Abstract

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+ or -apatite) with 7-20 mol.% RbAlSi 3 O 8 , grading outward into a Rb-dominant feldspar with 66 mol.% RbAlSi 3 O 8 (20 wt.% Rb 2 O); (2) an intermediate layer of non-porous, inclusion-free, end-member 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 during growth at a temperature too low for subsequent alkali-cation diffusion or (Al,Si)-ordering.

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