Geochemical and morphological features of beryl from the Bikita granitic pegmatite, Zimbabwe

The geochemical evolution of beryl in the Bikita rare-element granitic pegmatite, in Zimbabwe, was examined by analyzing this mineral for alkali elements in 23 samples from eight zones in this giant deposit. From the outer to the inner zones of the pegmatite, generally corresponding to the least up to the most evolved ones, the alkali contents of near-homogeneous crystals of primary beryl increase from 0.09 to 1.08 wt.% Li₂O, from 0.39 to 1.56 wt.% Na₂O and from 0.10 to 2.23 wt % Cs₂O. The entry of Li into the Be-populated tetrahedra is compensated initially by incorporation of Na into the channels of the structure, but in the more evolved samples from the inner zones of the pegmatite body. Cs becomes the main charge-balancing cation. A single sample of subsolidus pinkish beryl, in thin platy crystals flattened parallel to {0001}, with a thickness-to-diameter aspect of up to 1:50, has a general geochemical signature that in part overlaps, in part departs from, that of the primary beryl. Crystals of the late beryl show an alkali-poor core overgrown by alkali-rich outer zones, containing up to 1.15 wt.% Li₂O, 2.01 wt.% Na₂O and 3.48 wt.% Cs₂O. However, the alkali contents generally decrease from the early to the late "generations" of the platy crystals. The extremely platy habit cannot be linked to the alkali content, and the factor that promoted this habit remains unknown.