Aravaite, Ba2ca18(Sio4)6(PO4)3(CO3)F3O: Modular Structure and Disorder of a New Mineral with Single and Triple Antiperovskite Layers ISSN 2052-5206
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mineralogical crystallography Aravaite, Ba2Ca18(SiO4)6(PO4)3(CO3)F3O: modular structure and disorder of a new mineral with single and triple antiperovskite layers ISSN 2052-5206 Biljana Kru¨ger,a* Hannes Kru¨ger,a Evgeny V. Galuskin,b Irina O. Galuskina,b Yevgeny Vapnik,c Vincent Oliericd and Anuschka Pauluhnd Received 2 June 2018 aInstitute of Mineralogy and Petrography, University of Innsbruck, Innrain 52, Innsbruck, 6020, Austria, bDepartment of Accepted 29 August 2018 Geochemistry, Mineralogy and Petrography, University of Silesia, Be˛dzin´ska 60, Sosnowiec, 41-200, Poland, cDepartment of Geological and Environmental Sciences, Ben-Gurion University of the Negev, PO Box 653, Beer-Sheva, 84105, Israel, and dPaul Scherrer Institute, Swiss Light Source, Villigen, 5232, Switzerland. *Correspondence e-mail: Edited by J. Lipkowski, Polish Academy of [email protected] Sciences, Poland The crystal structure of the new mineral aravaite Ba2Ca18- Keywords: modular structure; antiperovskite ˚ layers; aravaite; disorder; diffuse scattering. (SiO4)6(PO4)3(CO3)F3O[R3m, a = 7.12550 (11), c = 66.2902 (13) A, V = 2914.81 (8) A˚ 3, Z = 3] was solved from single-crystal diffraction data, collected CCDC reference: 1864463 using synchrotron radiation at the X06DA beamline of the Swiss Light Source. The unit cell of this modular mineral contains six layers of Supporting information: this article has 3.5À {Ba(PO4)1.5(CO3)0.5} (T-layer), three triple antiperovskite layers (tAP) supporting information at journals.iucr.org/b 4+ {(F2OCa12)(SiO4)4} , and three single antiperovskite layers (sAP) 3+ {(FCa6)(SiO4)2} . The structure refinement confirms a model with a layer sequence of T–sAP–T–tAP as an average structure of this mineral. However, one-dimensional diffuse scattering observed parallel to c* implies imperfections in the stacking sequence of the average structure. Qualitative modelling of disorder confirms that the alternating sequence of T–sAP and T–tAP blocks is disturbed. The blocks occurring in this new mineral are known from other so- called hexagonal intercalated antiperovskite structures: T–sAP (stracherite and zadovite group), T–tAP (ariegilatite and nabimusaite group). 1. Introduction Hexagonal antiperovskites are inorganic compounds built from units of anion-centred octahedra. Their structure can be considered as polytypic variations of the cubic perovskite structure, for example, as structures corresponding to the 2H, 4H and 9R polytypes reported by Ga¨bler & Niewa (2007). Many minerals can be described using antiperovskite units composed of face- and corner-sharing octahedra (Krivovichev, 2007, 2009). As they are not exclusively built from corner- sharing octahedra they are not considered as members of the perovskite supergroup (Mitchell et al., 2017). Additionally, some minerals show antiperovskite modules, which are inter- calated with intermediate ions and complexes (Krivovichev, 2007). The diversity of modular structures is well represented in the intercalated (or broken) antiperovskite minerals of the arctite supergroup (Galuskin et al., 2018a). This supergroup combines silicate minerals of the nabimusaite group (Galuskin et al., 2015a, 2018c; Galuskina et al., 2018) and zadovite group (Galuskin et al., 2015b, 2017, 2018b). The entire supergroup is characterized by modular structures containing single anti- perovskite (sAP) layers of composition {[WB6](TO4)2}, triple antiperovskite (tAP) layers {[W3B12](TO4)4}, and tetrahedral (T) layers A(TO4)2, where A = Ba, K, Sr ...; B = Ca, Na ...; 5+ 6+ 2À À # 2018 International Union of Crystallography T = Si, P, V ,S ,Al...; W =O ,F . The aforementioned 492 https://doi.org/10.1107/S2052520618012271 Acta Cryst. (2018). B74, 492–501 mineralogical crystallography layers are shown in Fig. 1. Minerals of the zadovite group contain sAP layers, whereas tAP layers are observed in minerals of the nabi- musaite group. The units addressed as layers above may also be denoted as modules using the modular clas- sification (Ferraris et al., 2004). Taking the chemical composition into account, we can describe the structure of the new intercalated antiperovskite mineral with alter- nating blocks of ariegilatite BaCa12- (SiO4)4(PO4)2F2O (Galuskin et al., 2018c) and stracherite BaCa6- (SiO4)2[(PO4)(CO3)]F (Galuskin et al., 2018b). These two minerals belong to the nabimusaite and zadovite groups, respectively. All of the mentioned structures show R- centred trigonal unit cells. Nabimu- saite-group minerals with a sequence of T–tAP (3Â)havea lattice parameter c of roughly 41 A˚ . The sequence T–sAP (3Â) as found in zadovite-group minerals sums up to c ’ 26 A˚ . The larger c parameter of circa 66 A˚ observed in aravaite is the sum of the sequence T–sAP–T– tAP (3Â). Figure 1 2. Occurrence The structure of aravaite consisting of blocks of ariegilatite and stracherite. The structure is formed by 3.5À Aravaite (IMA 2018-078) was found ordered intercalation of T-modules = {Ba(PO4)1.5(CO3)0.5} with antiperovskite layers of two types: 4+ 3+ tAP (triple antiperovskite module) = {(F2OCa12)(SiO4)4} and sAP = {(FCa6)(SiO4)2} (single in pyrometamorphic spurrite rocks antiperovskite module) in the sequence of T–sAP–T–tAP– ... of the Hatrurim Complex in the Negev Desert (311305800 N, 35160200 E) near Arad, Israel. These pyro- metamorphic rocks are represented by spurrite marbles, larnite pseudo- conglomerates, gehlenite hornfelses and different paralavas, occurring widely along the rift zone of the Dead Sea (Bentor, 1960; Gross, 1977; Vapnik et al., 2007; Sokol et al., 2008; Galuskina et al., 2014). Asso- ciated minerals are spurrite, calcite, brownmillerite, shulamitite, CO3- bearing fluorapatite, brucite, fluor- mayenite–fluorkyuygenite, peri- clase, barytocalcite and baryte. Moreover, in the same rock samples, Figure 2 two other new minerals with anti- SEM micrographs (backscattered electron mode). (a) Aravaite intergrown with stracherite. A fragment perovskite modules were found: of this crystal was used for the structural study. This is a rare example of spurrite marble, in which stracherite, BaCa (SiO ) [(PO )- ariegilatite is absent. (b) Epitaxial intergrowth of three intercalated antiperovskites: aravaite, 6 4 2 4 ariegilatite and stracherite. Arv is aravaite, Ari is ariegilatite, HSi is undiagnosed hydrosilicates, May (CO3)]F, a mineral with single anti- is fluormayenite–fluorkyuygenite, Shl is shulamitite, Spu is spurrite, Sta is stracherite. perovskite (sAP) layers (Galuskin et Acta Cryst. (2018). B74, 492–501 Biljana Kru¨ger et al. Aravaite, Ba2Ca18(SiO4)6(PO4)3(CO3)F3O 493 mineralogical crystallography Table 1 Chemical composition as averaged from 41 (aravaite), 42 (stracherite) and eight (ariegilatite) spot measurements. Note, that T1 and T2 denote the total of tetrahedral sites in the antiperovskite- and in the T-layers, respectively. Aravaite Stracherite Ariegilatite Wt% Average E.s.d. Range Average E.s.d. Range Average E.s.d. Range SO3 0.98 0.17 0.57–1.18 1.93 0.10 1.69–2.14 0.26 0.09 0.13–0.38 V2O5 0.21 0.09 0.06–0.43 n.d. P2O5 9.37 0.39 8.59–10.29 11.15 0.37 10.01–11.72 9.13 0.56 8.18–9.76 TiO2 0.13 0.05 0.06–0.26 n.d. 0.23 0.05 0.14–0.31 SiO2 18.61 0.29 18.07–19.22 15.14 0.15 14.89–15.45 19.55 0.35 18.99–19.98 Al2O3 0.10 0.03 0.05–0.18 0.09 0.02 0.04–0.13 0.14 0.03 0.09–0.19 BaO 14.63 0.45 13.58–15.60 19.84 0.23 19.19–20.23 12.67 0.14 12.40–12.92 MnO 0.17 0.08 0–0.34 n.d. 0.18 0.07 0.08–0.31 CaO 50.88 0.59 49.65–52.43 45.46 0.17 45.07–45.74 54.23 0.43 53.80–55.03 MgO 0.10 0.08 0.04–0.56 n.d. K2O 0.04 0.01 0–0.07 0.12 0.02 0.08–0.17 Na2O 0.46 0.12 0.18–0.93 0.05 0.02 0–0.11 0.25 0.08 0.17–0.45 CO2 2.30 4.60 1.15 F 3.30 0.13 3.00–3.62 2.46 0.09 2.20–2.63 2.89 0.08 3.20–3.47 —O F 1.39 1.04 1.41 Total 99.90 99.80 99.48 Ba 1.86 0.96 1.01 K 0.02 0.02 Na 0.12 0.01 A 2.00 1.99 1.01 Ca 17.73 6.01 11.86 Na 0.17 0.10 Mn2+ 0.05 0.03 Mg 0.05 B 18.00 6.01 12.99 4À (SiO4) 5.93 1.87 3.92 5À (AlO4) 0.04 0.01 0.03 4À (TiO4) 0.03 0.04 3À (PO4) 0.12 0.01 T1 6.00 2.00 4.00 3À (PO4) 2.58 1.04 1.57 2À (CO3) 1.02 0.78 0.32 2À (SO4) 0.24 0.18 0.04 4À (SiO4) 0.12 0.07 3À (VO4) 0.04 T2 4.00 2.00 2.00 F 3.23 0.96 1.87 O 0.77 0.04 1.16 W 4.00 1.00 3.03 The empirical formulas are calculated on the basis of 30 (aravaite), 11 (stracherite) and 19 (ariegilatite) cations pfu and CO2 by charge balance: (1) (Ba1.86K0.02Na0.12)Æ2- 2+ (Ca17.73Na0.17Mg0.05Mn 0.05)Æ18{(SiO4)6.05(PO4)2.58(CO3)1.02(SO4)0.24(AlO4)0.04(TiO4)0.03(VO4)0.04}Æ10(F3.23O0.77)Æ4, (2) (Ba0.96K0.02Na0.01)Æ0.99Ca6.01{(SiO4)1.87(PO4)0.12(AlO4)0.01}Æ2- 2+ {(PO4)1.04(CO3)0.78(SO4)0.18}Æ2(F0.96O0.04)Æ1 and (3) Ba1.01(Ca11.86Na0.10Mn 0.03)Æ11.99{(SiO4)3.92(PO4)0.01(AlO4)0.03(TiO4)0.04}Æ4{(PO4)1.57(CO3)0.32(SO4)0.04V0.07}Æ2(F1.87O1.16)Æ3.03.