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

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⁺, Fe_total, 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.