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

Published online by Cambridge University Press:  05 July 2018

D. K. Teertstra ,
P. Černý  and
F. C. Hawthorne
D. K. Teertstra
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
P. Černý
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
F. C. Hawthorne
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
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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-feldsparRb-feldsparrubidiumsanidinegranitic pegmatite
Type
Research Article
Information
Mineralogical Magazine , Volume 63 , Issue 3 , June 1999 , pp. 313 - 320
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1999

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