Sulfur-bearing monazite-(Ce) from the Eureka carbonatite, Namibia: oxidation state, substitution mechanism, and formation conditions

Author(s)

Broom-Fendley, Sam ^{[1, 2]} ; Smith, Martin P. ^[3] ; Andrade, Marcelo B. ^[4] ; Ray, Santanu ^[3] ; Banks, David A. ^[5] ; Loye, Edward ^{[1, 2, 6]} ; Atencio, Daniel ^[7] ; Pickles, Jonathan R. ^{[1, 2]} ; Wall, Frances ^{[1, 2]}

Affiliation

^[1] Univ Exeter, Camborne Sch Mines, Penryn Campus, Penryn TR10 9FE, Cornwall - England

^[2] Univ Exeter, Environm & Sustainabii Inst, Penryn Campus, Penryn TR10 9FE, Cornwall - England

^[3] Univ Brighton, Sch Environm & Technol, Cockcroft Bldg, Lewes Rd, Brighton BN4 2GJ, E Sussex - England

^[4] Univ Sao Paulo, Sao Carlos Inst Phys, POB 369, BR-13560970 Sao Carlos, SP - Brazil

^[5] Univ Leeds, Sch Earth & Environm, Leeds LS2 9JT, W Yorkshire - England

^[6] E Tech Met Ltd, Bristol BS32 4JY, Avon - England

^[7] Univ Sao Paulo, Inst Geociencias, Dept Minerai & Geotecton, Rua Lago 562, BR-05508080 Sao Paulo, SP - Brazil

• Document type: Journal article
• Source: MINERALOGICAL MAGAZINE; v. 84, n. 1, p. 35-48, FEB 1 2020.
• Web of Science Citations: 1

Abstract

Sulfur-bearing monazite-(Ce) occurs in silicified carbonatite at Eureka, Namibia, forming rims up to similar to 0.5 mm thick on earlier-formed monazite-(Ce) megacrysts. We present X-ray photoelectron spectroscopy data demonstrating that sulfur is accommodated predominantly 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 suggested 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 O-2(-). 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. (AU)

• FAPESP's process: 14/50819-9 - SoS RARE: multidisciplinary research towards a secure and environmentally sustainable supply of critical rare earth elements (Nd and HREE)
• Grantee: Daniel Atencio
• Support Opportunities: Research Projects - Thematic Grants