1,720,967 research outputs found
Coexisting cummingtonite-sodic amphibole pair in metaquartzites from the ophiolite's sedimentary cover (St. Marcel Valley, Italian Western Alps): A X-ray structure refinement and petrology study
No full textA contribution to the study of FTIR spectra of opals
No full textA FTIR spectra collection of a large number of various micro-crystalline (C and CT) and non-crystalline (A) opal samples is here presented. The suite of the investigated specimens consists of the most important natural and synthetic opal samples, with a gemological significance, present on the market today, including both with and without play of color. The opal-C type is clearly identifiable on the basis of the sharp peak at about 620 cm-1, whereas the FTIR spectra of opal-CT and -A differ mainly for the position of the 790 cm-1 SiO band. Other phases, such as i.e. clay minerals, present in opal samples and related to different genesis and localities, can be easily detected by mid-infrared spectroscopy, in addition to X-ray powder diffraction data
A study on the characteristics of some C- and CT-opals from Brazil
No full textPhysical properties of blue and fire opals found in Piauí state, Brazil, have been characterized through optical analysis, specific gravity measurements, XRPD and IR spectroscopy, whereas chemical composition has been determined by LA-ICP-MS and SEM-EDS. Values obtained for refractive index and specific gravity are n = 1.430-1.461 and G = 1.98-2.28, in agreement with literature. The results of the XRPD and IR-spectroscopy show the cristobalite presence (C and -CT type). Trace quantities of chromophore elements are correlated with the variations of colour. The particular "coralloid islands" structure, built up by microspherules of amorphous silica, observed by SEM analyses could explain the particular effects of iridescence shown by some blue specimens
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
Aquamarine from the Masino-Bregaglia Massif, Central Alps, Italy
No full textAquamarine from granitic pegmatites of the Masino-Bregaglia Massif, Central Alps, Italy, is investigated by classical gemological methods, LA-ICP-MS chemical
analyses, and UV-Vis-NIR and mid-IR spectroscopy.
This beryl has typical gemological properties
for aquamarine, including iron absorption features,and is characterized by a low alkali content and both types I and II water molecules. Although so far only a small amount of this aquamarine has been
polished into gems, it represents an attractive gem material of potential economic interest
Caratterizzazione di berilli in pegmatiti del plutone di Val Masino-Bregaglia (Alpi Centrali)
No full textTre campioni di berillo provenienti da filoni pegmatitici del plutone alpino di Val Masino-Bregaglia (Alpi Centrali) sono stati studiati mediante indagini ottiche, diffrattometriche, spettroscopiche e chimiche. Dai risultati ottenuti è stato possibile trarre le seguenti conclusioni: i) tutti e tre i campioni appartengono alla varietà “acquamarina” e nei loro canali strutturali sono presenti molecole di acqua; ii) dal punto di vista chimico sono caratterizzati da una prevalenza del contenuto di sodio (Na = 0,40 – 0,53 % in peso) e di ferro (Fe = 0,81 – 0,96 per cento in peso) più elevata rispetto a tutti gli altri elementi determinati (Li, Rb, Cs, Mg, Ca, Ti, V, Cr, Mn, Zn). Questi ultimi sono infatti tutti presenti in quantità inferiore a 0,1% in peso. La predominanza del ferro (presente nei due stati di valenza, Fe2+ e Fe3+) sugli altri elementi cromofori determinata nei campioni esaminati è tipica dei berilli di varietà acquamarina e giustifica l’ipotesi di considerare questo elemento come principale responsabile della colorazione azzurra
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.
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