The structure of an Al-rich titanite has been refined in several space groups to provide data on a natural titanite and to evaluate the structural effects of the coupled substitution (Al,Fe)3+ + (OH,F) = Ti4+ + O2−. Titanite from Grisons, Switzerland, Ca(Ti0.904+Al0.093+Fe0.013+)O0.98SiO3.96(OH0.08F0.02), is monoclinic with a = 7.050(3), b = 8.681(4), and c 6.539(2)Å and β = 113.90(3)°. Anisotropic refinement in space group A2/a (Rw = 0.048) yielded results similar to those obtained by Mongiorgi and di Sanseverino (1968) for titanite from Zillertal except that the mean Ti–O bond length is smaller due to substitution of Al for Ti in the octahedral site. The orientation of the maximum thermal vibrational displacement for most atoms subparallel to a reflects positional disordering due to the development of a domain texture. The details of the P21/a refinement (Rw = 0.065) are comparable to those of synthetic P21/a titanite (Speer and Gibbs, 1976). Despite extensive Al3+ → Ti4+ substitution, the alternating long and short Ti–O(1) bond lengths, (1.970 and 1.768Å, Fig. 1) which reflect the amount of displacement of the Ti atom from the geometric center of the Ti octrahedron are the same as for the synthetic P21/a titanite at 25°C (Taylor and Brown, 1976). The extreme distortion of some of the Si and Ti polyhedra and unreasonable bond distances cast doubt on the reliability of refinement in space group P21 constrained by only 21 weak maxima of type h0l, h = odd. The weak maxima are the result of diffuse h-relplanes. It is possible that the h-relplanes are due either to linear positional ordering or to chemical cation ordering in the octahedral sites.

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