Geochemistry of a New Zealand nephrite weathering rind

Abstract

A dark-green nephrite boulder collected from Pleistocene glacial outwash gravels, West Coast of New Zealand, has a 6.1 cm thick weathering rind composed of an inner pale-green-white bleached zone (3.5 cm) and an outer brown oxidised zone (2.6 cm). The primary mineralogy of the boulder is monomineralic actinolite with small amounts of secondary Al-substituted goethite within the weathering-rind. Outwards from the unweathered nephrite core, SiO2, FeO, MgO, CaO, Na2O and density decrease, while Fe2O3, Al2O3, Cr, H2O+, H2O− and bulk oxidation ratio increase. The generalised weathering reaction can be characterised as: actinolite+H2O+H+=Fe-hydroxide+silicic acid+(Ca2+, Mg2+, Na+)aq. Assuming that alumina was immobile during weathering, isocon analysis indicates a mass decrease of −24% and a volume change of −26% in the bleached zone and a mass decrease of −52% and volume change of −58% in the oxidised zone with 7–84% depletion of all elements except Fe3+ and H2O− which indicates gains of 47–51% and 42–64%, respectively. Using H2O− values to estimate relative porosity, increasing Fe2O3, Al2O3, Cr and oxidation ratio and decreasing SiO2, FeO, CaO, MgO and rock density occur with increasing H2O−. Non-linear trends suggest that higher porosity created by actinolite dissolution results in an exponential increase in the amount of weathering (consistent with dissolution and secondary precipitation). An abrupt change in nephrite density at H2O+ values between ~2.38 and 2.55 wt % is probably related to incipient dissolution and cation loss as well as lower amounts of Fe-hydroxide formation. Depending on the deposition age of the gravels from which the nephrite boulder was derived, the weathering rind may be the result of 360–250 ka of weathering. The intensity of the green colouration of the nephrite reflects total iron content and oxidation ratio (mol.2Fe2O3×100/[2Fe2O3+FeO]). Combined with compositional data of other West Coast nephrites, a colour change from green or dark-green (unweathered) to pale-green-white (weathered) occurs where the oxidation ratio is >7; a change to brown colour occurs where oxidation ratios are >17.