No abstract

The crystal structure of the a-form of Ag,NSeO, shows it to not a salt but to have discrete molecules with a 3-fold axis of symmetry. THE structure of Ag3NSe03 has been in doubt ever since it was prepared in two crystal modifications (a, p) by DostAl and RhiiEka.1 1.r. studies2 lent some support to the view that it might be formulated Agi+NSeO:-.

two cyclopentadienyls in (C,H,) ,Ti(C,H,), to titanium with a mean Ti-C distance of 2.31 (5) ,& and the two phenyls are o-bonded with Ti-C distance of 2.27 (1) A.(C6H5),Ti(C6H5), WAS made according to the method of Summers and U1oth.l The crystals were grown from a toluene-heptane solution a t -5" yielding yellow-orange, diamond shaped tabular plates which slowly decomposed under X-rays. The crystals are orthorhombic and systematic absences indicate two possible space groups Crnca and C2cb. The unit cell dimensions are as follows: a = 12.854 = 1.27 g ~m -~, D, = 1.28 g ~m -~(for 2 = 4), , u (Cu-K,) = 42-0 cm-l, space group C2cb from structure analysis.Cu-K, data were collected on a Picker four circle automated diffractometer, scaled using standard reflections and corrected for absorption., The structure was solved by the heavy-atom method and refined by full-matrix anisotropic

Four natural gypsum rock samples were prepared and certified for major elements and some trace elements by the Analytical Group of Domtar Inc., Research Centre in Senneville (Montréal), Québec, Canada with the cooperation of Domtar Gypsum Products Laboratory in Caledonia, Ontario, Canada. The analytical round‐robin results received from 29 participating laboratories were statistically evaluated, summarized, and form the basis of this paper. These certified reference materials are primarily intended as calibration standards for the determination of major and minor elements in gypsums and gypsum related minerals and products. As certified gypsum rock samples were not available on the North‐American or world markets, these may be particularly important not only to the gypsum and cement industries, but also to geochemists, geologists and analysts of minerals and ores.

The crystal structure of the a-modification of the "red salt" of trisilver amidoselenate, Ag3NSe03 has been determined from MoKa X-ray diffraction data, and refined to R = 3.19% from 337 unique absorption-corre~ted observed reflections. The structure is rhombohedral, R3c (No. 161), with (hexagonal) a = 8.462(4), c = 11.372(6) A. The formula unit has point symmetry 3 (C3), with Se and N on the threefold axis, each approximately tetrahedrally coordinated; Se by tkee oxygen atoms (at 1.651(8) A) and N (at 1.70(2) A), and N by Se and three silver atoms (at 2.164 (8) IntroductionThe formation of an orange-red precipitate, during the reaction of ammonium amidoselenate NH4Se03NH2 with silver nitrate in dilute NH40H, was first observed by Dostal and KrejEi (1) in 1958. Later, Engelbrecht and Clementi (2) described this reaction in their work on Se-N compounds. They suggested that the formation of the title compound most probably proceeds via substitution of the amino-group hydrogens by silver. Further studies of the compounds of the type Ag-N-X (where X is S or Se) were carried out by Dostal and RdiiEka (3), RdiiEka (4), and Paetzold, Dostal, and RdiiEka (5). It was Dostal and RdiiEka (3) who first confirmed the existence of two different modifications of Ag3NSe03, and named them a and P. The a-modification results from the reaction given in ref. 3, while the P-modification results when the orange-red precipitate is kept at room temperature in the mother liquor for several days. The p-form can also be obtained as a product of the hydrolysis of Ag3NSe03. 3NH3. 2H20. Both modifications are highly explosive, and can be distinguished easily by their different X-ray powder patterns.Paetzold, Riiiitka, and Dosthl (5) studied a number of silver compounds containing NX03 groups (X = S, Se) by means of IR spectroscopy. They proposed possible structural models for