| Apatite is a common accessory mineral in igneous rocks that is sensitive to minor changes in rare earth element (REE) concentrations. Hydroxylapatite and less common fluorapatite, in eight carbonatite and four silicate rocks, were confirmed by electron microprobe analyses in samples from the Oka Carbonatite Complex, Quebec. These minerals contained significant amounts of Ca, P, Sr, La, Ce, Si, OH and F, as well as less important amounts of Na, S and Cl. Barium, Pr, Nd, Sm and HREE were below the detection limit, and C could not be detected with the available instrumentation. In the Oka apatite, the cations Na +, Sr2+, La3+ and Ce3+ partly replaced Ca2+. The cations Si4+, S 6+ and possibly C4+, partly replaced P5+. The anions F- and Cl- partly replaced (OH)-.; The REEs are incorporated into the apatite crystal structure primarily by the coupled substitution Ca2+ + P5+ ↔ REE3+ + Si4+.; Electron backscattered images of apatite from Oka shows oscillatory and/or sector zonation not seen in transmitted light. Electron microprobe analysis has revealed that the zones within the apatite vary in LREE+Si content. (Abstract shortened by UMI.)... |
