The compositional evolution of pollucite from African granitic pegmatites

Occurrences of pollucite are known from nine highly fractionated granitic pegmatites in Zimbabwe, Namibia and Mozambique. The pollucite is typically 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 muscovite and spodumene, and replacement by feldspar and clay minerals are well-developed in African pollucite. The crystal chemistry, compositional ranges, character of subsolidus re-equilibration, and veining, alteration plus metasomatic features correspond to those observed at other pollucite localities, documenting general uniformity of pollucite crystallisation and subsolidus reactions in granitic pegmatites. Veining, replacement and cryptic processes such as analcimisation may significantly reduce Cs grades in apparently monomineralic pollucite ore, but late cation exchange to analcime is limited at the localities axamined. This suggests that leaching of Cs from undisturbed capsules of ceramic radwaste, which utilise a pollucite phase for containment of radioactive Cs isotopes, will be negligible in a Na poor environment. (C) 1997 Elsevier Science Limited.