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CRYSTAL-CHEMISTRY OF STROMBOLI CLINOPYROXENE - A COMPARISON WITH ANALOGS FROM OTHER AEOLIAN ISLANDS (ITALY)
No full textSingle clinopyroxene crystals from four different volcanic suites on Stromboli (Aeolian Islands, Italy) were studied by X-ray diffraction and microprobe analysis in order to obtain information on the relations between clinopyroxene crystal chemistry and the petrology of the host rocks, and to provide comparisons with the nearby islands of Vulcano, Filicudi and SaIina.
Crystal chemical study reveals the coexistence on Stromboli of rocks with two clinopyroxene compositions with C2/c symmetry, diopside and augite, both of which show wide ranges in structural and chemical parameters. Unit cell and M1 site volumes are relatively large, a feature that is indicative of low crystallization pressure, as already observed in clinopyroxene from Vulcano and Salina. Comparable clinopyroxene from Filicudi are characterized by lower volumes and interpreted as having crystallized in a deeper magma chamber (Malgarotto et al., 1993).
The intracrystalline Mg-Fe2+ ordering reaction shows closure temperatures for Stromboli clinopyroxene ranging between 620 and 1100 degrees C. This wide range indicates different cooling rates related to eruptive mechanisms
Single-crystal thermometric calibration of Fe-Mg order-disorder in pigeonites
No full textThe single-crystal X-lay technique was used to calibrate a new intracrystalline geothermometer based on equilibrium Mg-Fe* fractionation (Fe* = Fe2+ + Mn2+) between M1 and M2 sites of natural P2(1)/c pigeonite, Suitable crystals free of exsolution textures and sharp diffraction maxima were selected by careful TEM and XRD investigations from a large number of samples. Two single crystals, PCA82506-3 (Wo(6)En(76)Fs(18)) from the Pecora Escarpment 82506 Antarctic ureilite, and BTS308-2 (Wo(10)En(47)Fs(43)) from the BTS308 Parana rhyodacite, were annealed at temperatures ranging from 600 to 1000 degrees C. The TEM investigation, carried out on fragments of selected single crystals both before and after thermal treatment, shows heating-induced texture modifications preliminary to spinodal decomposition in both crystals and a size increase in antiphase domains in BTS308-2, The two geothermometric equations calculated by linear regression of In K-D* vs. 1/T are:
In K-D* = -3291(+/- 269)/T(K) + 0.971(+/-0.253); (r(2) = 0.974)
ln K-D* = -2816(+/- 83)/T(K) + 0.542(+/-0.083); (r(2) = 0.995)
respectively for PCA82506-3 (X-Fe* = 0.20) and BTS308-2 (X-Fe* = 0.49) [X-Fe* = Fe*/(Fe* + Mg)]. These results imply negligible compositional effects on Mg-Fe* site partitioning within the range of compositions encompassed by the samples. Comparison of the intracrystalline fractionation data of pigeonites with those of Pbca orthopyroxenes shows a similar degree of ordering for both at a given temperature. This result suggests only a small effect of Ca on Fe*-Mg ordering in pyroxene with Ca content up to Wo(10)
Le ossa: un potenziale mezzo di datazione mediante indagine cristallografica
No full textrecenti studi hanno messo in evidenza la relazione sistente tra le modificazioni cristallochimiche della frazione inorganica dell'osso, frazione principalmente costituita da idrossiapatite (Ca5(PO4)3(OH)), e l'età post mortem del reperto.
In questo lavoro viene allo scopo affinata la metodologia di studio mediante diffrazione X delle polveri, tecnica che, unitamente al pregio di essere rapida ed economica, si presenta quale promettente via della datazione dei reperti ossei. Viene proposto il metodo di acquisizione dei raggi diffratti mediante scansione "step by step": tale metodo permette una precisa misura del parametro Imax/Ifondo che mostra essere correlato con l'età del reperto. E' inoltre emerso che la scansione "step by step" permette di rilevare la presenza di fasi cristalline secondarie non rilevabili mediante scansione continua, tecnica più frequentememte utilizzata. Viene riconfermato ilfondamentale ruolo assunto dalla scelta delle variabili intrinseche (età del deceduto, sesso, stato sociale) ed estrinseche (clima e ambiente di sepoltura) ai fini dell'accuratezza dei risultati. L'analisi comparata dei risultati evidenzia il possibile ruolo assunto dalla variazione col tempo del rapporto quantitativo tra componente inorganica e organica dell'osso. tale variazione, massima nei primi millenni della storia diagenetica dell'osso, confermerebbe la rilevanza archeologica di tale indagine
Microchemical and crystallographic analysis of human bones from Nasca, Peru. A possible method of direct dating of archaeological skeletal material
No full textThis paper provides two archaeometric equations obtained by crystal–chemical parameters for dating human bone samples from the archaeological site of Nasca, Peru. Based on radiocarbon dating, the burials span from 500 BC to AD 1000. Crystal–chemical modifications in bone hydroxylapatite over time were compared with a twentieth century skeleton from Italy. Chemical analysis was carried out by means of electron microprobe along a profile from the periosteal to the endosteal border of the midshaft of four human femurs. The results indicate that the Ca/P ratio is linearly correlated with time according to the equation t=7880.68Ca/P−12805.90 (r=0.97; r2=0.95). Bone apatite X-ray powder diffraction analysis showed a systematic increase in the (002) reflection intensity with time, according to the equation t=250.49h/a−1961.86 (r=0.98; r2=0.97). The two independent archaeometric equations permit good accuracy in dating osteoarchaeological remains from the south coast area of Peru
Clinopyroxene from Lipari: Comparison with analogues from other Aeolian Islands, Italy
No full textClinopyroxene phenocrysts from the volcanic rocks of Lipari, one of the Aeolian Islands (Italy), were characterized by single-crystal X-ray diffraction and electron-microprobe analysis. They form three groups, related to different volcanic cycles. Although there is no evidence of tholeiitic rocks in the archipelago, the structural framework of these clinopyroxenes is close to those of clinopyroxene from subalkaline basalts of tholeiitic affinity. This feature may be ascribed to contamination of the magma by older gabbros in the lower crust, or to crystallization from an early magma of tholeiitic composition. Comparisons with analogues from other Aeolian Islands (Vulcano, Salina, Filicudi and Stromboli) highlight the fact that the Lipari clinopyroxene are low-pressure phenocrysts
Microtextures and crystal chemistry in P2(1)/c pigeonites
No full textA single-crystal X-ray investigation was performed on crystals of P21/c natural pigeonite with varying Ca and Fe* ( = Fe2+ + Mn2+) contents, in order to verify the effect of microtextural disorder on structure refinements and to constrain the crystal chemistry of pigeonite. Antiphase domains and exsolution lamellae affect differently the refinement results. In a crystal free of exsolution the structure obtained after refinement with all reflections is an average of that of the antiphase domains and of their boundaries, whereas in an exsolved crystal it represents only the structure of the prevailing pigeonite lamellae.
The refinement using only h + k odd reflections seems to give the structure of the Ca-free pigeonite characteristic of the antiphase domains rather than that of Ca-rich domain walls. The ratio of the scale factors in refinements with all reflections and with only h + k odd reflections allows the ratios of the exsolved augite and pigeonite phases to be estimated.
The crystal chemistry of the investigated samples follows the trends outlined by data on Ca-free and Fe-free synthetic samples. In particular, it is shown that Ca and Fe* substitution for Mg induce similar changes in the average structure, i.e. both induce an expansion in the M1 polyhedron and decrease the difference between the M2–O3 distances
Potenzialità dell'indagine XRF nella diagnostica di opere pittoriche ad affresco
No full textRiassunto presentato al Convegno Nazionale AIAr, " INNOVAZIONI TECNOLOGICHE PER I BENI CULTURALI IN ITALIA", Caserta, 16-18 febbraio 200
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