Chromite in the Twin Sisters dunite, an alpine-type ultramafic body some 90 km2, occurs as minor disseminated euhedral-subhedral crystals or in clusters forming pods and lenses. Modal proportions of chromite in the pods and lenses are frequently as high as 80 percent. Ferritchromite occurs as rims on chromite crystals that are bordered by chlorite and (where still available) olivine.
Microprobe analyses of chromite, ferritchromite, chrome-magnetite, olivine, and chlorite show that the chromite-ferritchromite boundary is sharp (less than 10 microns) and that, relative to chromite, ferritchromite is enriched in Cr, Fe, Ni, and Mn and impoverished in Mg and Al. However, when chrome-magnetite, found only in association with serpentine, is the alteration product, there is a marked decrease in chromium.
Chromite alteration appears to take place above the upper stability limit of serpentine and below that of chlorite.