1,721,012 research outputs found
Ferrouginous concretions in a Late Cretaceous karst bauxite: composition and conditions of formation
No full textThe karst bauxites from the Apulian carbonate platform southern Italy. have an ooidic texture. The concretions, which
are formed at the expense of the kaolinitic matrix, are composed of Al-hematite in the core and of Al-hematite and boehmite
in the banded cortex. The Al-hematite in the core has an average formula Al0.12Fe0.88.2O3 and the Al is within the range of
values observed in hematite formed through in situ epigenetic replacement of kaolinite. The Al substitution in hematite
involves a water-unsaturated environment, possibly promoted by fluctuations of the groundwater table followed by intense
drainage and evaporation. In the banded cortex, boehmite may form instead of Al-hematite at lower water activity values.
Further, the boehmite stability increases for high values of the a 3qra3
Al Hq ratio, which is mostly controlled by the pH of the
solution. The stability of the Al-hydroxide decreases with increasing pH
X-ray fluorescence: chemical characterization of materials by X-ray spectrometry.
No full textX-ray fluorescence techniques are widespread since the half of the last century for chemical investigation on rocks, minerals, industrial products, construction materials, precious materials, environmental pollutants, metals, paints, etc. Actually, they can be used to analyse almost every kind of solids and in many cases also liquid or gelatinous substances. XRF allows to obtain chemical analyses, in elements or in oxides, expressed as percent atoms or atoms in percent weight, alternatively as oxides, stoichiometrically binding the oxygen to the dosed cation. Depending on the accuracy of the desired data and on the sample characteristics, the XRF can be used as partially destructive or as absolutely non-destructive technique; consequently, the sample required for an analysis varies from few tens of milligrams up to about 12 g, depending on the selected analytical procedure and the type of instrumentation. In the last decades, the diffusion of the ED silicon drift detectors, together with the development of very accurate and high specialised software for quantitative analysis, increased the diffusion of portable spectrometers offering new possibilities for in-situ and very rapid specimen characterizations, useful during forensic investigations, particularly on samples that cannot be removed for legal reasons or difficult to transport as being too large. The theoretical physical principles and the main components of X-ray spectrometers, in energy dispersion (ED) and wavelength dispersion (WD), are described also comparing the advantages and disadvantages of each analytical technique. Some forensic case studies are also presented
Fluorescenza di raggi X
No full textLa tecnica di analisi spettrochimiche per fluorescenza di raggi X (FRX)
permette di ottenere le analisi chimiche, in elementi o in ossidi; tali analisi
possono essere espresse come atomi per cento o come atomi in peso per
cento. Spesso, per consuetudine, soprattutto nelle analisi di minerali e rocce,
le analisi vengono espresse come ossidi, legando stechiometricamente
l’ossigeno al catione dosato.
La FRX è considerata una tecnica distruttiva perché normalmente il
campione deve essere macinato e successivamente trattato con leganti
organici o con altri componenti per poterne ottenere un disco su cui effettuare
le misure: difficilmente si potrà recuperare il campione originale,
una volta ottenuto il provino per le analisi FRX. Cosa importante è che lo
stesso provino lo si potrà rianalizzare, con un altro spettrometro FRX o
anche con altra strumentazione analitica come ad esempio la microscopia
elettronica a scansione, anche a distanza di molti anni, purché esso sia
stato ben conservato.
Nel seguito saranno descritti i casi in cui è possibile utilizzare questa
metodica anche in modo assolutamente non distruttivo.
La quantità di campione necessaria per un’analisi chimica mediante FRX
varia, a seconda del risultato qualitativo o quantitativo che si voglia ottenere
ed a seconda della metodica e della strumentazione utilizzata, da poche decine
di milligrammi (circa 40 mg, Acquafredda et al., 2018) fino a circa 12 g.
I campioni che si possono analizzare in FRX sono i più disparati, dalle
rocce, ai minerali, ai manufatti industriali, ai materiali da costruzione, ai
metalli, alle vernici, ecc. In pratica si può analizzare quasi tutto quello che
è una sostanza solida ed in molti casi anche sostanze liquide o gelatinose.
La preparazione del campione dipenderà dalla metodica analitica che
si vorrà utilizzare; in alcune occasioni, come per le analisi non distruttive,
il campione potrà essere analizzato senza alcun trattamento preventivo
o addirittura senza essere spostato dalla sua sede naturale. Quest’ultima
possibilità apre un campo molto vasto di applicazioni, in special modo in
ambito forense o dei Beni Culturali, lì dove potrebbe essere necessario
analizzare campioni non rimovibili per motivi legali o difficilmente trasportabili
perché di grandi dimensioni; stesso discorso vale per la caratterizzazione
di oggetti preziosi la cui parziale distruzione diminuirebbe
notevolmente il valore dell’oggetto da analizzare.
Negli ultimi anni, l’utilizzo di spettrometri FRX portatili, sempre più
spesso consente di rilevare la presenza: sul territorio di inquinanti, le cui
analisi puntuali potranno essere successivamente confermate da opportuni
campionamenti, cui seguiranno più accurate misure in laboratori qualificati
al rilascio di certificati di analisi
Aspetti petrografici degli altari in marmi policromi in Puglia
No full textIl lavoro presenta i risultati di alcune indagini petroarcheometriche svolte su altari in marmi e pietre policrome, antiche e moderne
Weathered biotites from granitoids: the fractionation of REE, Th and transition elements and the role of accessory and secondary phases
No full textOssidiane neolitiche di Masseria Candelaro (FG): primi dati sulle provenienze da Lipari e Palmarola
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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