Sulfur-bearing monazite-(Ce) from the Eureka carbonatite, Namibia: oxidation state, substitution mechanism, and formation conditions.
BROOM-FENDLEY, Sam; SMITH, Martin P.; ANDRADE, Marcelo Barbosa de; RAY, Santanu; BANKS, David A.; LOYE, Edward; ATENCIO, Daniel; PICKLES, Jonathan R.; WALL, Frances.
BROOM-FENDLEY, Sam; SMITH, Martin P.; ANDRADE, Marcelo Barbosa de; RAY, Santanu; BANKS, David A.; LOYE, Edward; ATENCIO, Daniel; PICKLES, Jonathan R.; WALL, Frances.
Abstract: 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 data demonstrating that sulfur is accommodated predomin- antly in monazite-(Ce) as sulfate, via a clino-anhydrite-type coupled substitution mechanism. Minor sulfide and sulfite peaks in the X-ray photoelectron 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 sug- gested is accommodated by auxiliary substitution of OH? for O2? . However, Raman data show no indication of OH? , and instead we suggest charge imbalance is accommodated through F? substituting 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, Mo, Zr and V. In contrast to S-bearing monazite-(Ce) in other carbonatites, S-bearing monazite-(Ce) at Eureka formed via a dissolution?precipitation 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.
@article={002993735,author = {BROOM-FENDLEY, Sam; SMITH, Martin P.; ANDRADE, Marcelo Barbosa de; RAY, Santanu; BANKS, David A.; LOYE, Edward; ATENCIO, Daniel; PICKLES, Jonathan R.; WALL, Frances.},title={Sulfur-bearing monazite-(Ce) from the Eureka carbonatite, Namibia: oxidation state, substitution mechanism, and formation conditions},journal={Mineralogical Magazine},note={v. 84, n. 1, p. 35-48},year={2020}}