The crystal structure of synthetic titanite, CaTiOSiO4, and the domain textures of natural titanites

The crystal structure of synthetic titanite (a : 7.069(2), b -- 8.722(5), c = 6.566(8), P : 113.86(2)', P2r/a) has been refined by least-squares methods to an unweighted R : 0.043. Chains of corner-sharing TiO octahedra running parallel to the a-cell edge are crosslinked by silicate tetrahedra to form a TiOSiOn framework that accommodates Ca in irregular 7-coordination polyhedra. Diffraction data of the type k 1- / 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 A) and short (1.766 A) 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 P2,/a titanite is antiferroelectric. Long-exposure single-crystal photographs of several natural titanites show diffuse ft + / odd diffraction data, indicating that natural specimens may consist of domains of P2t/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.