Micaceous kimberlites from South Africa and Canada contain two types of groundmass mica less than 1 mm across. Very rare Type I micas are relatively iron-rich with mg [ = Mg/(Mg + Fe)] 0.45-0.65, TiO 2 3-6 wt%, Al 2O 3 14-16wt%, no Fe 3+ required in tetrahedral sites, low NiO (~0.02 wt%), and relatively high na [Na 2O/(Na 2O + K 2O)] 0.02-0.03. The much more abundant Type II micas are variable in composition, but relative to Type I micas are more magnesium ( mg 0.80-0.93), lower in TiO 2 (0.7-4.0 wt%) and Al 2O 3 (6.8-14.2 wt%), have substantial Fe 3+ in tetrahedral sites, and have relatively low na. Both types may have rims with compositions indicative of mica-'serpentine' mixtures resulting from reaction with a highly aqueous fluid. The petrographically-determined 'serpentine' is chemically of two types: Fe-rich serpentine and Fe-rich talc. Associated phases in the ground-mass vary from one kimberlite to another: calcite, dolomite, diopside, chromite, Mg-ilmenite, perovskite, barite, pyrite, pentlandite, millerite?, heazlewoodite?, quartz. Inter-grain variations in composition of Type II micas may result from establishment of local reservoirs on a mm scale, consequent upon mechanical mixing and competition of other phases for minor elements (e.g. chromite for Cr, serpentine for Ni). Type I micas may result from an intrusive precursor (carbonatitic?) to kimberlite, perhaps genetically related, which was incorporated into a later pulse of kimberlite from which the Type II micas crystallized.