12,770 research outputs found
The 2M1 dioctahedral mica polytype: a crystal chemical study
No full textThe structure of dioctahedral true micas such as muscovite and celadonitic muscovite (2M1 polytype, Space Group C2/c) is mostly affected by variations of the octahedral Al content ([vi]Al). Crystals with greater Mg, Fe substitutions (i.e., celadonitic muscovite) reduce the dimensional difference between the larger trans-oriented M(1) site and smaller cis-oriented M(2) octahedral site. The octahedral anionic position O(4) is displaced from the center of the hexagon, defined by O(31) and O(32) oxygen atoms (i.e., “octahedral hexagon”), both on and off the (001) plane. The distance between interlayer cation A and O(4) is smaller in more substituted species, thus providing different orientations of the O(4)-H vector, as a function of [vi]Al. Octahedral distances and are expressed as a function of cell parameters and [vi]Al content, thus allowing an approximate estimate of site dimensions. These approximations are useful when a detailed structural refinement is not available. In celadonitic muscovite, the octahedral hexagon mean edge is not significantly affected by [vi]Al content. The [vi]Al increase produces both a decrease in cell lateral dimensions and a distorted “octahedral hexagon”. The decrease in a and b is consistent with a decrease of , whereas the distortion of the “octahedral hexagon” is consistent with an increase of , because an irregular hexagon produces a longer mean edge than a regular hexagon of equal area.The tetrahedral mean basal edge is reduced as celadonitic substitution progresses. The tetrahedral rotation angle was thus found to increase from celadonite to muscovite. However in muscovite with [vi]Al content between 1.8 and 2.0 atoms per formula unit (apfu), approaches a saturation value, thus showing a proportional increase of tetrahedral and octahedral sheet lateral dimensions. Furthermore, alpha variation allows a coarse approximation of the threshold [vi]Al content, below which celadonitic substitution may not progress
Crystal chemistry of the 1M mica polytype: The octahedral sheet
No full textThis work provides a crystal-chemical description of the trioctahedral sheet in 1M mica polytypes of C2/m symmetry. Some octahedral parameters were found to be related strictly to octahedral chemical composition, whereas others were related to the overall layer chemistry. As a general rule, the M I site is more affected by octahedral chemical composition than the M2 site, and octahedral distortions are more affected than octahedral size. This behavior is attributed to the constraints related to edge-sharing octahedra and octahedral-tetrahedral coupling, which involves the sharing of the apical tetrahedral oxygen atom between the tetrahedron and octahedron. A more quantitative explanation was sought by considering expressions that relate octahedral parameters as a function of atomic coordinates and unit-cell parameters. This procedure identified those parameters affected by local distortions and those that are a function of a more global effect and whose variation greatly affects the layer. Local distortions are obviously more likely a result of local chemical composition. The size and distortion of the octahedral sheet seem to be independent for the database of samples considered. The alpha angle was found to be a complex parameter required to: (1) fit tetrahedral and octahedral lateral dimensions; (2) fit tetrahedral topology to its local chemical composition, and (3) fit the interlayer cation electrostatic interactions
Crystal structure and crystal chemistry of lithium-bearing muscovite-2M(1)
No full textCrystal-structure refinements were done on Li-bearing muscovite-2M(l) crystals from microgranite and granitic pegmatite rocks in order to characterize their crystal chemistry and their relationships with muscovite and trioctahedral lithium-containing micas. In addition to the substitution mechanism Li-[6](+) Al-[6](-1)3+ Al-[4](-2)3+ Si-[4](2)4+, Li-bearing muscovite shows additional substitutions, such as mechanism Li-[6](+) Fe-[6](2+) Al-[6](-1)3+ ([6])(square -1), indicating that the structure deviates from ideal dioctahedral character. Single-crystal X-ray-diffraction data were collected for five crystals in space group C2/c; the agreement factor, R-obs, varies between 0.033 and 0.042. The mean tetrahedral cation-oxygen atom distances range from 1.637 to 1.646 Angstrom and from 1.629 to 1.647 Angstrom for T1 and T2 sites, respectively. Variation in distances is associated with the Li+ / (Li+ + Al3+) ratio, octahedral M2 site expansion (9.30 less than or equal to volume(M2) less than or equal to 9.90 Angstrom (3)) and reduction in size of the M1 site. Moreover, as the Li+ / (Li+ + Al3+) ratio increases, the silicate ring becomes less distorted (5.9 less than or equal to alpha less than or equal to 11.4 degrees), the basal oxygen-atom planes become less corrugated (0.147 less than or equal to Deltaz less than or equal to 0.232 Angstrom), and the interlayer separation narrows (3.337 less than or equal to interlayer separation less than or equal to 3.422 Angstrom)
MABS validation through repeated execution and data mining analysis
No full textAgent Based Modelling is the most interesting and advanced approach for simulating a complex system: in a social context, the single parts and the whole are often very hard to describe in detail. Besides, there are agent based formalisms which allow to study the emergency of social behaviour with the creation and study of models, known as artificial societies. Thanks to the ever increasing computational power, it's been possible to use such models to create software, based on intelligent agents, which aggregate behaviour is complex and difficult to predict, and can be used in open and distributed systems. Data mining is born in the last decades in order to help users in finding useful knowledge from the otherwise overwhelming amount of data available nowadays from the web and the data collected every day by companies. Data Mining techniques can therefore be the keystone to reveal non-trivial knowledge expressed by the initial assumption used to build the micro-level of the model and the structure of the society of agents that emerged from the simulation
Facades of the Libreria di San Marco in Venice, The: An Interpretation of the Design Process
No full text"A new work in which I propose an interpretation of the design process Sansovino used to create the magnificent facades of the Libreria di San Marco in Venice, a masterpiece of Renaissance architecture." Sent to Marquand librarian by author Dec. 202
Art without an Author: Vasari’s Lives and Michelangelo’s Death
No full textMonografia sulla rappresentazione di Michelangelo nelle due edizioni delle Vite, sulla storia del libro e la questione della sua paternitàBook dedicated to the representation of Michelangelo in Vasari's Lives of the Artists, to the history of the book, and to the problem of its authorshi
Crystal chemical modeling and XANES spectroscopy of K in interlayer site of micas
No full textA detailed description of the interlayer site in trioctahedral true micas is presented based on a statistical appraisal of crystal-chemical, structural, and spectroscopic data determined on two sets of trioctahedral micas extensively studied by both X-ray diffraction refinement on single crystals (SC-XRD) and X-ray absorption fine spectroscopy (XAFS) at the potassium K-edge. Spectroscopy was carried out on both random powders and oriented cleavage flakes, the latter setting taking advantage of the polarized character of synchrotron radiation. Such an approach (AXANES) is shown to be complementary to crystal-chemical investigation based on SC-XRD refinement. However, the results are not definitive as they focus on few samples having extreme features only (e.g., end-members, unusual compositions, and samples with extreme and well-identified substitution mechanisms). The experimental absorption K-edge (XANES) for potassium was decomposed by calculation and extrapolated into a full in-plane absorption component (sigma (sub ) ) and a full out-of-plane absorption component (sigma ). These two patterns reflect different structural features: sigma (sub ) represents the arrangement of the atoms located in the mica interlayer space and facing tetrahedral sheets; sigma is associated with multiple-scattering interactions entering deep into the mica structure, thus also reflecting interactions with the heavy atoms (essentially Fe) located in the octahedral sheet. The out-of-plane patterns also provide insights into the electronic properties of the octahedral cations, such as their oxidation states (e.g., Fe (super 2+) and Fe (super 3+) ) and their ordering (e.g., trans- vs. cis-setting). It is also possible to distinguish between F- and OH-rich micas due to peculiar absorption features originating from the F vs. OH occupancy of the O4 octahedral site. Thus, combining crystal-chemical, structural, and spectroscopic information is shown to be a practical method that allows, on one hand, assignment of the observed spectroscopic features to precise structural pathways followed by the photoelectron within the mica structure and, on the other hand, clarification of the amount of electronic interactions and forces acting onto the individual atoms at the various structural sites
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