The structure of datolite has been refined (R = 0.059) using least-squares methods and 1297 three-dimensional counter-collected intensities in order to locate the hydrogen atom and provide more precise structural parameters, there by alleviating some of the ambiguities of previous structural studies. The datolite structure is based on sheets of four- and eight membered rings of alternating SiO4 and BO3(OH) tetrahedra with the sheets bonded together by calcium atoms. The T-O(br) bond lengths of the SiO4, and the BO3(OH) tetrahedra are 1.648(2), 1.651(2), 1.661(2) Å and 1.475(3), 1.486(3), 1.464(3) Å, respectively, while the T-O(nbr) bond lengths are 1.570(2) and 1.496(3) Å. The hydrogen atom is located at a distance of 0.75(5) Å from O(5) in the approximate position predicted on the basis of infrared (Sahl, 1966) and nuclear magnetic resonance (Sugitani, Watanabe, and Nagashima, 1972) studies. The SiO4 and BO3(OH) tetrahedra are more distorted than those of Pavlov and Belov (1959) but less distorted than those of Pant and Cruickshank (1967). The parameters

mean Si-O(br) bond length, and Si-O-B angle fit the data presented by Brownand Gibbs (1970) for a model that includes covalent bonding. An extended Hü ckel molecular orbital calculation for the SiO4 tetrahedron suggests that Mulliken Si-O bond overlap populations, n(Si-O), may be used to order Si-O bond length variations in silicates where all oxygen atoms are not charge balanced and where the neighboring cations and anions are ignored.

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