A sample of the zeolite mineral ferrierite was found to have unit-cell dimensions a= 19-16, b= 14-13, c=7"49/~ (unit-cell content approximately Nat.sMgzSia0.sA15.5072.18H20) and to belong to space group Immm. The crystal structure was determined from three-dimensional X-ray data. Channels, whose size is defined by a ring of ten tetrahedra, exist parallel to the c axis in the aluminosilicate framework. Another prominent feature is the existence of large cavities which contain Mg(H20)~ + ions. Structurally, ferrierite most closely resembles mordenite and dachiardite. There is no evidence for ordering of metal atoms in the metal positions. It was not possible to locate the Na ÷ ions or six of the eighteen water molecules. PHILIP A. VAUGHAN 985

The compounds ZrOC12. 8I-I20 and ZrOBr~. 8tt20 are isomorphous and belong to the tetragonal space group D~x-P421c, with Z --8. For the chloride a ----17-08 and c -~ 7.689 /~, and for the bromide a ----17-65 and c -~ 7.95 /~. The zireonyl group consists of a complex in which four zirconium atoms are at the corners of a slightly distorted square and are linked along each edge of the square by two OH groups--one above and one below the plane of the square. Four water molecules are bound to each zirconium in such a manner that the arrangement of the eight oxygens about each zirconium is a distorted square antiprism. There are no zirconium-halogen bonds in the structure.

X-ray scattering by aqueous solutions of MOX2·8H2O (M=Zr and Hf, X=Cl and Br) can be explained if one assumes the existence of a complex [M4(OH)8 (H2O)16]X8. The metal atoms in this complex are arranged in a square and are held together by double OH bridges along each edge. In addition, each metal atom is bound to four additional water molecules in such a manner that the configuration about the metal is a square Archimedes antiprism. The halogen ions are not bound directly to the metal. They do, however occupy definite positions in the complex, and are presumably held in place by electrostatic forces. The structure is very similar to that which has been found in the crystalline oxyhalides.

ASYMMETRIC X-RAY REFLEXION FROM NATURAL QUARTZ CRYSTALSnatural quartz crystals is similar to that predicted by the dynamical theory for perfect, absorbing crystals. Using the results of the theoretical investigation, it has been possible to obtain experimental F values with an accuracy of ~10% from one crystal, without the necessity for making detailed assumptions about the degree of perfection of its texture. From the combination of observations from a series of crystals it may be possible to increase the accuracy of this method. Higher accuracy may be obtained if the absorbing effect of any projections on the crystal surface can be found from the experimental variation of J(fl) with fl for a weak reflexion.A disadvantage of the method is that the crystals used must be quite large, and show faces suitable for mounting. However, wherever applicable the method leads to far more accurate results than any previous investigation.It is a pleasure to thank Prof. Sir Lawrence Bragg and Dr W. H. Taylor, in whose laboratory this work was carried out, for their constant help and encouragement. I am indebted to Dr S. 0. Agrell, Curator of the Museum of the Department of Mineralogy and Petrology, Cambridge, for the provision of the two quartz crystals. I would also like to thank my colleagues, Dr P. B. Hirsch and Dr J. S. Thorp, for their invaluable help and advice throughout this work. This research was carried out while the author was in receipt of a maintenance grant from the Department of Scientific and Industrial Research.References DARWIN, C. G. (1914 Revised values for carbon--oxygen and carbon-nitrogen double bonds are suggested, and these values are used to discuss the effect of resonance on interatomic distances in urea and related compounds.

2157bond can be formed in the latter two cases due to the separation of the hydrogen bonding atoms. As the data in this work show, the spectra can be explained in terms of the dipolar species existing in DzO.In summary, by assuming the dipolar form for the three acids as the principal species in aqueous solution, a variety of solution infrared results can be explained.It has previously been proposed that I represented picolinic acid in D20.4 Internal hydrogen bonding of Acknowledgment.The solubility of A1203 in rutile Ti02 was determined in the temperature region of 1200-1426' from the change of the c unit-cell parameter. Solubility increased with equilibration temperature, ranging from 0.62% A1203 by weight at 1200' to 1.97% at 1426'. Corundum was the equilibrium solute phase below, and AlzTiOs above, 1240'. The density of a rutile sample equilibrated at 1426' containing 1.6% by weight A1203 was 4.199 g/cma, not significantly different from 4.202 g/cm3 measured for a corresponding pure rutile sample. These results are consistent with an interstitial solubility model in which AIS+ and 02-in 1 : 2 proportion are placed on the respective rutile lattice sites, and the remaining A13+ occupies interstitial spaces. Treatment of the temperature dependence of the solubility data gave values for AH' of solution of 35 kcal/mol and 100 kcal/mol for A12Ti05 and AlzOs, respectively. The heat of formation of AlzTiOs computed from these results is -559 kcal/mol near the transition temperature.Solubility also increased with the addition of NblOS.