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

1
Department of Crystallography, Institute of Earth Sciences, St. Petersburg State University, Universitetskaya Nab. 7/9, 199034 St. Petersburg, Russia
2
Nanomaterials Research Center, Kola Science Center of Russian Academy of Sciences, Fersman Str. 14, 184209 Apatity, Russia
3
Centre for Geo-Environmental Research and Modelling, Saint-Petersburg State University, Ulyanovskaya ul. 1, 198504 St. Petersburg, Russia
4
The Mining Museum, Saint Petersburg Mining University, 2, 21st Line, 199106 St. Petersburg, Russia
*
Author to whom correspondence should be addressed.
Crystals 2020, 10(6), 464; https://doi.org/10.3390/cryst10060464
Received: 1 May 2020 / Revised: 21 May 2020 / Accepted: 25 May 2020 / Published: 1 June 2020
(This article belongs to the Special Issue Mineralogical Crystallography)
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. View Full-Text
Keywords: stanfieldite; phosphate; crystal structure; merrillite; meteorite; pallasite; mesosiderite; luminophore; bioceramics; powder diffraction; Raman spectroscopy stanfieldite; phosphate; crystal structure; merrillite; meteorite; pallasite; mesosiderite; luminophore; bioceramics; powder diffraction; Raman spectroscopy
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Britvin, S.N.; Krzhizhanovskaya, M.G.; Bocharov, V.N.; Obolonskaya, E.V. Crystal Chemistry of Stanfieldite, Ca7M2Mg9(PO4)12 (M = Ca, Mg, Fe2+), a Structural Base of Ca3Mg3(PO4)4 Phosphors. Crystals 2020, 10, 464.

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