Subsolidus Rubidium-Dominant Feldspar from the Morrua Pegmatite, Mozambique: Paragenesis and Composition

D. K. Teertstra, P. Černý and F. C. Hawthorne
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2

Abstract: At the Morrua pegmatite, Mozambique, alkali feldspar has replaced pollucite under low-temperature (250–150°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-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.

Keywords: K-feldspar • Rb-feldspar • rubidium • sanidine • granitic pegmatite

Mineralogical Magazine; June 1999 v. 63; no. 3; p. 313-320; DOI: 10.1180/002646199548538
© 1999, The Mineralogical Society
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