SPATIAL DISTRIBUTION OF THE Fe2+ IN THE TETRAHEDRAL STRUCTURAL POSITION OF Be2+ IN CRYSTALS OF NATURAL BERYL

Abstract

Variously colored gem-quality iron-bearing beryls from two Brazilian localities, Lavra do Abilio (Minas Gerais) and Garimpo do Cercadinho (Bahia), were studied by polarized optical absorption spectroscopy and microprobe analysis. The purpose of this study was to investigate the spatial distribution of Fe2+ in the tetrahedral site that is normally occupied by Be2+ along the c-axis of the crystals. This was done by measuring the intensity of the E^c-polarized electronic spin-allowed band of BeFe2+ that occurs at ~12 000 cm–1. The beryl samples from the two localities are different because crystals from Lavra do Abilio show a homogeneous distribution of Fe2+, whereas Fe2+ in Garimpo do Cercadinho beryls strongly varies along c. This indicates different physico-chemical conditions of crystallization. No correlation between BeFe2+ and Fetotal and Fe2+ at the octahedral Al-site was deteremined by microprobe analysis. The latter (VIFe2+) causes the blue color of aquamarine and the green color of "green" beryl. Heliodor centers also affect color. An analysis of weak narrow spin-forbidden bands of octahedral VIFe3+ at the nominal Al site and narrow molecular H2O (located in structural channels) vibrational modes indicates that both are distributed homogenously in both beryl types. Investigation of a light-blue synthetic hydrothermally grown beryl, which was studied as well, shows that the BeFe2+ distribution along c and perpendicular to c is constant in value.