An untwinned crystal of perovskite (CaTiO3) from the Benitoite Gem mine, San Benito County, California, was investigated by polarized IR spectroscopy and by single-crystal X-ray methods. The unit-cell parameters are a 5.380(1), b 5.440(1), c 7.639(1) .ANG., V 223.56(7) .ANG.3, Z = 4, space group Pbnm; the crystal structure was refined to a final R of 2.7%. At. coordinates and anisotropic displacement parameters are in good agreement with previously published refinements of synthetic crystals of CaTiO3 perovskite grown at high temp. IR spectra show two OH absorption bands centered at 3394 and 3326 cm-1. These bands are weakly pleochroic in the (001) section, but show a more distinct anisotropic behavior in the (110) section, with a stronger component of absorption perpendicular to [001]. This finding suggests an alignment of the OH vectors preferentially parallel to the [110] direction (a of the pseudocubic cell). The proposed mechanism of hydrogen incorporation, with O2 (= OH) atoms acting as donor oxygen atoms in hydrogen bonding, is explained by cation substitutions coupled with cation vacancies. Even though the approx. H2O content derived from the intensity of the absorption bands is rather low (ca. 70 ppm), similar concns. are considered to occur in natural (Mg,Fe)SiO3 perovskite in the Earth's lower mantle.
