Exotic aluminium phosphates, natromontebrasite, brazilianite, goyazite, gorceixite and crandallite from rare-element pegmatites in Namibia

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(PO (sub 4) )(OH,F) is reported from the Daheim pegmatite. In the Okatjimukuju pegmatite, montebrasite has been replaced by a number of phases including crandallite CaAl (sub 3) (PO (sub 4) ) (sub 2) (OH) (sub 5) .H (sub 2) O and brazilianite NaAl (sub 3) (PO (sub 4) ) (sub 2) (OH) (sub 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 SrAl (sub 3) (PO (sub 4) ) (sub 2) (OH) (sub 5) .H (sub 2) O and gorceixite BaAl (sub 3) (PO (sub 4) ) (sub 2) (PO (sub 3) OH)(OH) (sub 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.