Be, Fe2+-substitution in natural beryl: an optical absorption spectroscopy study

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Marco E. Ciriotti
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Be, Fe2+-substitution in natural beryl: an optical absorption spectroscopy study

Messaggio da Marco E. Ciriotti » mar 13 ago, 2019 16:16

▪ Taran, M.N. & Vyshnevskyi, O.A. (2019): Be, Fe2+-substitution in natural beryl: an optical absorption spectroscopy study. Physics and Chemistry of Minerals, 46, (in press).

Collection of variously colored gem quality iron-bearing beryl from two Brazilian deposits, Lavra do Abilio (Minas Gerais) and Garimpo do Cercadinho (Bahia) was studied by polarized optical absorption microspectroscopy and microprobe analysis. The main attention was paid to spatial distribution of Fe2+ in tetrahedral structural site of Be2+, estimated by intensity of E⊥c-polarized electronic spin-allowed band of BeFe2+ at ~ 12,000 cm−1, mainly along c-axis of the crystals. It is found that in the samples from the two deposits these characteristics are different: in beryl from Lavra do Abilio the distribution is nearly homogeneous, while in those from Garimpo do Cercadinho BeFe2+-content strongly varies along the crystal’s elongation that may indicate the different physico-chemical conditions of crystallization. It is found that there is no correlation between distribution of BeFe2+, Fetotal, detected by microprobe, and Fe2+ in octahedral Al-site, which causes blue color of aquamarine and green—of “green” beryl, wherein heliodor centers are also involved in process of coloration. Judging from intensity of weak narrow spin-forbidden bands of octahedral Fe3+ in Al-sites and narrow near-infrared lines of vibration transition of H2O-molecules in the structural channels, these two admixtures are distributed homogenously in beryl from both deposits. A light-blue synthetic hydrothermal beryl was also studied for comparison. It is found that BeFe2+-distribution along and across elongation of the sample is essentially homogeneous.
Marco E. Ciriotti

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