1,720,967 research outputs found
Investigation on the gemological, physical and compositional properties of some green opals from Eastern Rhodopes, Bulgaria
No full textAbstract: The physical and compositional properties of some common opals from Eastern Rhodopes, Bulgaria have been investigated through several methodologies such as optical and gemological equipments, X-Ray Powder Diffraction (XRPD), Scanning Electron Microscopy (SEM), Raman Spectroscopy and Laser Ablation Microprobe (LA-ICP-MS). The investigated opals show different colors such as light-apple and yellowish green; the diaphaneity is opaque or opaque/translucent with luster waxy/dull/greasy. They are inert to the long and short wavelength UV radiation (366–254 nm). Refractive index and specific gravity values are between 1.445–1.470 and 1.96–2.09 g/cm3, respectively. XRPD analyses show the opals are CT type and in some cases contain quartz and clay minerals (saponite). Fe and, subordinately, V appear the main chromophores that determine the green-yellowish color, while Ni is responsible for the apple and light green. Other detected trace elements are Al, Ca, K, Mg, Na, B, Zn, Rb. SEM analysis shows three types of structures already observed in the CT opals like randomly distributed nanograins and lepispheres, mammillary structure, channels structure made up by bundles of fibers. Raman spectra confirm the samples are made up by para-crystalline CT-opal, generally considered of volcanic origin, with significative content of saponite. Graphic abstract: [Figure not available: see fulltext.
Characterization of recently discovered common green opals from Anosy (Madagascar)
No full textThe recently discovered common green opals from Anosy in Madagascar were studied for their physical, chemical and gemological properties and gemological relevance. The color of the opals is yellowish green but not very homogeneous for the presence of cavities, dark lamellae and spots, the diaphaneity is translucent / opaque with greasy luster and are inert to long and short wavelength UV radiation (366-254 nm). Refractive index and specific gravity values range between 1.435-1.460 and 2.03-2.07, respectively. The opals are CT type with tridymite more abundant than cristobalite and contain clay minerals (saponite). The main chromophores that determine the color are Fe and, subordinately, V and Cu. Other detected trace elements are Mg, Al, Ca, K, Na, Ni and Cr. The high contents of Ba, probably deriving from mica and feldspars, are noteworthy and can represent a geochemical marker. Three types of microstructures were observed: homogeneous and very fine microspheres, lepispheric cauliflower structures and globular aggregates of different sizes formed by laminae. Our analyses suggest that opals formed under similar genetic conditions, with a slow or moderate growth rate. Anosy opals would certainly have a good commercial value as semi-precious materials, although lower than those from other regions of Madagascar (Bemia)
Petrology and crystal chemistry of retrograde amphiboles from Fe-Ti eclogite gabbros, Western Ligurian Alps, Italy
No full textEvaluation of the Gemological Properties of Datolites from the Campotrera Deposit in the Northern Apennines (Italy)
No full textThis work investigates the gemological properties of the datolite from the famous field of Campotrera near Reggio Emilia in Italy, for a possible commercial use in the market. This mineral occurs in widespread multi-centimeter veins, together with calcite and prehnite, within polygenic breccias in basaltic ophiolites. The most common form for this datolite is the double wedge with a prism (110) and a pinacoid (001). The gems obtained are mixed or carré cut, colorless or salmon pink, transparent, with a vitreous luster and weight between 1 to 5 carats. They have high brilliance, transparency and birefringence, glassy luster, absence of cleavage. The major chromophore is probably Fe, which occurs as inclusion of hematite and ilmenite. Raman investigations highlighted different fluid inclusions. The primary are randomly distributed or, in some cases, follow the growth zones, while the secondary form aligned tracks along the microcracks. Fluid inclusions can be biphasic and made up by liquid + gas (L + G), generally >10 mm in size, and more rarely, monophasic, composed only by liquid (L) generally <10 mm. The gems extracted from the rough sample are very valuable but their delicacy requires attention in the cutting and preparation of the jewels
Black Quartz from the Burano Formation (Val Secchia, Italy): An Unusual Gem
No full textThe Burano Formation in Val Secchia in the province of Reggio Emilia is rich in black or very dark quartz. The crystals are often pitted by scars, rarely shiny, often opaque, rarely translucent, and about 3 cm long. However, they have a beautiful color and are generally euhedral and bi-terminate with simple habitus and well developed faces—for these characteristics, they are very sought after by collectors. Micro-Raman analyses showed the quartz contains abundant inclusions of anhydrite and graphite. The inclusions of anhydrite are responsible for the chromatic inhomogeneity, while the black color is linked to the presence of disordered graphite inclusions. LA-ICP-MS analyses did not show a significant presence of chromophore elements. Black quartz formed in the original evaporite deposits of gypsum, which, due to diagenesis and the increase in temperature during deep tectonic burial conditions, lost water of crystallization and subsequently transformed into anhydrite. After the formation of quartz crystals, the Burano Formation was exhumated and rehydrated, resulting in gypsification at conditions close to the surface. The black quartzes of the Burano Formation represent an appreciable gemological material for the development of local craftsmanship, even if the difficulties in reaching the deposit limit their marketing
The crystal structure of vicanite-(Ce), a borosilicate showing an unusual (Si3B3O18)15- polyanion
No full textThe crystal structure of holotype vicanite-(Ce) has been solved and refined to R = 1.8% for 1398 observed reflections with the aid of a new crystal from the same locality (Tre Croci, Vetralla, Italy), found more than 10 years after the first. The new unit formula is (Ca,REE,Th)15Fe3+(SiO4)3 (Si3B3O18)(BO3)(As5+O4)(As3+O3)x(NaF3)1–xF7·0.2H2O with x = 0.4. The structure is trigonal, R3m, Z = 3, a = 10.8112(2), c = 27.3296(12) Å, and layered along [001] with three distinct layers.
Layer A at z ca. 0 (1/3, 2/3) contains an Fe(SiO4)6 group and a threefold B3O9 borate ring. Each
15–
tetrahedron of the ring shares one oxygen atom with one Si tetrahedron, forming an unusual Si3B3O18
polyanion.
Layer B at z ca. 1/9 (4/9, 7/9) contains an AsO4 tetrahedron and a BO3 triangle.
Layer C at z ca. 2/9 (5/9, 8/9) represents the disordered part of the structure, containing two very close (0.85 Å) As3+O3 2– polyhedra, the occurrence of which is mutually exclusive and
3– and NaF3 statistically disordered. A 3-dimensional network of M-(O,F)n polyhedra (M = Ca, REE, Th; 8 < n < 10) provide connections
among neighboring layers
RHODONITE-PYROXMANGITE FROM TANATZ ALP, SWITZERLAND
No full textThe chemical, physical, and gemological properties of attractive rhodonite-rich rocks from Tanatz Alp (Switzerland) were investigated using classical gemological methods, petrographic observation, and analytical techniques such as X-ray powder diffraction, Raman spectroscopy, and scanning electron microscopy. The color of the samples varied from pale pink to purplish pink; they were opaque and contained braunite, rhodochrosite, and spessartine. The gem-quality samples have an attractive color and are compact with no fractures. Tanatz Alp rhodonites are characterized by the presence of other minerals such as kutnohorite, spessartine, ankerite, rhodochrosite, khristovite-(Ce), pyroxmangite, and chlorite. The presence of the polymorph pyroxmangite was also confirmed by micro-Raman analyses. The samples had a chemical composition of MnO (45.97–48.64 wt.%) and SiO2 (46.70–47.92 wt.%), with very low amounts of MgO (0.64–1.65 wt.%) and CaO (<5 wt.%). Rhodonites from Tanatz Alp have some economic importance because of their use in jewelry and ornamental objects
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
- …
