Volume 134, Issue 5-6

Quenselite, PbMnO 2 OH, is monoclinic, P 2/ a , with a = 5.61, b = 5.70, c = 9.15 Å, and β = 93.0°; Z = 4. The structure and space group proposed by B yström are not correct. The correct structure, which has been refined to an R value of 10.6%, consists of four hexagonal closest-packed oxygen layers stacked normal to [001] with Pb +2 ions nearly occupying anion sites. Brucite-type layers parallel to (001) are composed of distorted MnO 6 octahedra characteristic of the Jahn-Teller effect. Sandwiched between these are infinite chains of PbO(OH) 3 pyramids resembling those in yellow PbO. A system of weak hydrogen bonds linking the chains and the brucite-type layers is proposed.

Almandite, a natural garnet, orders antiferromagnetically at T N = 7.5°K. The spin structure has the magnetic symmetry Ia 3 d ′. The oxygen parameters are x = − 0.03398, y = 0.04935, z = 0.15284 at 300°K; they have practically the same values at 4.2°K.

Mössbauer spectra taken from almandite show that the orthorhombic symmetry around Fe 2+ (24 c position) is maintained in the magnetically ordered state. At 4.2°K the magnetic field at the nucleus is 228 ± 4 kG, and for the electrical field gradient we find (− 2.21 ± 0.06), (1.24 ± 0.04) and (0.97 ± 0.04) × 10 18 V/cm 2 along the three proper axes, respectively.

The crystal structure of p -toluene sulfonic acid monohydrate, C 7 H 8 O 3 S · H 2 O, has been determined from counter-diffractometer data and refined by least squares to an R value of 0.069. The crystals are orthorhombic with a = 20.427 (1), b = 6.266(1), c = 7.063(1) Å. A space-group ambiguity was resolved in favor of noncentric Pna 2 1 with four molecules in the cell. The structure consists of tosylate and hydronium ions linked in a hydrogen-bonded array. The hydronium ions are approximately tetrahedral with one coordination position open.

The crystal structure of vrbaite has been determined. Four chemical units of Hg 3 Tl 4 As 8 Sb 2 S 20 are contained in the orthorhombic unit cell: a = 13.399, b = 23.389, c = 11.287 Å, symmetry C 2 ca – C 2 v 17 . The structure was solved through a kind of superposition method using the three-dimensional Patterson function. The final discrepancy index R is 4.8%. The structure of vrbaite is characterized by infinite As 2 SbS 5 chains parallel to c and by Hg 3 As 4 S 10 sheets perpendicular to b . The former sandwich the Tl atoms and make slabs perpendicular to the b axis. These slabs and the Hg 3 As 4 S 10 sheets, sandwiching the other Tl atoms, are arranged alternately along the b direction. The good cleavage parallel to (010) can be well explained by the structure. Vrbaite is the first structure with mixed (As,Sb) chains.

Heat of solution measurements of the enthalpies of Na-K mixing, Al-Si ordering, and fusion for crystalline solutions and end-member phases in the system Na(AlSi 3 O 8 )–K(AlSi 3 O 8 ) were obtained with a vacuum-jacketed, isoperibolic calorimeter using 20.1 percent hydrofluoric acid at temperatures ranging from 40 to 60°C. Heat of solution data for artificially prepared microcline (–low-albite) crystalline solutions at 49.7°C are represented by [unk] sol = −149.408 + 5.230 N 2 − 5.928 N 1 N 2 2 − 8.457 N 2 N 1 2 kcal/mole based on 39 measurements of 14 different bulk compositions, where N 1 and N 2 denote the mole fractions of NaAlSi 3 O 8 and KAlSi 3 O 8 respectively. The thermochemical uncertainty of the above least-squares formulation is ± 0.032 kcal/mole. Unit-cell parameters determined by least-squares refinement of diffraction data indicate that the excess volumes of Na-K mixing of these microcline solutions are positive over the entire composition range. Combining the calorimetric and crystallographic data leads to the following expression for the variation of molar enthalpy of mixing with pressure: [unk] ex = (5928 + 0.110 P) N 1 N 2 2 + (8457 + 0.110 P ) N 2 N 1 2 cal/mole. The sign and magnitude of [unk] ex determined by calorimetry agree closely with [unk] ex calculated from a third-order Margules formulation of high-temperature two-phase equilibrium data in the critical region.