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Acquamarine, maxixe e berilli blu sintetici idrotermali: analisi ed identificazione
No full textIl berillo varietà acquamarina, i berilli Maxixe e tipo Maxixe (irradiato) e i berilli blu sintetici idrotermali attualmente presenti sul mercato (produzione Tairus e Malossi) sono stati studiati con tecniche di analisi gemmologica standard e metodologie analitiche sofisticate (chimiche e spettroscopiche), al fine di identificarli univocamente. Queste varie tipologie di berilli blu possono essere riconosciute e distinte tra loro sulla base della combinazione delle proprietà gemmologiche, con la composizione chimica e le caratteristiche spettroscopiche.
La distinzione tra la varietà acquamarina e i berilli Maxixe e tipo Maxixe si basa in primo luogo sul loro dicroismo opposto, sulla eventuale fluorescenza anomala ai raggi ultravioletti della varietà tipo Maxixe e sulla loro composizione chimica. Tutte le acquamarine contengono ferro che agisce come elemento cromoforo, oltre che concentrazione variabili di vari elementi alcalini e alcalino terrosi, quali litio, sodio, potassio, rubidio, cesio, magnesio e calcio. Questi ultimi possono anche essere presenti in basse concentrazioni, come in alcune acquamarine provenienti dalla Nigeria. I berilli Maxixe e tipo Maxixe, invece, hanno una composizione chimica caratterizzata da assenza di ferro e presenza di alcali, quali sodio e cesio. Questo determina differenti spettri di assorbimento nel range ultravioletto-visibile-infrarosso vicino (UV-Vis-NIR) per queste due importanti varietà di berilli azzurro-blu: l’acquamarina ha il tipico spettro contenente i contributi del ferro, sia nello stato bivalente (Fe2+), che trivalente (Fe3+), mentre, invece, il berillo Maxixe/tipo Maxixe presenta una serie di segnali tra 500 e 700 nm, dovuti a centri di colore, causa della loro colorazione, generalmente instabile alla luce e al calore.
Per distinguere le acquamarine naturali dai corrispondenti prodotti sintetici idrotermali attualmente in commercio, risulta indispensabile l’indagine delle pietre al microscopi ottico. Infatti, i materiali sintetici, generalmente, presentano una serie di inclusioni peculiari e indicative del processo di sintesi idrotermale, quali: diffuse e marcate disomogeneità di crescita, eventuali residui della lamina seme, utilizzata nella sintesi, e, in alcuni casi, anomale inclusioni puntiformi nerastre. L’analisi chimica risulta molto utile per distinguere il berillo blu sintetico idrotermale di produzione Malossi, poiché esso è caratterizzato dalla presenza di rame, assente nei berilli naturali, oltre che ferro, manganese, sodio e litio. Nel caso del sintetico Tairus, la sua composizione ricca in ferro e priva di alcali, consente la distinzione con le acquamarine naturali con un contenuto medio-alto di elementi alcalini, ma non è significativa nel caso di acquamarine povere di tali elementi. Infine, la spettroscopia UV-Vis-NIR fornisce dati utili alla separazione nel caso del berillo sintetico Malossi, il cui spettro rappresenta il risultato di contributi di più elementi cromofori (rame, ferro e manganese), mentre non è diagnostica per il sintetico Tairus, che possiede un tipico spettro di acquamarina.
Questo lavoro è il risultato di una collaborazione tra il Dipartimento di Scienze della Terra dell’Università degli Studi di Milano, l’Istituto Gemmologico Italiano e l’American Gemological Laboratories ed è pubblicato su Gems & Gemology (2008, Vol. 44, No., 3, pp. 214-226)
Natural and lab-grown minerals: the beryl case
No full textThe distinction between natural and synthetic gemstones is one of the most relevant issue in the gemological field, due to the need of protecting the integrity of natural gems respect to the synthetic ones. Innovations, in the last two decades, have been enormous and pervasive, in particular in the field of synthesis, allowing new synthetic productions, with significant impact on the trade. In fact, a variety of synthetic gems (e.g., diamond, corundum, beryl, quartz, spinel, crysoberyl and opal), obtained by different synthetic processes, such as, flux-growth, hydrothermal synthesis, Verneuil and Czochralski techniques, and, in the case of diamond, HPHT (high pressure and high temperature) and CVD (chemical vapor deposition) processes, are permanently present on the market.
Synthetic beryl represents a relevant synthetic gemstone, in terms of quantity produced and availability. A remarkable number of synthetics, mainly emerald, but also aquamarine and different colored (e.g., pink, orange, red, purple) samples, grown either by the flux method or hydrothermal process, are available on the market. The different colorations are obtained using various chromophoric dopants, such as, vanadium, chromium, manganese, iron, cobalt, nickel, copper, also together.
The research of diagnostic markers is essential to fully characterize and identify the various natural and synthetic beryls and it is possible using a multimethodological approach, resulting in a combination of standard gemological testing and advanced analytical techniques, mainly non destructive, such as chemical and spectroscopic methods.
The following features, obtained by various analytical techniques, provide means to distinguish between various colored natural and synthetic beryls.
1. Gemological properties. Refractive indices, birefringence, specific gravity and UV fluorescence in a few cases can be different between natural and synthetic beryl.
2. Microscopic features. The study of inclusions at the optical microscope can provide important information for the separation of the synthetic beryl from the natural counterpart. In particular, in most cases, synthetic beryl exhibits diagnostic internal features, consisting of strongly inhomogeneous growth structures, residues of seed plates, metallic and flux inclusions, and phenacite crystals. These inclusions prove its artificial nature and allow for a rapid identification.
3. Chemical composition. The various natural and synthetic beryls can be separated on the basis of their chemical composition. Minor and trace elements, such as alkaline and terrous-alkaline elements (Na, Li, K, Mg, Ca, Cs..), chomophore elements (V, Cr, Mn, Fe, Co, Ni, Cu,...), chlorine and metallic elements (Ni, Mo, Pt, Rh,..), are differently present in natural and synthetic stones, allowing for a rapid distinction, in most cases.
4. Spectroscopic features. Spectroscopic techniques are an useful tool in the analysis and identification of the different natural and synthetic beryls. UV-Vis-NIR spectroscopy provides information about the causes of colors, which can be different in natural and synthetic stones, highlighting the presence of “exotic” chromophore ions (e.g. Cu, Ni, Co), typical of some synthetics. Mid-infrared spectroscopy is useful to determine the presence of water molecules, besides to impurities of chlorine and ammonium, which are often relevant in the distinction between natural and synthetic beryl. Moreover, in some cases, this technique can be diagnostic if one is able to establish the nature and the orientation of water molecules (H2O types) in the structural channels of beryl
Nephrite jade from Val Malenco, Italy: Review and update
No full textAlpe Mastabia, in the Val Malenco district of northern Italy, has been a source of nephrite jade since the early 2000s. Twenty-one samples from this locality were investigated by classical gemological methods; X-ray powder diffraction, combined with quantitative phase analysis; scanning electron microscopy in combination with energy-dispersive spectrometry; electron microprobe analysis; mass spectrometry; and mid-infrared spectroscopy. From a mineralogical standpoint, this jade consists mainly of tremolite amphibole, with variable amounts of other constituents, especially calcite (up to approximately 30 wt.%), but also pyroxene, apatite, and sulfide minerals. Its pale green color is related to the low iron content of the tremolite amphibole, whereas the other minerals are responsible for different colors (calcite for white, molybdenite and galena for gray). On the basis of minor and trace-element composition, we can classify this jade as dolomite-related nephrite (para-nephrite). Although new material could be recovered from this area, future production will probably be limited by access difficulties
A MULTI-METHODOLOGICAL STUDY OF A GEM-QUALITY SYNTHETIC DARK BLUE BERYL
No full textBeryl is an accessory mineral commonly found in pegmatitic rocks, with ideal chemical formula Be3Al2Si6O18 and crystal structure consisting of six-membered rings of Si-tetrahedra, linked by Al-octahedra and Be-tetrahedra, forming a three-dimensional framework. The “extra-framework” content (alkali cations, water and carbon dioxide molecules) lies in the six-membered ring channels parallel to [0001].
Because of the peculiar beryl’s commercial value, a remarkable number of synthetic samples, emeralds and other various specimens with “exotic” colourations, are permanently present on the market [1].
In the present work a multi-methodological investigation of a synthetic Cu/Fe-bearing dark blue beryl [IV(Be2.86Cu0.14)Σ=3.00 VI(Al1.83Fe3+0.14Mn2+0.03Mg0.03)Σ=2.03 IV(Si5.97Al0.03)6.00 O18 (Li0.12Na0.04 0.40H2O)] has been performed by means of gemmological standard testing, combined with electron microprobe analysis, laser ablation inductively coupled plasma mass spectroscopy, thermogravimetric analysis, infrared spectroscopy and single-crystal X-ray diffraction. The aim of this work is to provide a full characterization of this material, covering gemmological properties, crystal structure and crystal chemistry.
The investigated 2.70 ct gem is uniaxial negative with refractive indices =1.590 and =1.582 and birefringence 0.008; the measured density is 2.77 g/cm3. These properties are the same reported for the natural aquamarine beryl [2]. Only the characteristic internal growth pattern can be useful for the separation of this gem material from its natural counterparts [3]. The chemical analyses reveal significant contents of iron and copper, the latter never found in any natural aquamarine beryl. The X-ray single-crystal structural refinements confirm that the gem maintains the space group P6/mcc and the general structural arrangement of the natural beryls, with unit-cell parameters: a~9.25 and c~9.22 Å. The analysis of the difference Fourier maps of the electron density suggests that Cu is located at the tetrahedral site (Wyckoff 6f-position), along with Be, whereas Fe shares the octahedral site with Al (4c-position). The channel content is distributed in two extra-framework sites: the first one occupied by water molecules (2a-position) and the second one (2b-position) mainly by alkali cations, in agreement with previous studies of natural beryls [4]. Infrared spectra show that the H2O molecules in the channel are present with two different configurations: one with the H•••H vector oriented //[0001] (“type I”) and the other with H•••H vector oriented perpendicular to [0001] (“type II”) [5, 6].
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References.
[1] J.I. Koivula, M. Tannous, K. Schmetzer, Gems & Gemology, 36, 360-379, 2000; [2] R. Webster, Gems: Their sources, Description and Identification, 6th ed., Butterworth-Heinemann, Oxford, 2006; [3] I. Adamo, A. Pavese, L. Prosperi, V. Diella, D. Ajò, G.D. Gatta, Gems & Gemology, submitted; [4] G.D. Gatta, F. Nestola, G.D. Bromiley, S. Mattauch, American Mineralogist, 91, 29-34, 2006; [5] D.L. Wood, K. Nassau, Journal of Chemical Physics, 42, 2220-2228, 1967; [6] D.L. Wood, K. Nassau, American Mineralogist, 53, 777-800, 1968
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
The profile of trace elements, including the REE, in gem-quality green andradite from classic localities
No full textCrystals of gem-quality andradite (variety “demantoid”) from Val Malenco, Italy, have been analyzed for major, rare-earth and other trace elements (Sc, Ti, V, Cr, Co, Ni, Zn, Sr, Y, Zr), together with samples from Aosta Valley, Italy, and from the classic deposits of the world (Russia, Iran, Pakistan, Namibia, Madagascar). They are all of gem quality and vary from bright green to yellow-green in color. The samples are homogeneous within the limit of the analytical error and cover a restricted range of composition from almost pure andradite (Adr ≥ 98 mol.%) to members of andradite–uvarovite (Adr81–96Uv3–18) or andradite–grossular (Adr92–94Grs6–8) solid solution, with the sole exception of the sample from Namibia, showing a composition varying from pure andradite to almost pure grossular (Adr11Grs89). All the samples have a low abundance of most trace elements, except for Cr, which ranges from a few ppm to more than 5 wt% Cr2O3. The substitution of Fe3+ and Cr3+ for Al at the [Y] site significantly controls the geometry of the structural sites and the incorporation of the REE. In particular, (i) the samples showing a composition close to pure andradite exhibit LREE-enriched and HREE-depleted patterns with a strong positive Eu anomaly, whereas (ii) the uvarovite-enriched samples show flatter patterns with a small positive Eu anomaly, and (iii) grossular-rich samples are LREE-depleted with no Eu anomaly or a negative one. However, such a compositional variation may also arise from differences in the bulk composition of the host rocks and from changes in the physicochemical conditions during growth
A 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
Tsavorite and other grossulars from Itrafo, Madagascar
No full textSince 2002, tsavorite and other grossular varieties have been recovered from a primary deposit at Itrafo, a village in the Andrembesoa area of central Madagascar. Twenty-two samples from this locality were investigated by classical gemological methods, chemical analysis, and UV-Vis-NIR and mid-IR spectroscopy. The garnets' chemical composition was nearly pure grossular (>92 mol.%), with iron and vanadium as the main chromophores. Their iron content and Fe2O3:V2O3 ratio were higher than those generally found in tsavorite from well-known deposits. Although the Itrafo deposit is relatively large, and new veins could be discovered, future production will be limited by access difficulties and security concerns
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