Crystal Chemistry of Stanfieldite, Ca7M2Mg9(PO4)12 (M = Ca, Mg, Fe2+), a Structural Base of Ca3Mg3(PO4)4 Phosphors.

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 Ca_8.04Mg_9.25Fe_0.72Mn_0.07P_11.97O₄₈. 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)ų. The general formula of the mineral can be expressed as Ca₇M₂Mg₇(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 ≡ Ca₄Mg₅(PO₄)₆. The mineral from the Lunar sample "rusty rock" 66095 approaches the M = Mg end member, Ca₇Mg₂Mg₉(PO₄)₁₂. In lieu of any supporting analytical data, there is no evidence that the phosphor base with the formula Ca₃Mg₃(PO₄)₄ does exist. [ABSTRACT FROM AUTHOR]