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 Ca8.04Mg9.25Fe0.72Mn0.07P11.97O48. 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)Å3. 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.
Highlights
It is known that the speciation of chemical elements in meteoritic substance significantly differs from their speciation in contemporary terrestrial lithosphere [1]
Concerning phosphorus, the main geochemical factors governing the diversity of terrestrial phosphorus-bearing minerals are (1) highly oxidative conditions typical of the present Earth and (2) the aquatic environment, which dramatically multiplies the number of possible pathways for phosphate geosynthesis
A series of Mg-rich phosphates are characteristic of stony-iron meteorites—pallasites and mesosiderites [6,7]
Summary
It is known that the speciation of chemical elements in meteoritic substance significantly differs from their speciation in contemporary terrestrial lithosphere [1]. The most common meteoritic phosphates are the minerals related to the join merrillite–ferromerrillite, Ca9 NaMg(PO4 )7 –Ca9 NaFe2+ (PO4 )7 [3,4]. They are the typical accessories of ordinary chondrites, lunar rocks, martian meteorites, iron and stony-iron meteorites [3,4,5,6]. Ca7 Mg9 (Ca,Mg) (PO4 ) was shown to have the same cell metrics as natural stanfieldite but it crystallizes in a different space group [9,40]. We present the results and try to resolve some ambiguities related to a crystal chemistry of natural stanfieldite and its synthetic analogues
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