1477 The Canadian Mineralogist Vol. 48, pp. 1477-1485 (2010) DOI : 10.3749/canmin.48.5.1477 THE SINGLE-CRYSTAL X-RAY STRUCTURES OF BARIOPHARMACOSIDERITE-C, BARIOPHARMACOSIDERITE-Q and NATROPHARMACOSIDERITE SIMON L. HAGER, PETER LEVERETT§ AND PETER A. WILLIAMS School of Natural Sciences, University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia STUART J. MILLS Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada DavID E. HIBBS School of Pharmacy, University of Sydney, Sydney, NSW 2006, Australia MATI RAUDSEPP Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada ANTHONY R. KAMPF Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, California 90007, USA WILLIAM D. BIRCH Geosciences Section, Museum Victoria, GPO Box 666, Melbourne, Victoria 3001, Australia ABSTRACT The crystal structures of two polymorphic forms of bariopharmacosiderite have been determined. Bariopharmacosiderite-C, from Robinson’s Reef, Clunes, Victoria, Australia, (Ba0.47K0.04Na0.02)(Fe3.97Al0.03)[(As0.72P0.28)O4]3(OH)4•2.52H2O, is cubic, space group P43m, a 7.942(1) Å, Z = 1, R = 0.089. Bariopharmacosiderite-Q, from the Sunny Corner mine, Sunny Corner, New South Wales, Australia, Ba0.5Fe4(OH)4(AsO4)3•6.16H2O, is tetragonal, space group P42m, a 7.947(1), c 8.049(2) Å, Z = 1, R = 0.050. In the cubic polymorph, Ba ions are disordered over all faces of the unit cell, whereas in the tetragonal polymorph, Ba ions are centered on the 001 face. In both cases, the Ba ions are 12-coordinate, with eight bonds to arsenate oxygen atoms; four H2O groups complete the coordination sphere, with longer bonds to Ba in the cubic polymorph. Additional H2O groups are hydrogen-bonded to each other and to the H2O groups that coordinate Ba. Some of these exhibit “zeolitic” behavior. By analogy to the properties of synthetic pharmacoalumite, KAl4(AsO4)3(OH)4•nH2O, the cubic polymorph appears to be more stable than the tetragonal one, although two further body-centered polymorphs are known. In addition, the crystal structure of natropharmacosiderite from the Gold Hill mine, Utah, has been determined. Natropharmacosiderite, (Na0.75K0.14Ba0.11)S1.00 Fe4(AsO4)3(OH)3.89O0.11•4H2O, is cubic, space group P43m, with a 7.928(9) Å, Z = 1 and R = 0.0654. The Na position is displaced by ~0.2 Å from the Wyckoff 3c site, and is coordinated by four H2O groups and eight oxygen atoms of arsenate groups. A new general formula for “excess cation” pharmacosiderite is proposed, involving deprotonation of bridging hydroxide ions. Keywords: bariopharmacosiderite, natropharmacosiderite, crystal structure. SOMMAIRE Nous documentons la structure cristalline de deux formes polymorphiques de la bariopharmacosidérite. La bariopharmacosidérite-C, provenant de Robinson’s Reef, Clunes, Victoria, en Australie, (Ba0.47K0.04Na0.02)(Fe3.97Al0.03) § E-mail address: [email protected] 1478 The caNADIAN MINeralOGIST [(As0.72P0.28)O4]3(OH)4•2.52H2O, est cubique, groupe spatial P43m, a 7.942(1) Å, Z = 1, R = 0.089. La bariopharmacosidérite-Q, provenant de la mine Sunny Corner, à Sunny Corner, Nouveau Pays de Galles, en Australie, Ba0.5Fe4(OH)4(AsO4)3•6.16H2O, est tétragonal, groupe spatial P42m, a 7.947(1), c 8.049(2) Å, Z = 1, R = 0.050. Dans le polymorphe cubique, les ions Ba sont désordonnés sur toutes les faces de la maille élémentaire, tandis que dans le polymorphe tétragonal, les ions Ba sont centrés sur la face 001. Dans les deux cas, les ions Ba possèdent une coordinence 12, dont huit liaisons aux atomes d’oxygène de groupes arsenate; quatre groupes H2O viennent compléter la sphère de coordinence, avec des liaisons plus longues au Ba dans le polymorphe cubique. Des groupes H2O additionnels sont rattachés par liaisons hydrogène l’un à l’autre et aux groupes H2O entourant le Ba. Certains de ceux-ci font preuve d’un comportement “zéolitique”. Par analogie avec les propriétés de la pharmacoalumite synthétique, KAl4(AsO4)3(OH)4•nH2O, le polymorphe cubique semble être plus stable que le polymorphe tétragonal, quoique l’on connaisse deux autres formes polymorphiques à maille centrée. De plus, nous décrivons la structure cristalline de la natropharmacosidérite provenant de la mine Gold Hill, au Utah. La natropharmacosidérite, (Na0.75K0.14Ba0.11)S1.00 Fe4(AsO4)3(OH)3.89O0.11•4H2O, est cubique, groupe spatial P43m, avec a 7.928(9) Å, Z = 1 et R = 0.0654. La position Na est déplacée d’environ 0.2 Å de la position Wyckoff 3c, et l’atome est coordonné à quatre groupes H2O et huit atomes d’oxygène des groupes arsenate. Nous proposons une nouvelle formule générale pour les membres du groupe de la pharmacosidérite ayant un excédent de cations; cette formule implique une déprotonation des ponts d’hydroxyle. (Traduit par la Rédaction) Mots-clés: bariopharmacosidérite, natropharmacosidérite, structure cristalline. INTRODUCTION tetragonal, P42m and I42m, with a ≈ c ≈ 8 and 16 Å, respectively. We note that bariopharmacosiderite, as Pharmacosiderite, KFe4(AsO4)3(OH)4•nH2O, n = originally described, is tetragonal, a = 7.97, c = 8.10 Å; 5–6 (sensu stricto), has been known for more than the optical character is uniaxial negative with = 1.718 200 years, but until recently, several of its structural and v = 1.728 (Walenta 1966). Whereas single-crystal peculiarities have remained to be elucidated. It was X-ray structures for a number of synthetic AlP, MoP, originally proposed to be cubic, although anomalous TiSi, TiGe, (TiNb)Si and GeGe analogues of (FeAs) optical properties have been noted for many samples pharmacosiderite have been reported (Xu et al. 2004, of the mineral (Palache et al. 1951) and its congeners. and references therein), no satisfactory structure of any In line with early observations that it could be cation- pharmacosiderite-group minerals has appeared in the exchanged (Hartley 1899), bariopharmacosiderite literature, save for that of hydroniumpharmacosiderite. (Walenta 1966), natropharmacosiderite [renamed by However, the structure of a synthetic cubic phase with Burke (2008); originally described as sodium pharma- the pharmacosiderite structure, {[Rb1.94(H2O,OH)3.84] cosiderite; Peacor & Dunn (1985)], and hydronium- (H2)0.1}{Al4(OH)4[PO4]3}, was recently described pharmacosiderite (Mills et al. 2010a, 2010b) have since (Yakubovich et al. 2008). Here we report the structures been described. In addition, an Al-analogue of pharma- of cubic natropharmacosiderite, space group P43m, cosiderite, pharmacoalumite, is known (Schmetzer et bariopharmacosiderite-C and bariopharmacosiderite- al. 1981; for nomenclature, see Rumsey et al. 2010), as Q, space groups P43m and P42m, respectively. The are the Al-analogue of bariopharmacosiderite (Walenta suffixes -C and -Q refer to cubic and tetragonal poly- 1966) and natropharmacoalumite (Rumsey et al. 2010). morphs, respectively. Zemann (1947, 1948) reported the first structure of cubic pharmacosiderite, space group P43m; whereas EXPERIMENTAL the study established the overall framework of the structure, the K atoms could not be located. Similarly, Pale yellow crystals of bariopharmacosiderite from Buerger et al. (1967) reported a single-crystal structural the Sunny Corner mine, Sunny Corner, New South study confirming the basic lattice structure, but noted Wales, Australia, were analyzed (SEM, EDS, stan- that the structure in space group P43m is averaged, dardless) and gave Ba:Fe:As proportions very close and the K atoms could not be found. Furthermore, a to the ideal stoichiometry, 0.5:4:3. Only traces of Al spectrographic analysis failed to reveal the presence and other cations (Na, K) were detected. The crystals of any alkali metals; it may be that the crystal studied changed from pale yellow to green when left exposed was in fact the hydronium analogue, known previ- to the atmosphere for two weeks, and the pale yellow ously from ion-exchange studies (Hartley 1899, Heide color was restored upon heating at 100°C for 24 hours. 1928, Mutter et al. 1984). The careful study by Mutter Immersion in water for five minutes caused them to et al. (1984) did resolve ambiguities associated with become green again, indicating that at least part of reported anomalous optical properties and showed that the water of crystallization is “zeolitic” in nature. pharmacosiderite and its congeners could crystallize Pale orange-brown, optically isotropic crystals of in four related space groups; two are cubic, P43m and bariopharmacosiderite from Robinson’s Reef, Clunes, I43m, with a ≈ 8 and 16 Å, respectively, and two are Victoria, Australia (Museum Victoria specimen number barIOpharMacOSIDerITE, NATROpharMacOSIDerITE 1479 M7966) were analyzed (n = 5) with an electron micro- subsequent difference-Fourier syntheses, starting with probe (EMP) (WDS mode, 15 kV, 20 nA, 5 mm beam the coordinates of Zemann (1947, 1948) and Buerger et diameter). We used as standards albite (Na), synthetic al. (1967) and refined onF 2 by full-matrix least-squares KTaO3 (K), anandite (Ba), hematite (Fe), corundum methods using SHELXL (Sheldrick 2008). (Al), fluorapatite (P) and arsenopyrite (As). Analytical The unit cell of bariopharmacosiderite-Q is pseudo- results [average, (range, SD)] were Na 0.07 (0.01–0.26, cubic; we expected to find the Fe, As and O atoms 0.10), K 0.18 (0.05–0.54, 0.18), Ba 8.55 (7.96–10.0, in similar positions to those in the cubic pharmaco- 0.75), Fe 26.16 (24.58–27.02, 1.01), Al 0.08 (0.07–0.09, siderite-type structure of Buerger et al. (1967). This 0.02), As 19.96 (19.55–20.39, 0.34), P 3.21 (2.80–3.60, was confirmed, and the model refined isotropically to 0.27), total 85.39 wt%. Insufficient material was avail- an R1 value of 0.21.