Modern X-ray and neutron diffraction techniques can give precise parameters that describe dynamic or static displacements of atoms in crystals. However, confusing and inconsistent terms and symbols for these quantities occur in the crystallographic literature. This report discusses various forms of these quantities, derived from probability density functions and based on Bragg diffraction data, both when the Gaussian approximation is appropriate and when it is not. The focus is especially on individual atomic anisotropic displacement parameters (ADPs), which may represent atomic motion and possible static displacive disorder. The first of the four sections gives background information, including definitions. The second concerns the kinds of parameter describing atomic displacements that have most often been used in crystal structure analysis and hence are most commonly found in the literature on the subject. It includes a discussion of graphical representations of the Gaussian mean-square displacement matrix. The third section considers the expressions used when the GaussJan approximation is not adequate. The final section gives recommendations for symbols and nomenclature.

Abstract. (UO2)2(SiO4).. A least-squares refinement of the structure of soddyite from a new set of X-ray data shows that accurate results can be obtained even for crystals containing very heavy elements; the strong absorption can be satisfactorily accounted for if appropriate procedures are used. Here, in particular, it was even possible to locate the water H atoms and refine their coordinates and thermal parameters. The structure shows chains of pentagonal bipyramids centred on the U atoms, joined by edge sharing: the chains are parallel to [110] and are cross linked through SiO4 tetrahedra. The cohesion within the resulting threedimensional structure is further enhanced by a pattern of hydrogen bonds involving the water molecules and the uranyl O atoms.

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The crystal structure of copper(II) glutamate dihydrate, CuCsH7NO4.2H20, has been determined and refined by three-dimensional least-squares methods. The crystals are orthorhombic, space group P 212121, with a= 11-084, b= 10.350, c= 7.238/~ and four molecules per unit cell. Intensity data were collected visually from Weissenberg photographs about all three crystal axes; of 1087 reflections within the effective sphere of copper radiation all but 16 were strong enough to be measured. The final R index is 0"032; the standard deviations are about 0-004 A for the light-atom positions.The coordination about the copper atom is approximately square planar, involving two oxygen atoms and a nitrogen atom of glutamate groups and a water molecule; the Cu-O and Cu-N distances range from 1.97 to 2.00 A. Two additional glutamate oxygen atoms at 2.30 and 2.59/~ complete a severely distorted octahedron. Five of the six available protons are involved in hydrogen bonding. The glutamate group is in an extended configuration and has normal bond distances and angles.

Important data concerning chemical composition of a mineral of the gadolinite group can be obtained from a refinement of its crystal structure. In spite of the complexities, an entirely crystallographic procedure performed on a single grain can afford a practical way to identify and characterize these species; such a procedure is often successful, provided the quality of the crystal allows it. We consider the crystal-chemical mechanisms corresponding to the most important substitutions, and apply such a method to identify the Fe-poor and Ca-rich gadolinite-group minerals from Cuasso al Monte, Varese, Italy. These crystals can be ascribed to hingganite-(Y), with minor amounts of gadolinite (0.26-0.31 molar fraction) and datolite components (0.15-0.25 molar fraction) in solid solution. The component minasgeraisite-(Y) is not present, even as a minor one, in the samples studied.