Original paper

Beryl from the granitic pegmatite at Namivo, Alto Ligonha, Mozambique

Neiva, Ana M.R.; Neiva, João M.C.

Abstract

In an elongated, concentrically zoned granitic pegmatite from Namivo, Mozambique, a geochemical evolution was found from green and bluish green beryl of the outer zone to pink beryl in the inner and core zones. This evolution is caused by magma fractionation and marked in beryl by an increase in Al, Li, Na, Cs, Al/(Fe + Mn + Mg),∑REE and a decrease in Be, Fe, Mg, Be/Li, Be/(Na + Cs), Be/(Li + Na + Cs), Na/Li and Li/Cs. Beryl has low REE contents and shows enrichment of the middle REE with a negative Eu anomaly. The colour change in beryl from the outer zone to the core zones is attributed to an increase in the Mn/Fe ratio and a possible decrease in the Fe3+/Fe2+ ratio. The octahedral substitution of Fe2+, Mg2+ and Mn2+ for Al3+ pre-dominates in beryl from the outer zone, while the tetrahedral substitution of Li for Be mainly takes place in beryl from the other more fractionated zones. Both substitutions are charge balanced by Na+ and Cs+ in channels, but Cs≥0.009 apfu is only important in beryl from the inner and core zones. These substitutions cause an increase in unit-cell dimension c, cell volume and c/a ratio in beryl from the outer zone to the other zones. A compositional gap occurs between beryl from the outer zone and the other zones at Be 2.75 apfu and unit-cell dimension c at 9.210 Å.In a single zoned crystal from the outer zone, Li, Na, Cs, Al/(Fe + Mn + Mg), unit-cell dimension c, cell volume and c/a ratio increase and Be, Be/Li, Na/Li, Li/Cs, Be/(Na + Cs) and Be/(Li + Na + Cs) decrease from core to rim. The core represents the primary beryl, while the rim has a composition similar to that of beryl from the inner and core zones of the pegmatite.

Keywords

berylcrystal chemistrycell parametersevolutionpegmatitemozambique