TRIVALENT IODINE IN THE CRYSTAL STRUCTURE OF SCHWARTZEMBERGITE, Pb2+5 I3+ O6 H2 Cl3

Abstract

The crystal structure of schwartzembergite from the San Rafael mine, Sierra Gorda, Chile, tetragonal, a 3.977(1), c 12.566(4) A, I 4/ mmm , has been refined to an R index of 1.8% based on 103 unique observed ( I > 5σ I ) reflections. Schwartzembergite is optically biaxial, but long-exposure precession photographs show no evidence of non-tetragonal symmetry. The tetragonal structure consists of interleaved PbO and Cl sheets stacked along the four-fold axis. The Pb and I sites are both coordinated by four O atoms arranged in a square to one side of the cation, with Pb–O = 2.407(3) A and I–O = 2.15(1) A. The Pb and I sites are only 0.54(4) A apart, and hence cannot both be occupied at the local scale. The Pb site is also coordinated by four Cl atoms at a distance of 3.331(3) A and arranged on the opposite side of the site to the O atoms; this very asymmetric coordination is characteristic of Pb 2+ exhibiting stereoactive lone-pair behavior. Bond-valence summation around the I site gives a value of 2.92 vu (valence units), indicating the iodine to be trivalent (rather than pentavalent, as has been assumed in previous studies); this is the first recorded occurrence of trivalent I in a mineral. Single-crystal polarized infrared and 1 H MAS NMR spectroscopies indicate the presence of structural H. Chemical analysis by electron microprobe and H-line extraction (1.20 wt.% H 2 O) gave the formula Pb 2+ 5 I 3+ O 6 H 2 Cl 3 . Electron diffraction showed numerous superlattice reflections, suggesting a C - or I -centered orthorhombic supercell with a = 2 a s , b = 6 b s , c = c s ( i.e. , an 8 × 24 A supercell), where the subscript s denotes the (pseudo-) tetragonal subcell. The observed substructure and local bond-valence requirements indicate that the (I 3+ O 4 ) groups cannot polymerize, and that I 3+ must occur on both sides of the oxygen layer in each (Pb,I)O sheet. These constraints result in five possible motifs for the superstructure; one of these sheets somewhat resembles the ordered sheet in nadorite, Pb 2+ Sb 3+ O 2 Cl, suggesting that this may be the pattern of order in schwartzembergite.