Kassite from the Prairie Lake (Ontario) and Iron Mountain (Wyoming) and its relation to perovskite alteration

Kassite [CaTi2O4(OH)2] is a rare mineral typically found in silica-undersaturated environments. Localities where it was previously recognized include the Afrikanda alkaline-ultrabasic complex in Kola Peninsula (Russia), Magnet Cove alkaline complex (Arkansas) and an ultrabasic Fe-ore deposit in Wuyang (Henan, China). Identification of kassite is not trivial because it is dimorphous with cafetite, and can be easily confused with titanite, perovskite or TiO2 polymorphs during a routine petrographic examination. The primary objective of the present study was to determine the nature of Ca-Ti oxide phases commonly observed to replace perovskite in alkaline and carbonate-rich rocks (including carbonatites, kimberlites and nephelinites). For detailed analysis, we selected relatively large (100 ?m - 1 mm) grains of hydrous Ca-Ti oxide from silicocarbonatite from the Prairie Lake complex (Ontario) and serpentine-calcite kimberlite from Iron Mountain (Wyoming). Both samples were examined using electron-microprobe analysis, Raman micro-spectroscopy and X-ray micro-diffraction. For comparison, cafetite from Khibiny (Kola, Russia) was investigated using the same techniques. Our data show that the major product of perovskite alteration in a CO2-rich environment (prior to the development of anatase and/or ilmenite) is kassite, i.e. a monoclinic (space group P21/a) polymorph of CaTi2O4(OH)2. The refined cell parameters of the Prairie Lake material are: a= 5.285(1) Å, b= 8.990(2) Å, c= 9.549(3) Å, and ?: 90.42(2)°. This mineral can be readily distinguished on the basis of its Raman spectrum, which contains well-defined lines at 169, 219, 340, 440, and 695 cm-1 not observed in its dimorphous cafetite. In addition to providing constraints on the conditions of peroskite alteration in CO2-rich systems, the present study has implications for safe disposal of fission products in perovskite-based ceramics and applicability of radiometric and other isotopic data obtained on perovskite concentrates.