Subsolidus rubidium-dominant feldspar from the Morrua pegmatite, Mozambique: paragenesis and composition

At the Morrua pegmatite, Mozambique, alkali feldspar has replaced pollucite under low-temperature (250-150 degrees C) hydrothermal conditions. Fluids invading a fracture system in pollucite formed round granular aggregates of (K-Rb)-feldspar in three stages: (1) a compositionally heterogeneous core of the feldspar cluster (+cookeite +/- apatite) with 7-20 mol.% RbAlSi3O8, grading outward into a Rb-dominant feldspar with 66 mol.% RbAlSi3O8 (20 wt. % Rb2O); (2) an intermediate layer of non-porous, inclusion-free, end-membe r K-feldspar; (3) an outer layer of porous end-member K-feldspar. Feldspars of all three stages seem to be monoclinic and disordered, with metastable sanidine structure. Zoning in K/Rb, preserved on a fine scale, was formed d uring growth at a temperature too low for subsequent alkali-cation diffusio n or (Al, Si)-ordering.