Bachelor of Science (Honours)
School of Earth, Atmospheric and Life Sciences
Weall, Tara, Causes of Colour Variation in Smithsonite from Tsumeb, Namibia, Bachelor of Science (Honours), School of Earth, Atmospheric and Life Sciences, University of Wollongong, 2023.
Smithsonite, ZnCO3, is an abundant, variably coloured secondary mineral in the Tsumeb Cu-Pb-Zn-Ag deposit in Namibia where it formed during oxidation of the sulfide ore. Previous studies suggest that atomic substitution for Zn and/or minute inclusions of other minerals are responsible for the wide range of colours shown by smithsonite. Fifty-two specimens of variously coloured smithsonites were analysed by EMPA and LA-ICP-MS techniques; they fall into two groups defined by Fe and Cu content. Group I (29 specimens) is Fe-rich (mean 0.0446 APFU) and Cu-poor (mean 0.0004 APFU) and includes most of the colourless, pink, red-pink, yellow and brown smithsonites. Group II (23 specimens) is Cu-rich (mean 0.0362 APFU) and Fe-poor (0.0001 APFU) and includes mainly green and blue-green smithsonites. Colourless smithsonites have low concentrations of chromophore elements such as Co, Cu and Mn. Pink smithsonites are coloured by Co and/or Mn; a Co content of ca 100 ppm (0.0002 APFU) appears to be sufficient to confer a subtle rose-pink hue in the absence of competing chromophores, while the Mn content required to develop a characteristic salmon-pink colour is an order of magnitude greater and possibly as high as ca. 1950 ppm (0.0045 APFU). Blue-green and green smithsonites are coloured principally by Cu, with a minimum content of ca 4000 ppm (0.0080 APFU) required for colour development or for suppression of colour produced by Co or Mn (if present). The blue-green colour appears to result from Cu when Mn is absent (or present in the 100 ppm range), while richer green colours may be caused by an interaction between Cu and Mn in the 1000+ ppm range. There is no evidence that the colour of pink, red-pink, green or blue-green smithsonite is caused by inclusions of other minerals and the chromophores Co, Cu and Mn are believed to be in solid solution, partly replacing Zn in the smithsonite lattice. Yellow and brown smithsonites, however, are coloured by minute inclusions of Fe oxides and hydroxides. Inclusions are visible under low (x40) magnification in some specimens but in other examples may be too small for resolution even with SEM/BSE imagery. None of the studied specimens contained CdS inclusions to which yellow and brown colouration has previously been attributed. Thirteen (25%) of the smithsonites studied fluoresce under either LW or SW ultraviolet radiation; eleven of them are Group I specimens (Fe-rich; Cu-poor). There is circumstantial evidence that Mn is the principal activator among these specimens and that Cu is generally a fluorescence quencher, but further work is needed to confirm this.
FoR codes (2020)
370505 Mineralogy and crystallography
Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.