The phase diversity of nickel phyllosilicates from New Caledonia : an investigation using transmission electron microscopy (TEM)

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Authors
Cymes, Brittany A.
Advisor
Nicholson, Kirsten N.
Issue Date
2016-05-07
Keyword
Degree
Thesis (M.S.)
Department
Department of Geological Sciences
Other Identifiers
Abstract

New Caledonia is a French territorial island in the Southwest Pacific with an economy heavily dependent upon nickel-mining, being the 5th largest producer worldwide. The nickel deposits result from tropical lateritic weathering of ophiolite units that were emplaced during the late Eocene. The ultramafic units weathered and continue to weather into bright green Ni-rich phyllosilicates colloquially referred to as ‘garnierite’. Detailed investigations of ‘garnierite’ using transmission electron microscopy (TEM) have been carried out in other locations to characterize important nanoscale features; however, none have been undertaken in New Caledonia. In this investigation, ten samples of ‘garnierite’ from New Caledonia were examined with powder X-ray diffraction (XRD) and TEM with energy dispersive spectrometry (EDS). The XRD results show the phyllosilicate material to be comprised of highly disordered talc- and serpentine-like minerals with minor chlorite indicated by significantly broad reflections and two- dimensional diffraction bands, consistent with published data. TEM analysis, however, revealed new information. Talc-like minerals occur as fluffy aggregates of crystals with lattices with approximately 10 Å d-spacing and significant crystallographic disorder and also as aggregates of larger crystals with more orderly lattices; these two varieties represent the kerolite-pimelite series and the talc-willemseite series, respectively. Lizardite, chrysotile, and antigorite, with approximately 7 Å basal spacing, were observed in approximately equal measure among the samples. Chlorite occurs as crystals with lattices having approximately 14 Å basal spacing with both high amounts of crystallographic order, indicated by continuous lattices, and as crystals with highly disordered lattices, as observed in the kerolite-pimelite series. Large percentages of silver nanoparticles occur within matrices of amorphous silica within two of the samples. A bimodal size distribution is observed among the nanoparticles, with particles 2 to 30 nm in diameter comprising 75 % of the textures and particles 150 to 650 nm in diameter comprising 25% of the textures. These findings have several broad implications; (1) the antigorite observed indicates that the ophiolite unit experienced high-temperature alteration at one point in time, which is in disagreement with accepted supergene interpretation of its tectonic history; (2) the high relative chrysotile percentage and the fiber size (50-700 nm) may relate to the high incidence of lung disease among island civilians; and (3) the silver nanoparticles may have significant implications for both the economy of New Caledonia, being of great potential value, and the environment, being very toxic to aquatic and terrestrial flora and fauna and also humans.

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