Kimberlitic chromian pocroilmenities with intergrowths of titanian chromite and rutile

A suite of chromian picroilmenite macrocrysts (up to 13 wt% Cr203 %) from kimberlite which contain lameliar intergrowths of titanian chromites (23–42 wt% TiO2, 7–16 wt% Cr2O3) and rutile are described. Using the mineral chemistry of coexisting phases in the natural assemblages as well as the preliminary results of an ongoing study on the system, ilmenite-spinel, it has been possible to construct a phase compatibility diagram for systems containing ilmenite-spinel-Cr2O3-rutile relevant to upper mantle conditions. A model is developed whereby the spinel and rutile lamellae intergrowths may be formed by an initial step involving reduction “exsolution” followed by isochemical breakdown during cooling.