Beryl 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 . 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 . 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 . Only the characteristic internal growth pattern can be useful for the separation of this gem material from its natural counterparts . 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 . Infrared spectra show that the H2O molecules in the channel are present with two different configurations: one with the H•••H vector oriented // (“type I”) and the other with H•••H vector oriented perpendicular to  (“type II”) [5, 6]. --------------------------------------------- References.  J.I. Koivula, M. Tannous, K. Schmetzer, Gems & Gemology, 36, 360-379, 2000;  R. Webster, Gems: Their sources, Description and Identification, 6th ed., Butterworth-Heinemann, Oxford, 2006;  I. Adamo, A. Pavese, L. Prosperi, V. Diella, D. Ajò, G.D. Gatta, Gems & Gemology, submitted;  G.D. Gatta, F. Nestola, G.D. Bromiley, S. Mattauch, American Mineralogist, 91, 29-34, 2006;  D.L. Wood, K. Nassau, Journal of Chemical Physics, 42, 2220-2228, 1967;  D.L. Wood, K. Nassau, American Mineralogist, 53, 777-800, 1968.
|Titolo:||A MULTI-METHODOLOGICAL STUDY OF A GEM-QUALITY SYNTHETIC DARK BLUE BERYL|
|Autori interni:||GATTA, GIACOMO DIEGO|
|Data di pubblicazione:||set-2008|
|Parole Chiave:||Synthetic Cu/Fe-bearing dark blue beryl, electron microprobe analysis, laser ablation inductively coupled plasma mass spectroscopy, thermogravimetric analysis, infrared spectroscopy, single-crystal X-ray diffraction.|
|Settore Scientifico Disciplinare:||Settore GEO/09 - Georisorse Miner.Appl.Mineral.-Petrogr.per l'amb.e i Beni Cul|
|Citazione:||A MULTI-METHODOLOGICAL STUDY OF A GEM-QUALITY SYNTHETIC DARK BLUE BERYL / I. Adamo, G.D. Gatta, N. Rotiroti, V. Diella, A. Pavese. ((Intervento presentato al 1. convegno SIMP-AIC Joint Meeting "Learning from and for the Planet Earth" tenutosi a Sestri Levante nel 2008.|
|Appare nelle tipologie:||14 - Intervento a convegno non pubblicato|