The total valence of the bonds between acceptors and the H atoms bonded to a water molecule (s d) is taken equal to the bond valence (st) received by the donor 0 atom from coordination bonds. For an OH-group, s d = sr-l.

(100)twinning is always present in the fl monoclinic form. Final R = 0.093 and 0.052, using 1359 and 752 observed counter reflexions, respectively. In the tz' modification the SiO 4 tetrahedron is unusually small (average Si-O bond length = 1.61 A), and the Sr(1) atom is loosely bonded to its coordination environment (average Sr-O distance = 2.85 A) owing to the constraints of mirror symmetry; bond-valence calculations prove these two features to be consistent. The loss of the (100) symmetry plane in the fl structure allows Sr(1) to shift so as to improve its bond-valence balance, and the SiO 4 tetrahedron can attain a normal size. A large component of anisotropic thermal motion is observed normal to the (100) plane in the ~t' form, and is discussed in relation to possible disorder.

It is shown that one of the main reasons for most failures of the methods for calculating distance-dependent bond strengths is related to the distortion of the coordination polyhedra. The charge distribution (CD) method which depends on only one universal empirical parameter (contraction parameter) is modified to include: (i) an iterative calculation of the effective coordination number (ECoN), to deal with structures containing very distorted coordination polyhedra; (ii) a specific contraction parameter to treat structures containing any type of hydrogen bond; (iii) scale factors for coordination subshells, to treat structures with hetero-ligand polyhedra. The contraction parameter for the hydrogen bonds was obtained from 119 well refined structures based on neutron diffraction data. Examples of the application of the iterative charge distribution (CD-IT) are presented to show the efficiency of the new method in dealing with distorted (including hydrogen bonding) and hetero-ligand polyhedra. In particular, analysis of a series of 74 structures with pentacoordinated cations shows that deviations from overall trends are related to structure instability. The possible failure of the method with polyionic structures and 'dynamic' structures is discussed.

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