Sulfur-bearing monazite-(Ce) occurs in silicified carbonatite at Eureka, Namibia,
forming rims up to ~0.5 mm thick on earlier-formed monazite-(Ce) megacrysts. We
present X-ray photoelectron spectroscopy (XPS) data demonstrating that sulfur is
predominantly accommodated in monazite-(Ce) as sulfate, via a clino-anhydrite-type
coupled ...
Sulfur-bearing monazite-(Ce) occurs in silicified carbonatite at Eureka, Namibia,
forming rims up to ~0.5 mm thick on earlier-formed monazite-(Ce) megacrysts. We
present X-ray photoelectron spectroscopy (XPS) data demonstrating that sulfur is
predominantly accommodated in monazite-(Ce) as sulfate, via a clino-anhydrite-type
coupled substitution mechanism. Minor sulfide and sulfite peaks in the XPS spectra,
however, also indicate that more complex substitution mechanisms incorporating S2-
and S4+ are possible. Incorporation of S6+ through clino-anhydrite-type substitution
results in an excess of M2+ cations, which previous workers have suggested is
accommodated by auxiliary substitution of OHfor O2-
. However, Raman data show
no indication of OH-
, and instead we suggest charge imbalance is accommodated
through Fsubstituting for O2-
. The accommodation of S in the monazite-(Ce) results
in considerable structural distortion that may account for relatively high contents of
ions with radii beyond those normally found in monazite-(Ce), such as the heavy rare
earth elements (REE), Mo, Zr and V. In contrast to S-bearing monazite-(Ce) in other
carbonatites, S-bearing monazite-(Ce) at Eureka formed via a dissolutionprecipitation mechanism during prolonged weathering, with S derived from an
aeolian source. While large S-bearing monazite-(Ce) grains are likely to be rare in
the geological record, formation of secondary S-bearing monazite-(Ce) in these
conditions may be a feasible mineral for dating palaeo-weathering horizons.
