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

¹ CIG-R, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal and² CGS-UMR 7517, EOST, 1 rue Blessig, 67084 Strasbourg Cedex, France

^* E-mail: masbraga{at}dct.uminho.pt

(Received 15 December 2007; revised 27 August 2008)

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 µm 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 ± cookeite-smectite interstratification ± beidellite + montmorillonite ± illite-smectite interstratification, represents a late-stage secondary paragenesis, generated by hydrothermal alteration.

KEYWORDS: cookeite, cookeite-smectite interstratification, beidellite, montmorillonite, hydrothermal alteration, gem-bearing pegmatites, Mozambique