The crystal structure of synthetic titanite (a = 7.069(2), b = 8.722(5), c = 6.566(8), β = 113.86(2)°, P21/a) has been refined by least-squares methods to an unweighted R = 0.043. Chains of comer-sharing TiO6 octahedra running parallel to the a-cell edge are crosslinked by silicate tetrahedra to form a TiOSiO4 framework that accommodates Ca in irregular 7-coordination polyhedra. Diffraction data of the type k + l odd which violate the diffraction rules of the previously reported A2/a space group are ascribed to an “off-centered” displacement of the titanium atom from the geometrical center of each octahedron, resulting in long (1.974 Å) and short (1.766 Å) Ti-O bonds alternating along the chains. The displacement is such that Ti is shifted in the +a-direction in one-half of the chains and in the —a-direction in the other half. This arrangement implies that P21/a titanite is antiferroelec-tric. Long-exposure single-crystal photographs of several natural titanites show diffuse k + l odd diffraction data, indicating that natural specimens may consist of domains of P21/a titanite related by a half-turn parallel to b. The coupled substitution of Fe and Al for Ti and OH for O appears to favor domain formation.

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