科研之友
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摘要
Despite many studies reporting the presence of S-bearing apatite in igneous and hydrothermal systems, the oxidation states and incorporation mechanisms of S in the apatite structure remain poorly understood. In this study, we use ab initio calculations to investigate the energetics and geometry of incorporation of S with its oxidation states S6+, S4+, and S2- into the apatite end-members fluor-, chlor-, and hydroxylapatite, [Ca-10(PO4)(6)(F,Cl,OH)(2)]. The relative stability of different oxidation states of S in apatite is evaluated by using balanced reaction equations where the apatite host and a solid S-bearing source phase (e.g., gypsum for S6+ and troilite for S2-) are the reactants, and the S-incorporated apatite and an anion sink phase are the products. Here, the reaction energy of the balanced equation indicates the stability of the modeled S-incorporated apatite relative to the host apatite, the source, and sink phases. For the incorporation of S into apatite, coupled substitutions are necessary to compensate for charge imbalance. One possible coupled substitution mechanism involves the replacement of La3+ + PO43- <-> Ca2+ + SO42-. Our results show that the incorporation of SO42- into La- and Na-bearing apatite, Ca8NaLa(PO4)(6)(F,Cl,OH)(2), is energetically favored over the incorporation into La- and Si-bearing apatite, Ca9La(PO4)(5)(SiO4)(F,Cl,OH)(2) (the difference in incorporation energy, Delta E-rxn, is 10.7 kJ/mol). This thermodynamic gain is partially attributed to the electrostatic contribution of Na+, and the energetic contribution of La3+ to the stability of SO42- incorporated into the apatite structure. Co-incorporation of SO42- and SO32- is energetically favored when the lone pair electrons of SO32- face toward the anion column site, compared to facing away from it. Full or partial incorporation of S2- is favored on the column anion site in the form of [Ca-10(PO4)(6)S] and [Ca-20(PO4)(12)SX2)], where X = F, Cl, or OH. Upon full incorporation (i.e., replacing all column ions by sulfide ions), S2- is positioned in the anion column at z = 0.5 (halfway between the mirror planes at z = 1/4 and z = 3/4) in the energy-optimized structure. The calculated energies for partial incorporation of S2- demonstrate that in an energy-optimized structure, S2- is displaced from the mirror plane at z = 1/4 or 3/4, by 1.0 to 1.6 angstrom, depending on the surrounding species (F-, Cl-, or OH-); however, the probability for S2- to be incorporated into the apatite structure is highest for chlorapatite end-members. Our results describe energetically feasible incorporation mechanisms for all three oxidations states of S (S6+, S4+, S2-) in apatite, along with structural distortion and concurring electronic structure changes. These observations are consistent with recently published experimental results (Konecke et al. 2017) that demonstrate S6+, S4+, and S2- incorporation into apatite, where the ratio of S6+/Sigma S in apatite is controlled by oxygen fugacity (f(O2)). The new computational results coupled with published experimental data provide the basis for using S in apatite as a geochemical proxy to trace variations in oxygen fugacity of magmatic and magmatic-hydrothermal systems.
- 出版日期2017-8
