The crystal chemistry of titanite

The chemical substitutions of primary importance in natural titanites are (Al,Fe)³⁺ + (F,OH)^- ⇆ T₁⁴⁺ + O^2- , where total Al + Fe is not greater than 30 mol % and Al is usually predominant. Electron microprobe analyses and bond strength considerations suggest that Al prefers the octahedral site in natural titanites, whereas ubiquitous rare earths substitute for Ca. Ion microprobe mass analysis indicate that Pb, U, and Th are present in all but two specimens examined, both of which are from pegmatites. Trace level concentrations of elements not previously reported in natural titanites including Ru, Pd, Ag, and Te were detected by this technique.

The lattice parameters of titanites vary directly with the effective octahedral cation radius. As in zircon, partial metamictization of some titanites causes increases in unit cell dimensions and decreases in density. Under electron bombardment, the characteristic x-ray yields are lower for metamict titanites than for their structurally restored 0 equivalents which were heated at 1100 C for three hours.

The formation of out-of-step linear domains parallel to 100 (the octahedral chain direction) is favored by substitutions of Al + Fe for Ti. Pure CaT10SiO₄ is primitive (P2₁/a) with weak, but sharp k + ℓ odd reflections. Increasing Al + Fe substitution increases the frequency of domains and the k + ℓ odd reflections become diffuse, eventually disappearing as the average structure attains space group A2/a.