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Mineralogical Magazine; December 2000; v. 64; no. 6; p. 1147-1164
© 2000 Mineralogical Society of Great Britain and Ireland
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Exotic aluminium phosphates, natromontebrasite, brazilianite, goyazite, gorceixite and crandallite from rare-element pegmatites in Namibia

J. R. Baldwin1,*, P. G. Hill2, O. von Knorring3,{dagger} and G. J. H. Oliver1

1 Crustal Geodynamics Group, School of Geography and Geosciences, University of St Andrews, KY16 9AL, UK
2 Department of Geology and Geophysics, University of Edinburgh, EH9 3JW, UK
3 Department of Earth Sciences, University of Leeds LS2 9JT, UK

* E-mail: jrb2{at}st-andrews.ac.uk

Replacement phenomena in amblygonite–montebrasite in rare-element pegmatites from the Karibib-Usakos area, Namibia, have been investigated using the electron microprobe. The first African occurrence and analysis of the very rare mineral natromontebrasite NaAl(PO4)(OH,F) is reported from the Daheim pegmatite. In the Okatjimukuju pegmatite, montebrasite has been replaced by a number of phases including crandallite CaAl3(PO4)2(OH)5·H2O and brazilianite NaAl3(PO4)2(OH)4. In one example, montebrasite has been almost completely replaced by brazilianite which has also been found to contain not only crandallite but also its solid solution analogues: goyazite SrAl3(PO4)2(OH)5·H2O and gorceixite BaAl3(PO4)2(PO3OH)(OH)6. Apatite is common at the contacts with montebrasite and associated minerals and texturally is intimately intergrown with crandallite, goyazite and gorceixite at Okatjimukuju. The occurrence of these minerals offers insight into the chemistry of post-magmatic fluids in these pegmatites.

KEYWORDS: Namibia, exotic aluminium phosphates, lithium pegmatites, post-magmatic fluids




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