NEW Cu2+ COORDINATION POLYHEDRA IN THE CRYSTAL STRUCTURE OF BURNSITE, KCdCu7O2(SeO3)2Cl9

Abstract

The crystal structure of burnsite, KCdCu 7 O 2 (SeO 3) 2 Cl 9 , a new mineral species from fumaroles of the Tolbachik volcano, Kamchatka, Russia, has been solved by direct methods and refined on the basis of F 2 for all unique data collected with monochromatic MoK␣ X-radiation and a CCD (charge-coupled device) detector to an agreement factor (R1) of 1.9%, calculated using the 460 unique observed reflections (|F o | = 4 F). The mineral is hexagonal, space group P6 3 /mmc, a 8.7805(8), c 15.521(2) Å, V 1036.3(2) Å 3 , Z = 2. There are two non-equivalent Cu 2+ cations in the structure of burnsite. The Cu(1) cation is coordinated by three atoms of O and three of Cl; this is the first example of this type of mixed-ligand Cu 2+ 6 (: O 2-, Cl-) octahedron in a mineral. The octahedron is (2 + 4)-distorted owing to the Jahn-Teller effect, in contrast to the vast majority of Cu 2+ oxysalt minerals that contain (4 + 2)-distorted Cu 2+ octahedra. The Cu(2) cation is coordinated by two O and three Cl anions arranged at the vertices of a trigonal bipyramid; this is the first occurrence of a mixed-ligand Cu 2+ O 2 Cl 3 trigonal bipyramid in a mineral. The single symmetrically independent Cd 2+ cation is coordinated by six Cl anions located at the vertices of a regular octahedron. The structure contains one symmetrically independent K cation that is coordinated by six Cl anions in a trigonal prismatic arrangement. There is one unique Se 4+ cation that is strongly bonded to three O anions on one side of the cation, owing to the presence of a s 2 lone-electron pair. The structure is described both in terms of cation-centered polyhedra and oxocentered OCu 4 tetrahedra. The structure of burnsite is closely related to the structures of other natural copper oxide chloride selenites discovered in the Tolbachik fumaroles (chloromenite, georgbokiite, ilinskite).

Crystal Structures of Compounds Containing Ions Selenite

“…mol −1 . This energy range is comparable to that observed for weak hydrogen bonds [79] or other weakly bonded configurations, e.g., weak I-O bonds in α-HIO 3 [80], and anion-anion and cation-cation interactions [81,82]. Despite their weakness, these interactions provide additional stabilization of structural architectures.…”

Emulating exhalative chemistry: synthesis and structural characterization of ilinskite, Na[Cu5O2](SeO3)2Cl3, and its K-analogue

“…Of particular interest are copper oxoselenite chlorides, which, in addition to their mineralogical interest, possess rich and unique structural chemistry and special magnetic properties (Burns and Hawthorne 1995;Bastide et al 2000;Millet et al 2001;Burns et al 2002;Krivovichev et al 2006;Zhang et al 2010;Berdonosov et al 2013).…”

“…Most of naturally occurring copper selenite oxyhalide minerals contain Badditional^oxygen atoms that are coordinated solely by Cu 2+ cations, resulting in the formation of oxocentered (OCu 4 ) 6+ tetrahedra: allochalcoselite, Cu + [PbCu 2+ 5 O 2 ](SeO 3 ) 2 Cl 5 (Krivovichev et al 2006), burnsite, KCd[Cu 7 O 2 ](SeO 3 ) 2 Cl 9 (Burns et al 2002), chloromenite [Cu 9 O 2 ](SeO 3 ) 4 Cl 6 (Krivovichev et al 1998), francisite, [Cu 3 BiO 2 ](SeO 3 ) 2 Cl (Pring et al 1990), georgbokiite, α-[Cu 5 O 2 ](SeO 3 ) 2 Cl 2 (Krivovichev et al 1999a), nicksobolevite, [Cu 7 O 2 ](SeO 3 ) 2 Cl 6 (Vergasova et al 2014), parageorgbokiite β-[Cu 5 O 2 ](SeO 3 ) 2 Cl 2 , prewittite KPb 0.5 Cu[PbCu 5 O 2 ]Zn(SeO 3 ) 2 Cl 10 (Shuvalov et al 2013). It is the unique feature of Cu 2+ to form anion-centered units of different dimensionalities reviewed recently by Krivovichev et al (2013b).…”

Se–Cl Interactions in Selenite Chlorides: A Theoretical Study

“…However, it should be noted that the Equation (2) was originally proposed for hydrogen-bonded interactions and its applicability to closed-shell interactions is not fully justified. It is also of interest that the presence of the Se-Cl bonding interactions may at least in part explain the bond-valence deficiency frequently observed for Cl − anions in natural selenite chlorides [22,42,47,48]. Our conclusion about the closed-shell nature of the Se-Cl interactions is also supported by the fact that the total net charges on Se atoms (calculated for sofiite and georgbokiite) are in the range of +6.05-6.07, which indicates that there is no correlation between these values and the number of Se-Cl interactions per Se 4+ cation.…”

“…Burnsite, KCdCu 7 O 2 (SeO 3 ) 2 Cl 9 , was discovered in Tolbachik fumaroles in 2002 [18]. By analogy with Cu 5 O 2 (SeO 3 ) 2 Cl 2 polymorphs, its crystal structure contains additional O atoms tetrahedrally coordinated by four Cu atoms [48]. Two (OCu 4 ) tetrahedra share a common Cu atom to form a [O 2 Cu 7 ] dimer ( Figure 8a) similar to those recently observed in the crystal structure of Na 2 Cu 7 O 2 (SeO 3 ) 4 Cl 4 [72,73].…”