P. K. Sen Gupta

Joesmithite, a plumbous amphibole revisited and comments on bond valences

SummaryJoesmithite is a complex clinoamphibole which contains at least 14 components in its formula. Its ideal formula would be PbCa2 Mg3 Fe23+ (Si6Be2O22) (OH)2. It is monoclinic holosymmetric, an ordered derivative of theC2/m clinoamphiboles,a = 9.915(2),b = 17.951(4),c = 5.243(1)Å,β = 105.95(2)°, Z = 2, space groupP2/a, D(obs) = 3.83(1), D(cal) = 3.91 g cm-3. R = 0.056 for 2299 independent F0.The cells of joesmithite and pargasitic amphibole were compared according to differences, Δ, in their structure cells. The same procedure was applied to the margarosanite, PbCa2Si3O9 and walstromite, BaCa2Si3O9 pairs. In both pairs, the anion positions differed by 0.2 Å at most, but the Pb/A, Pb/Ba differences ranged from 0.4 to 0.6 Å. The lone pair Pb2+ cation responds according to the bond valence sums of its coordinating anions compared with its closed core counterpart.It appears that when the bond valence sums for the coordinating anions are different, Pb2+ moves toward the most underbonded anion. If the anion bond valence sums are equal for all vertices of the coordinating polyhedron (as in wulfenite, Pb2+Mo6+O4), then Pb2+ does not shift.ZusammenfassungJoesmithit ist ein komplex zusammengesetzter Klinoamphibol, der rumindest 14 Komponenten in seiner Formel aufweist. Die idealisierte Formel lautet PbCa2Mg3Fe23+ (Si6Be2O22)(OH)2. Er ist monoklin holoedrisch und von den geordnetenC2/m-Klinopyroxenen abgeleitet,a = 9,915(2),b = 17,951(4),c = 5,243(1) Å,ß = 105,95(2)°, Z = 2, RaumgruppeP2/a, D(beob) = 3,83(1), D(ber) = 3,91 g cm-3. R = 0,056 für 2299 unabhängige F0.Die Elementarzellen von Joesmithit und einem pargasitischen Amphibol wurden in Bezug auf deren Struktur verglichen, ebenso jene von Margarosanit, PbCa2Si3O9, und Walstromit, BaCa2Si3O9, In beiden Paaren differieren die Positionen der Anionen um maximal 0,2 Å, die Unterschiede Pb/A = (Na, K, □) und Pb/Ba betragen 0,4 und 0,6 Å. Das ein einsames Elektronenpaar aufweisende Kation Pb2+ folgt den Bindungsstärken der koordinierenden Anionen im Vergleich zu den Pendents mit abgeschlossener Elektronenschale.Differieren die Bindungsstärken der koordinierenden Anionen, bewegt sich Pb2+ offensichtlich auf das am schwächsten abgesättigte Anion zu. Falls die Summe der Bindungsstärken an den Anionen in allen Richtungen des Koordinationspolyeders gleich sind (wie im Wulfenit, Pb2+Mo6+O4), dann kommt es zu keiner Verschiebung des Pb-Atoms.

The crystal structure of philolithite, a trellis-like open framework based on cubic closest-packing of anions

Abstract The crystal structure of philolithite, Pb12O6Mn(Mg,Mn)2(Mn,Mg)4(SO4)(CO3)4Cl4(OH)12, P42/nnm, a = 12.627(9), c = 12.595(9) Å, V = 2008(2) Å3, Z = 2 has been solved by Patterson difference- Fourier syntheses and refined to R = 0.053 for 814 Fo > 4σFo using MoKα X-ray data. In the structure, MnO6 octahedra form straight chains parallel to [110] and [11̅ 0] by sharing opposite octahedral edges. Octahedra within the chains are further linked by sharing free corners with MnO4 and SO4 tetrahedra and CO3 triangles. The MnO4 and SO4 tetrahedra also form bridging struts between octahedral chains, connecting them in the [001] direction into an open framework. The Pb, Cl, and nonframework O atoms occupy the open spaces within the framework. The 10- and 12-fold coordinations of the Pb atoms exhibit the lone-pair effect. Pb atoms link via short bonds to non-framework O atoms to form chains parallel to [110] and [ 1̅10]. When viewed down [111], [ 1̅1̅̅ 1], [1̅1̅̅1], or [11̅1̅̅], [ 1̅11̅] the framework (less the CO3 groups) is seen to be based upon cubic closest-packing of anions. This open framework of composition [[6](Mn,Mg)122+[4](SO4)22-[4](Mn2+O4)6-2 O8(OH)24]32-, referred to as a closestpacked trellis, is the fundamental unit for the structure.

Hydrogen positions in dickite

The crystal structure of dickite was initially determined by Newnham and Brindley (1956) and refined from single crystal, film X-ray data by Newnham in 1961. Newnham was unable to locate the hydroxyl hydrogen atoms from his data which gave an R-factor of 7.5%. The unit cell of dickite contains two kaolinite layers, related by a glide plane. In each layer there are four hydroxyl groups; one is situated in the plane of atoms shared by tetrahedra and octahedra (inner hydroxyls), whereas the remaining three form the outer surface of the octahedral sheet (inner-surface hydroxyls) as distinct from hydroxyls on the surface of clay crystals. Controversy has existed regarding the orientation of these hydroxyl groups. Farmer (1964), Newnham and Brindley (1956), Newnham (1961), and Farmer ( 1974) suggested that weak hydrogen bonds exist between each of the inner surface hydroxyls and the corresponding basal oxygen in the next layer. Based on infrared studies, Wada (1967), Ledoux and White (1964), Serratosa et al. (1962), and Wolf (1963), however, stated that only some of these hydroxyls are involved in hydrogen bonding and that the location of the inner hydrogen was uncertain. Serratosa et al. (1962), however, in their study of kaolinite, considered the inner hydroxyl to be essentially perpendicular to the 1:1 layer and oriented toward the hexagonal hole in the tetrahedral sheet. Wolf (1963) shared this view, but Ledoux and White (1964) favored a model where that hydroxyl was directed towards the empty octahedral site. By a technique minimizing electrostatic energy, Giese and Datta (1973) calculated the positions of the hydrogen atoms. They concluded that the inner-surface hydroxyl hydrogens are almost normal to (001), whereas the inner hydrogen points toward the empty octahedral hole nearly parallel to (001). In micas, the inner O-H bond points away from the octahedral sheet (Giese, 1979). From neutron powder diffraction, Adams and Hewat (1981) refined Newnhams (1961) structure and located and refined hydrogen positions from a difference Fourier map. Significant discrepancies exist, however, between their refined hydrogen positions and those obtained from the calculations of Giese and Datta (1973). In addition, Rozdestvenskaya et al. (1982) refined the structure of dickite from X-ray diffraction data and found the hydrogen coordinates to be at variance with the theoretical calculations of both Giese and Datta (1973) and Bookin et al. (1982) and with the neutron values of Adams and Hewat (1981). We therefore undertook this study to obtain more accurate X-ray diffraction determined hydrogen positions in hopes of confirming one or more of the earlier work.

The structures of two nitro-substituted phenoxathiins: 1-nitrophenoxathiin and 9-nitro-1-azaphenoxathiin

The structures of two nitro-substituted phenoxathiin derivatives have been determined by single-crystal X-ray diffraction. 1-Nitrophenoxathiin, C~2H7NO3 S, is monoclinic, P2~, with a = 13.864 (3), b = 7.160 (2), c = 10.950 (2) A, fl = 111.20 (3) ° at 138 K and Z = 4. The final R factor is 0.033 for 2250 diffractometer data measured at 138 (2) K using Cu K~ radiation. The two molecules in the asymmetric unit are distinctly different in their folding about the central S-O vector. The dihedral angle between the non-central rings is 163.8 ° in molecule A, and 145.7 ° in molecule B. In molecule A, the orientation of the nitro group indicates an intramolecular non-bonding S . . . O interaction (2.606 A), while in molecule B the nitro group is rotated out of the plane of the benzene ring creating a less favorable situation for such an interaction. 9-Nitro1-azaphenoxathiin, CHH6N203S, is orthorhombic, Pbc2~, with a = 3.799 (1), b = 19.940(4), c = 25.874 (8) A at 138 K and Z = 8. The final R factor is 0.082 for 2057 diffractometer data measured at 138 (2) K using Cu K5 radiation. Both of the molecules in the asymmetric unit are nearly planar; the dihedral angle between the non-central rings is 178.2 ° for molecule A and 177.1 ° for molecule B. In both molecules, there are indications of non-bonding S . . . O interactions ( S . . . O distances of 2.590 and 2.570 A) which may help stabilize the near-planar conformations. However, the planarity of the tricyclic system in the present structure may primarily be due to the aza substitution at the 1-position.