The crystal structure of two possible new minerals within the apatite supergroup

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Marco E. Ciriotti
Messaggi: 24595
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
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The crystal structure of two possible new minerals within the apatite supergroup

Messaggio da Marco E. Ciriotti » mar 14 ago, 2018 14:06

Referenza:
▪ Biagioni, C., Hålenius, U., Pasero, M., Pieczka, A. (2018): The crystal structure of two possible new minerals within the apatite supergroup. IMA2018 Abstract book. Poster presentations. The ever-growing mineral diversity in the Earth and the Solar System: new minerals,
including nano- and biominerals, and related nomenclature/classification issues
, 526.

Abstract:
The framework topology of apatite displays a great flexibility, and is able to incorporate a wide combination of cations and anions. Despite its simple general formula, M12M23(TO4)3X, the apatite supergroup currently includes 40 mineral species, not to speak of hundreds of synthetic compounds with the apatite structure (e.g. White et al., 2005). The discovery of a new apatite-supergroup mineral may be related to subtle chemical differences along binary and ternary joins, e.g. to the dominant anion at the X site, for which the integration of single-crystal X-ray diffraction (SCXRD) and electron micro-probe analysis (EMPA) data may be crucial. We present herewith crystal structure data on two potentially new minerals within the apatite supergroup.
1. Hedyphane has ideal chemical formula Ca2Pb3(AsO4)3Cl. A natural phase with (OH) > Cl would represent a new mineral, i.e. the hydroxyl analogue of hedyphane. We got structural data on a specimen (from Sweden) which could meet this requirement. The refimenent of the site occupancies results in the following structural formula: (Ca1.44Pb0.56)Pb3[(As0.97P0.03)O4]3[(OH)0.52Cl0.48]. Unit cell parameters: a = 10.0581(6), c = 7.2865(5) Å. Refinement parameters: space group P-3, R1 = 0.0533, wR2 = 0.1293, 1252 unique reflections. Together with the acquisition of good-quality EMPA data, the accurate evaluation of occupancies at the split X sites along the channels of the structure does represent the key point for giving this phase the status of new mineral.
2. A mineral with ideal chemical formula Ca2Mn3(PO4)3(OH) would represent an unprecedented combination of dominant cations within the whole supergroup, and would deserve a new root name. We got structural data on a specimen (from Poland) which could meet this requirement. The refimenent of the site occupancies results in the following structural formula: (Ca1.25Mn0.74Na0.01)(Mn1.90Ca1.06Mg0.04)(PO4)3[(OH0.55Cl0.45], which closely matches the empirical chemical formula obtained through EMPA. Unit cell parameters: a = 9.4900(6), c = 6.4777(5) Å. Refinement parameters: space group P63/m, R1 = 0.0459, wR2 = 0.0946, 422 unique reflections. This should be the second apatite supergroup mineral, after pieczkaite, having manganese as a species-forming element. The possible existence of such new mineral was anticipated in Pasero et al. (2010). In that paper the ideal chemical formula was tentatively given as Mn2Ca3(PO4)3(OH), according to the general tendency of the M cation with larger ionic radius (in this case, Ca) to order at the M2 site. However, our structural information indicates an opposite situation, with Ca dominant at M1 and Mn dominant at M2, in agreement with Tait et al. (2015), who found a preferential partitioning of Mn at M2 in pieczkaite.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 24595
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

Re: The crystal structure of two possible new minerals within the apatite supergroup

Messaggio da Marco E. Ciriotti » mar 14 ago, 2018 14:53

Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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