Clay minerals in the Namacotche Pegmatite Group from Zambezia Province, Mozambique: main constituents of late-stage secondary paragenesis

Abstract

AB ST R ACT : Namacotche gem-bearing pegmatites of Alto Ligonha pegmatite district are heterogeneous, strongly fractionated, and have large Li and Ta and extremely large Cs contents. Clay samples were collected in fracture infillings and dilation cavities with gemstones and were studied using X-ray diffraction (XRD), polarized light microscope, scanning electron microscopy-energy dispersive spectroscopy, high-resolution transmission electron microscopy and chemical analyses. The <2 mm fraction contains cookeite, illite, illite-smectite and suggested irregular mixed-layer cookeite-smectite, beidellite, montmorillonite, kaolinite and goethite. The XRD patterns of chlorite and their d values suggest the presence of 'di-trioctahedral chlorite' similar to cookeite-Ia polytype. Cookeite chemical analyses show that Li contents range from 0.82 to 1.08 atoms per half unit cell.A close relationship has been established between occurrences of gemstones and clay minerals. Some important textures and crystal chemistry are discussed. The main gemstones related to the Namacotche Pegmatite are: morganite (pink cesian beryl), kunzite (spodumene) and elbaite tourmaline. As the mechanisms responsible for the gemstone formation take place at low temperature, the clay minerals paragenesis cookeite Ô cookeite-smectite interstratification Ô beidellite + montmorillonite Ô illite-smectite interstratification, represents a late-stage secondary paragenesis, generated by hydrothermal alteration.

Three-Dimensional Textural and Chemical Characterization of Polyphase Inclusions in Spodumene Using a Dual Focused Ion Beam - Scanning Electron Microscope (Fib-Sem)

“…Numerous previous investigators of inclusions have reported the presence of moderate-to low-salinity aqueous carbonic fluid inclusions in both normal and highly evolved pegmatites (e.g., Channer et al 1999, Fuertes-Fuente et al 2000, Beurlen et al 2001, Sirbescu & Nabelek 2003, Thomas et al 2010. Such aqueous carbonic fluids are known to have reacted with lithium aluminosilicates to produce a low temperature (250-300°C) cookeite-bearing assemblage of alteration minerals (Černý 1972, Sequeira Braga et al 2008. In their study of the alteration of spodumene in aplite-pegmatite suites from northern Portugal, Bobos et al (2007) showed that surface dislocations, cleavages, and fractures in spodumene are sites of excess energy, where the mineral dissolves more readily.…”