The eosphorite-childrenite series associated with the Li-Mn-Fe phosphate minerals from the Buranga pegmatite, Rwanda

Abstract

SummaryFrom the Buranga pegmatite, Rwanda, three occurrences of eosphorite-childrenite series minerals are described in assemblages of Li-Fe-Mn-bearing phosphates and in relationship with quartz-feldspar masses in close association with these assemblages.Electron-microprobe analyses of these terms (Mn,Fe) AlPO4(OH)2 · H2O afford the following Fet/(Fet + Mn) ratios: 0.548, 0.341, and 0.200. However, thanks to an investigation of the refractive indices and pleochroism, the presence of ferric iron is demonstrated and an oxidation phenomenon, similar to that previously known in rockbridgeite, is suggested to explain the existence of oxidized eosphorite. The oxychildrenite problem is raised again.The Fet/(Fet + Mn) ratios characterizing the eosphorite sensu lato would be correlated with the Fe and Mn content of the associated major phosphates, the chemical analyses of which are also provided. The investigation of the structural relationships shows that eosphorite might form a paragenesis with montebrasite and that these phosphates replace Na- or K-feldspars by a hydrothermal process. Owing to the apparent dependence of the chemical properties of the couple eosphorite-montebrasite on those of the earlier-formed phosphates, it seems likely that the formation period of eosphorite-childrenite series minerals was prior to the oxidation and ferrisickleriteheterosite formation.