A geochemical investigation of lithium minerals from four rare element pegmatites in the Damara Orogen, Namibia

Abstract

Abstract provided by author:

From laboratory and field evidence, a sequence of crystallisation was established in the pegmatites. The lithium minerals crystallised after the minerals in the outer parts of the pegmatites. Petalite crystallised first directly from the melt while montebrasite and lepidolite are secondary minerals, produced by late hydrothermal alteration

XRD provided mineral identification while atomic absorption was utilised to establish lithium concentrations. Montebrasite proved to be the most lithium rich mineral. EPMA confirmed the presence of montebrasite, with it's low fluorine levels, and showed it to contain veins of apatite (CaPO4) and probably crandallite (CaAlPO4). EPMA also established a solid solution series of the micas, from a dioctahedral low lithium muscovite to a trioctahedral high lithium lepidolite. Chemical trends in this series, with increasing Li, include, increasing F and Si, but decreasing Al

FTIR was able to distinguish the dioctahedral and trioctahedral micas by differing bands in the 550-850cm-1 region and by the OH band intensity. FTIR also resolved aspects of chemistry and structure of montebrasite and petalite. 7Li, 27A1 and 31P MAS-NMR studies were completed on the lithium minerals. Petalite was found to contain a single symmetric octahedral Li site and two tetrahedral Al sites. MAS-NMR on montebrasite produced a single broad asymmetric signal for 7Li, thought to arise from two merged octahedral Li site signals. A single sharp peak was produced for 27A1 NMR despite the apparent occurrence of two octahedral Al sites in montebrasite. Phosphorous NMR revealed three different P sites, for montebrasite, apatite and crandallite. The lithium micas were found to contain single asymmetric lithium sites and octahedral and tetrahedral Al sites. The tioctahedral and dioctahedral micas were distinguished by NMR, by a shift in the octahedral Al signal, from one structure to the other