THE SINGLE-CRYSTAL X-RAY STRUCTURES OF BARIOPHARMACOSIDERITE-C, BARIOPHARMACOSIDERITE-Q and NATROPHARMACOSIDERITE

Abstract

The crystal structures of two polymorphic forms of bariopharmacosiderite have been determined. Bariopharmacosiderite-C, from Robinson's Reef, Clunes, Victoria, Australia, (Ba 0.47 K 0.04 Na 0.02 )(Fe 3.97 Al 0.03 )[(As 0.72 P 0.28 )O 4 ] 3 (OH) 4 •2.52H 2 O, 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, Ba 0.5 Fe 4 (OH) 4 (AsO 4 ) 3 •6.16H 2 O, 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 H 2 O groups complete the coordination sphere, with longer bonds to Ba in the cubic polymorph. Additional H 2 O groups are hydrogen-bonded to each other and to the H 2 O groups that coordinate Ba. Some of these exhibit "zeolitic" behavior. By analogy to the properties of synthetic pharmacoalumite, KAl 4 (AsO 4 ) 3 (OH) 4 •nH 2 O, 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, (Na 0.75 K 0.14 Ba 0.11 ) S1.00 Fe 4 (AsO 4 ) 3 (OH) 3.89 O 0.11 •4H 2 O, 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 H 2 O groups and eight oxygen atoms of arsenate groups. A new general formula for "excess cation" pharmacosiderite is proposed, involving deprotonation of bridging hydroxide ions.

Mineralogy and origin of supergene mineralization at the Farbište ore occurrence near Poniky, central Slovakia

“…3) and its refined unit-cell parameters (Tab. 4) fit well with the data published by Hager et al (2010) Števko. c -Na-Ba-K (atomic ratios) ternary diagram for the Ba-dominant members of the pharmacosiderite group.…”

Bariopharmacoalumite, a new mineral species from Cap Garonne, France and Mina Grande, Chile

“…The tetragonal ''bariumzinc-alumopharmacosiderite'' from Cap Garonne can be considered bariopharmacoalumite-Qac using polytypic nomenclature (where the cubic polytypes are suffixed ÀCa). It is of note that both polytypes have also been reported for bariopharmacosiderite (Hager et al, 2010). The formula reported by Sarp et al (1994) for ''barium-zincalumopharmacosiderite'' is incorrect, as it is probable that Ba+Zn = 0.5 a.p.f.u., and that (OH) 4 4.…”

Ba–Cu ordering in bariopharmacoalumite-Q2a2b2c from Cap Garonne, France

“…The structure study began with an analysis of the subcell reflections corresponding to the tetragonal subcell with a = 7.702 Å and c = 7.776 Å . The starting model was that reported for bariopharmacosiderite-Q in space group P42m by Hager et al (2010). The model was refined to R 1 = 0.09 for 920 unique subcell reflections, using anisotropic thermal parameters.…”

“…According to a recent review of nomenclature for the minerals (Rumsey et al, 2010), pharmacos i d e r i t e , K [ F e 4 ( O H ) 4 ( A s O 4 ) 3 ] • 6 À7 H 2 O (Hausmann, 1813;Zemann, 1948) is used to define the mineral supergroup, with member groups including the pharmacosiderite, pharmacoalumite and ivanyukite groups, referring to the FeÀAs, AlÀAs and TiÀSi members. The group members are further distinguished by suffixes indicating the symmetry and periodicity of the unit cell (C for cubic, Q for tetragonal or T for trigonal; Rumsey et al, 2010;Hager et al, 2010;Yakovenchuk et al, 2009). The great flexibility of the structure to accommodate a large variety of substituting elements and valence states at all cation sites is indicated in published compositions for synthetic and ion-exchanged equivalents, which can be described by the general formula A 4Àx [B 4 O 4Ày (OH) y (TO 4 ) 3 ]•nH 2 O, with A = K + , Na + , Li + , Rb + , Cs + , H 3 O + , Tl + , NH 4 + , Ag + , Ba 2+ or Cu 2+ ; B = Fe 3+ , Al 3+ , Mo 3+ , Mo 4+ , Ti 4+ or Ge 4+ ; and T = As 5+ , P 5+ , Si 4+ , Ge 4+ or Al 3+ .…”

“…In all cases the distortion from cubic symmetry is very small. Crystal structure refinements have been conducted for examples of the trigonal (Yakovenchuk et al, 2009) and primitive tetragonal (Hager et al, 2010) polytypes, but no refinement of tetragonal mineral structures with doubled axes has been reported.…”