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Gem-quality zoisites from Merelani (Northeastern Tanzania): review and new data
No full textThe Merelani area (NE Tanzania) is the unique locality in the world for tanzanite, a violetish blue gem-quality variety of vanadium-rich zoisite. However, other coloured (brown, yellow, orange, etc.) zoisite occurs in this deposit, closely associated with tanzanite. This study reports a review and new data on physical and chemical properties of this material, obtained investigating five gem-quality samples, ranging in colour from yellowish brown and greenish yellow to violetish blue, by means of classical gemmological methods and by XRD, EMP and LA-ICP-MS analyses. The results confirm that the major element concentration of all the samples is almost identical, so their different colour is mainly dependent on the variation of some minor and trace elements. In particular, the main chromophore element is V but also the V/Ti ratio plays a significant role in the colour characteristics. All the faceted gems have been subjected to heat treatment in order to observe a possible change of their colour. The various coloured zoisite gems become permanently blue and change their pleochroic scheme from trichroic to dichroic at ∼ 500 °C, except for the blue specimen which is dichroic before and after heating and does not show any change of colour. The oddness of this sample could be due to an undeclared previous heat treatment, made to enhance the colour. All these characteristics observed in zoisite from Merelani derive from the geological history that makes it one of the most interesting and significant gem deposit of the world.
Physical and chemical properties of some italian opals
No full textThe physical and compositional properties of some opals from different parts of Italy have been investigated through several methodologies like optical analysis, specific gravity, refractive indices, XRPD, IR spectroscopy, LA-ICP-MS. The opals show different colors: white, brownish-white, yellowish-white, yellow-/greenish white and grey. Black and metallic inclusions, consisting of todorokite, are sometimes present. Play-of-color have not been observed but some opals show small iridescence zones; opals are inert to the long and short wavelength UV radiation (366 - 254 nm) with the exception of one sample; also phosphorescence is absent. Refractive index and specific gravity values are respectively 1.43 to 1.44 and 2.07 to 2.33 g/cm3 consistent with data available in the literature. XRPD analyses highlighted Italian opals are A, CT and C types, but most of them can be classified as opals- CT. IR spectroscopy data confirmed the opal classification. The most abundant impurities are Mg (between 400 and 900 ppm), Fe (35-400 ppm), Ca (72-96 ppm) and Ni (20-65 ppm). Similarly to what observed in other opals worldwide, Fe appears to be the principal factor that determines the brown to red to orange to yellow colors. Chromophore elements like V, Cr, Cu, Ti, Co are present in very low concentrations and probably do not influence the physical properties of the Italian opals analyzed. Ni is present in more relevant amounts and is probably related to clay minerals. Mn is clearly detected only in two samples and is related to the presence of dendrites. The investigated Italian opals show a rather homogeneous trace element composition which is in the range of worldwide opal’s composition, except for their noticeable amount in Ni
Tanzanite and other zoisites from Merelani (NE Tanzania): new gemmological and chemical data
No full textZoisite is an orthorhombic (space group: Pnma) polymorph of clinozoisite and is not considered a member of the epidote-group, which includes only monoclinic minerals. Tanzanite is the gem name for the violetish-blue coloured vanadium-bearing zoisite occurring in the Merelani mining deposit (northeastern Tanzania) located at the center of the Great Rift Valley region. The more than 5 km long area of the Merelani mineralization is occupied by late Proterozoic metasedimentary rocks and is divided into four government-controlled main blocks. Most gem-quality tanzanites occur in fault zones within outcrops of graphitic gneisses and schists, together with small quantities of transparent zoisite crystals of various colours (brown, yellow, green, pink, colourless). The colour of tanzanite and zoisite on the market today has generally produced by heat-treatment, although the response of the stones changes with the varying amounts of vanadium, chromium, and titanium.
In the present work we have investigated five rough samples of zoisite from the Merelani area, consisting of one specimen with the typical violetish-blue colour of tanzanite and four stones ranging in colour from yellow-green to yellow-brown. From this material, we obtained five faceted gems weighting from 0.22 to 0.52 ct (zoisites) up to 2.44 ct (tanzanite). The gems and the rough grains have been characterized by traditional gemmological tests combined with EMPA-WDS and LA-ICP-MS measurements in order to determine their optical, physical and chemical properties. The faceted gems have been also heated at temperature up to 800°C with steps of 50°C. The annealing time for every steps varied from 15 to 120 min, with the increase of the temperature. The gemmological tests have been repeated after each step.
Both tanzanite and zoisites examined have a composition close to the idealized formula Ca2Al3[Si2O7][SiO4]O(OH), but contain some other minor and trace elements substituting for calcium and aluminium. In particular, the tanzanite sample is enriched in vanadium and chromium (V=2625 ppm; Cr=224 ppm) respect to zoisites (V=1139-1571 ppm; Cr=81-140 ppm) whereas is depleted mainly in titanium and strontium (tanzanite: Ti=43 ppm, Sr=1266 ppm; zoisites: Ti=95-268 ppm, Sr=1451-2549 ppm). All the examined samples show comparable rare earth elements (REE) distribution patterns with a marked enrichment of LREE relatively to M-HREE, but the determined concentration is higher in tanzanite (ΣREE=384 ppm) than in zoisites (ΣREE=123-364 ppm). Before the heat treatment, all the examined gems were biaxial positive (refractive indices and birifringence ranging over 1.687-1.692 to 1.698-1.702 and 0.008-0.013, respectively) with a strong trichroism; the density varied from 3.20 to 3.46 g/cm3. When viewed with a gemmological microscope, most gems contain several inclusions such as fluid veils, opaque solid grains (probably graphite) and fractures. The four yellow-green to yellow-brown zoisites responded to heat treatment by turning violet-blue and dichroic at approximately 450/550°C. The violetish-blue tanzanite became dichroic, but showed no change in colour. According to the most recent data reported in literature, colour variations are probably due to a change of the oxidation state of vanadium and titanium (Ti3++V4+ converting to Ti4++V3+) with heating
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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