An ab-initio study of the energetics and geometry of sulfide, sulfite, and sulfate incorporation into apatite: The thermodynamic basis for using this system as an oxybarometer

Sulfur in the Apollo Lunar Basalts and Implications for Future Sample-Return Missions

“…Both observations confirmed that lunar rocks are very reduced, consistent with the presence of Fe metal in the same rocks. This is exciting, because it is the first measurement of a natural sulfide-bearing apatite, and confirms recent hypotheses about sulfide substitution into apatite made on the basis of ab initio computations and experimental work (Kim et al 2017;Konecke et al 2017a). Those hypotheses suggest that, if possible, sulfide substitutes for OH − , Cl − , and/or F − on the column site of the apatite mineral structure.…”

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

“…Recent micro X-ray absorption near-edge structure analyses of both laboratory-synthesised and natural samples have indicated that sulfide and sulfite can occur in apatite (Konecke et al , 2017 a , 2017 b ; Brounce et al , 2019; Sadove et al , 2019), which helps to support the concept that these species may occur in monazite-(Ce) as well. Ab initio studies of apatite support the mechanism for coupled substitution of S 6+ and S 4+ for P 5+ (Kim et al , 2017). Sulfite occurs as a trigonal pyramid with a lone electron pair replacing one of the O atoms in the substituted PO 4 (or SO 4 ).…”

“…The stability of sulfite substitution in apatite is affected by the presence of the anion column, which repels the lone pair of electrons in the sulfite trigonal pyramid. The highest stability occurs in chlorapatite owing to increased distance between the Cl – anion and the electron pair, relative to OH – and F – (Kim et al , 2017). The absence of the anion column in monazite-group minerals therefore suggests that sulfite substitution may be more stable than in apatite.…”

“…The absence of the anion column in monazite-group minerals therefore suggests that sulfite substitution may be more stable than in apatite. Conversely, however, S 2– is accommodated in the anion site in apatite (Kim et al , 2017), and no such site is available in monazite-(Ce). Substituting S 2– for O 2– is difficult to reconcile with the monazite-(Ce) structure owing to the considerable size difference between these two anions.…”

“…However, experimental and ab initio studies have shown that apatite, which has been assumed typically to accommodate sulfur as S 6+ only (e.g. Pan and Fleet, 2002), is also capable of accommodating more reduced varieties of S such as S 4+ and S 2- (Konecke et al, 2017a;Kim et al, 2017). There has been no direct investigation into the redox state of S in monazite-(Ce) to investigate its capability to accommodate S in different oxidation states.…”