Oltre il Segno/OltreMare

La realizzazione di un volume contenente le incisioni scelte all’interno della Scuola di Grafica d’Arte dell’Accademia di Belle Arti di Palermo, coordinata dai Proff. Giovanni D’Alessandro e Riccardo Mazzarino rappresenta motivo di orgoglio e di soddisfazione per la nostra Istituzione che costruisce i percorsi didattici dei propri corsi a partire dall’esperienza laboratoriale. L’incisione grafica è tra le tecniche artistiche più antiche ma nel contempo più contemporanee. La gestualità intrinseca al segno, che si manifesta nella carta, svela universi della visione inaspettati.(Mario Zito - Direttore dell’Accademia di Belle Arti di Palermo) Il segno è il risultato di un gesto a volte deciso, a volte contorto, a volte leggero, i cui risultati spesso sono inattesi e sorprendenti. Il volume contiene esemplari di incisioni fortemente caratterizzanti della scuola di Grafica d’Arte che vanta all’interno del proprio corso di studi docenti-artisti che consapevoli della ricchezza del loro bagaglio esperienziale offrono agli studenti gli strumenti necessari per far sì che l’arte del saper fare artigianale, si trasformi in mera poetica artistica

Mixing and ordering behaviour in manganocolumbite-ferrocolumbite solid solution: a single-crystal X-ray diffraction study

The structural changes upon cation substitution in natural AB2O6 columbites have been studied by means of single-crystal X-ray diffraction. Most of the structural variations across the MnNb2O6- FeNb2O6 solid solution in completely ordered samples can be simply understood in terms of ionic radii. The substitution of Fe for the larger Mn cation causes a linear decrease of all unit-cell parameters. Going from manganocolumbite to ferrocolumbite the site A is reduced in volume and becomes less distorted. The oxygen cage around the cation assumes a more regular arrangement since the mismatch between A and B chains decreases. At the same time, the divalent cation moves toward the barycenter of the polyhedron. The B site, which is not involved in the Fe-Mn cation substitution, maintains its geometry unchanged. Ordering of divalent cations at A sites and pentavalent cations at B sites causes linear variations of a and c cell parameters. A non-linear behavior is shown by the b cell parameter that shows a minimum at order parameter Qm ∼ 0.7. A discontinuity at this Qm value is also shown by other structural parameters. Cation ordering also causes volume variations of the two octahedral sites as a consequence of the different ionic radii of the various species. Octahedral bond-length distortion parameters show that the B site is in general more distorted than the A site; distortion of the B site increases with ordering due to higher cation-cation repulsion along the B octahedral chain and to the second-order Jahn-Teller (SOJT) effect. Octahedral chains respond to modiÞ cations of the polyhedra by folding along the common edge

Hydration/dehydration and cation migration processes at high temperature in zeolite chabazite

High temperature structural behavior of a natural chabazite of composition (Ca1.1Na0.4K0.7)- [Si8.6Al3.4O24] · 14.4H2O has been characterized by means of in situ HT single-crystal X-ray diffraction (SC-XRD) and thermogravimetric analysis. Lattice dimensions have been measured in the 25-700 °C range and crystal structure refined from XRD data collected at T ) 25, 100, 125, 175, 250, 300, 425, and 600 °C. Variations of unit-cell parameters as a function of temperature reveal two discontinuities at 100 and 200 °C, to which no symmetry changes are associated, and an overall volume reduction of 2.8%. Between 200 and 250 °C, a steep contraction of cell volume is associated with a significant broadening of diffraction profiles, which turn sharp and narrow at higher temperatures. As the dehydration process proceeds with increasing T, cationic sites partly coordinated by extraframework water molecules become unstable and cations migrate toward new positions. Cations occupying the C2, C3, and C4 sites at room temperature move first toward C2 while T is raised to 200 °C, and for T > 200 °C they start migrating toward smaller cavities where coordination is assured by oxygen atoms of the framework only, the sixmembered double ring (C1 site), and a peripheral position within the eight-membered ring. The latter position has been labeled as C5. Sites C1 and C5 are stable up to 700 °C. Reversal experiments demonstrated that the whole process is reversible under the conditions of this study; by decreasing temperature, water enters the structure again and cations migrate back to their original positions

Ca in orthopyroxene: structural variations and kinetics of the disordering process

The kinetics of the disordering process in Ca-rich orthopyroxenes was studied. The substitution of Ca for Mg and Fe in the orthopyroxene structure was investigated in order to evaluate the influence of this cation on the disordering process. Geometrical parameters obtained by X-ray single-crystal diffraction on different Ca-rich orthopyroxenes showed that Ca enters the octahedral M2 site of orthopyroxene, causing an enlargement of this polyhedron. As a consequence, both tetrahedral chains extend to allow matching between the tetrahedral and octahedral layers. The kinetic study was carried out at T = 730°C on an orthopyroxene from the volcanic rock sample L3. This orthopyroxene, with composition Wo4En60Fs36, contains thin exsolved augite lamellae but no Guinier-Preston zones. A series of isothermal annealing runs were performed on a single crystal in the presence of IW buffer followed by quenching. The ordering degree after each annealing run was determined by X-ray single-crystal diffraction. Analysis of the kinetic data using Mueller’s (1967) theory yielded a disordering rate constant of 2.69(8)·10-2 min-1. This value is in perfect agreement with that expected for a “normal” kinetic behaviour. The presence of Ca and augite lamellae in the orthopyroxene matrix do not seem to be responsible for the drastic change in the activation energy of the Fe-Mg exchange process observed in the Ca-rich orthopyroxene from the Johnstown meteorite