Chromite from the Blue Ridge province of North Carolina

Accessory chromite in dunite shows a variety of textures that indicate alteration. One group, type A, consists of four types of chromite: clean chromite; lattice chromite, in which the invading chlorite occurs along three directions in the (100) plane; optically zoned chromite; and poikiloblastic chromite. Most of type A chromites are surrounded by chromian clinochlore. The other group, type B, consists of euhedral to subhedral chromite grains which are included in olivine or pyroxene. The accessory chromites define a trend exhibited by chromite from other areas that have undergone metamorphism. Olivine-spinel geothermometry indicates equilibration temperatures near 700 degrees C, roughly consistent with mineral assemblages in the host gneisses. Thus, the Blue Ridge dunites are metamorphic rocks and not primary mantle peridotites.