Garnets from Val d’Ala Rodingites, Piedmont, Italy: An Investigation of Their Gemological, Spectroscopic and Crystal Chemical Properties

In Val d\u2019Ala (Piedmont,Western Alps, Italy), the more interesting rocks for the mineralogical research are represented by rodingites (rich in mineralized veins and fractures) associated with serpentinites in the eclogitized oceanic crust of Piemonte Zone, south of Gran Paradiso Massif. Among the vein-filling minerals, garnets are the most appreciated as mineral specimens and, in less degree despite their vivid and rich colors, for their potential as gem-quality materials. This study provides a complete gemological characterization of five faceted samples and others new information by means of Synchrotron X-ray computed micro-tomography imaging gem features. Electron-probe microanalysis (EMPA) and laser ablation\u2013inductively coupled plasma\u2013mass spectrometry (LA\u2013ICP\u2013MS) established that the chemical composition of garnets from different localities, resulted both close to pure andradite, enriched in light rare earth elements (LREE) with a positive Eu anomaly, and grossular-andradite solid solution (grandite), enriched in heavy rare earth elements (HREE). X-ray powder diffraction analyses indicate the possible coexistence of almost pure grossular and andradite. A spectroscopic approach, commonly used with gem-like material, by Raman and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, completes the characterization of the samples. The new data on the textural and geochemical features of the grandite and andradite garnets suggest local growth processes under various chemical and oxidation conditions of metasomatic and metamorphic fluids interacting with the host-rocks. Garnets represent long-lasting mineral records of the complex geological history of the Val d\u2019Ala rodingitic dikes during their oceanic- and subduction-related metamorphic evolution

A contribution to the study of FTIR spectra of opals

A 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

Physical properties of blue and fire opals found in Piau\ued 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

Aquamarine from the Masino-Bregaglia Massif, Central Alps, Italy

Aquamarine 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

Investigation on the gemological, physical and compositional properties of some green opals from Eastern Rhodopes, Bulgaria

Abstract: 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.

AVASPEC 2048: AN INNOVATIVE SPECTROSCOPIC METHODOLOGY TO DISCRIMINATE THE QUALITY GEM MINERALS

The AvaSpec 2048 is a spectrometer that allows to obtain the electromagnetic spectra in the  range between 400 and 1000 nm, i.e. in the wave band of the visible and close to the infrared. The spectrometer is equipped with an Avasoft software that convert the AvaSpec signals into absorption spectra. The absorption mode (or transmission mode) wavenumbers can be read by means of a USB port, reproduced on a spreadsheet and processed. At lower wavenumber the spectra are characterized by a strong background noise (from 350 to 430 nm) produced by the weak light source power. To solve this problem, the spectra were reproduced and cleaned of the noise, with the statistical software IGOR Pro. The analyses have been carried out on a large number of green minerals that also represent the most important natural and synthetic gemstones present today on the market: emeralds, tourmalines, garnets, opals, quartz, corundum. The aim of this work is to provide an original database for the gem-quality green minerals. All the samples have been previously investigated to determine the common gemmological properties (size, gravity measurement, colour, luster, cut, fluorescence) and subsequently analyzed through the AvaSpec 2048 equipment. We collected spectra of gemstones characterised by the same chromophore chemical elements in different legand field [2]: for example Cr, Fe, V responsible of the different shades of colour. We also collected several spectra on minerals like beryl aquamarine and zoisite tanzanite [1], that are precious gemstones frequently submitted to thermal treatment aimed to the improvement of the colour. These ameliorative treatments are very commonly carried out on the gems before the marketing. The spectra obtained on the minerals before and after the thermal treatment show several and relevant differences, and therefore represent a useful and non destructive method to recognize the colour change that take place through the heating. We also investigated some synthetic Beryl Emeralds (AGEE, Biron) and artificial material: it resulted that the spectra are very different and easily recognizable from the natural ones

Physical, geochemical and gemological properties of opals from Faroe islands

The opals from Faroe Islands are commercially not very relevant but are important from the historical point of view. Nice yellowish-red color fire opal and common opals from these islands are also very appreciated by collectors and can reach high prices in the markets. The analysed opals derive from Hoyvik near Torsshavn, in the island of Streymoy. Parent rocks are represented by the "middle basalt" formation. Investigated opals generally show vitreous-dull luster and specific gravity values comparable with data in literature. They are generally CT type with variable contents of cristobalite and tridymite and, in some cases, chalcedony. In order of abundance, trace elements are represented by Fe, Al, Ca, Mg, Na, P and K; Fe contents are very variable. The distribution of the elements is dictated expecially by cristallochemical substitution in the pseudo crystalline structure of the opals. A distinct feature of the opals from Faroe is represented by the presence of P; this element probably derives from metasomatic fluids. Trace elements probably derive from feldspars and pyroxenes in the host basaltic rocks. SEM investigations highlighted different textures like random piling of individual nanograins, lepispheres, tablets and plates that are typical of magmatic CT opals. The variable chemical composition and physical properties, and the different microtextures, would indicate the opals from Faroe Islands formed from different solutions, through variable growth rates