c Klinopyroxen der bisher mit Röntgen-Einkristallverfahren untersucht wurde, nämlich Augit 12052) (Takeda, 1972a). Vergleichsanalysen der beiden lunaren Klinopyroxen-Strukturen lassen ihre gemeinsame Hochtemperatur-Entstehung erkennen, sowie den — im Gegensatz zu der natürlichen Probe — deutlichen Einfluß hoher Drucke auf die experimentell hergestellte Probe.
Beide lunare Proben unterscheiden sich deutlich von den untersuchten terrestrischen Klinopyroxenen mit Diopsid-artiger C2/c Struktur: der Hauptunterschied zwischen lunaren Klinopyroxenen und ihren terrestrischen Analogen ist der hohe AlIV (T) (AlIV > 0.20 a.f.u.) trotz niedrigem Ca(M2). Im Vergleich mit der idealen Diopsidstruktur zeigte sich daß, Si4+ AlIV und Ca Mg + Fe2+ Substitutionen an T resp. M2 Plätzen nicht kompatibel mit terrestrischen Klinopyroxenen sind
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