MANGANESE-RICH FE-MONTICELLITE FROM THE O KA CARBONATITES, QUEBEC

High-Mn (up to 9.5 wt %MnO) monticellite was first detected in the dee p-seated carbonatites of the Oka Complex, Quebec. The Physical propert ies (refractive indices, density), interplanar spacings, unit-cell par ameters (a(0) = 6.440, b(0) = = 11.100, c(0) = 4.840), and chemical co mposition (Ca-1.01(Mg0.48Fe0.302+Mn0.21)(0.99)Si1.00O4.00) of this min eral arec intermediate between analogous values for minerals of the mo nticellite group: CaMgSiO4 (monticellite), CaFeSiO4 (kirschsteinite), and CaMnSiO4 (glaucochroite). Our data and preexisting data on melt an d fluid inclusions in monticellite and associated minerals indicate th at the following: (1) the monticellite-calcite carbonatites were gener ated from heterogeneous (melt + fluid) magmatic systems; (2) the cryst allization temperatures of monticellite in the deep-seated carbonatite complexes are higher (up to 890 degrees C) than those of volcanic roc ks (600-700 degrees C), and (3) the monticellite sodiccarbonatite of t he Oldoinyo Lengai were formed at a temperature no more than 550 degre es C; (4) the fluid pressure under which the Oka carbonatite was forme d is estimated at 4.6 kbar. The data reported in this paper suggest th at monticellite is stable only under low pressures.