Crystal chemistry of stanfieldite, Ca7 M2 Mg9 (PO4)12 (M = Ca, Mg, Fe2+), a structural base of Ca3 Mg3 (Po4)4 phosphors

Abstract

Stanfieldite, natural Ca-Mg-phosphate, is a typical constituent of phosphate-phosphide assemblages in pallasite and mesosiderite meteorites. The synthetic analogue of stanfieldite is used as a crystal matrix of luminophores and frequently encountered in phosphate bioceramics. However, the crystal structure of natural stanfieldite has never been reported in detail, and the data available so far relate to its synthetic counterpart. We herein provide the results of a study of stanfieldite from the Brahin meteorite (main group pallasite). The empirical formula of the mineral is Ca8.04 Mg9.25 Fe0.72 Mn0.07 P11.97 O48. Its crystal structure has been solved and refined to R1 = 0.034. Stanfieldite from Brahin is monoclinic, C2/c, a 22.7973(4), b 9.9833(2), c 17.0522(3) Å, β 99.954(2)◦, V 3822.5(1)Å3. The general formula of the mineral can be expressed as Ca7 M2 Mg7 (PO4)12 (Z = 4), where the M = Ca, Mg, Fe2+. Stanfieldite from Brahin and a majority of other meteorites correspond to a composition with an intermediate Ca≈Mg occupancy of the M5A site, leading to the overall formula ~Ca7 (CaMg)Mg9 (PO4)12 ≡ Ca4 Mg5 (PO4)6. The mineral from the Lunar sample “rusty rock” 66095 approaches the M = Mg end member, Ca7 Mg2 Mg9 (PO4)12. In lieu of any supporting analytical data, there is no evidence that the phosphor base with the formula Ca3 Mg3 (PO4)4 does exist.