The crystal chemistry and space groups of natural and synthetic titanites

The chemical substitutions of primary importance in natural titanites are (Al,Fe)S+ * (F,OH)-eTia+ + 02-, where total Al * Fe is not greater than 30 mole percent, and Al is usually predominant. Electron microprobe analyses and bond strength considerations suggest that Al and Fe occupy octahedral sites in natural titanites, whereas ubiquitous rare earths substitute for Ca. 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 CaTiOSiOn is primitive (P2Ja),with weak but sharp k + / odd reflections. Increasing Al + Fe substitution increases the frequency of domains and the & + / odd reflections become diffuse, eventually disappearing as the average structure attains space Eroup A2/a. 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, possibly accompanied by the inclusion of water in the recoildamaged structure.