THE COMPOSITIONAL EVOLUTION OF POLLUCITE FROM AFRICAN GRANITIC PEGMATITES

Occurrences of pollucite are known from nine highly fractionated grani tic pegmatites in Zimbabwe, Namibia and Mozambique. The pollucite is t ypically associated with spodumene, petalite, amblygonite, quartz and feldspar. The primary pollucite has Si/Al (at.) ratios of 2.47 to 2.57 and CRK = 100(Cs+Rb+K)/(Cs+Rb+K+Na+Ca+Li) of 73.5 to 87.7. Extensive re-equilibration generated blebs and veins of (Cs, Al) rich pollucite with compositions trending towards end-member pollucite CsAlSi2O6 with Si/Al = 2.00 and CRK = 100. Characteristic sequences of alteration by coarse veins of lepidolite, quartz and feldspars, fine veins of musco vite and spodumene, and replacement by feldspar and clay minerals are well-developed in African pollucite. The crystal chemistry, compositio nal ranges, character of subsolidus re-equilibration, and veining, alt eration plus metasomatic features correspond to those observed at othe r pollucite localities, documenting general uniformity of pollucite cr ystallisation and subsolidus reactions in granitic pegmatites. Veining , replacement and cryptic processes such as analcimisation may signifi cantly reduce Cs grades in apparently monomineralic pollucite ore, but late cation exchange to analcime is limited at the localities axamine d. This suggests that leaching of Cs from undisturbed capsules of cera mic radwaste, which utilise a pollucite phase for containment of radio active Cs isotopes, will be negligible in a Na poor environment. (C) 1 997 Elsevier Science Limited.