Core from exploration boreholes at the Karee platinum mine of the western Bushveld Complex, in South Africa, became iron-stained and friable upon surface exposure. This alteration mostly occurs in pyroxenites that are immediately above and below the UG2 chromitite layer and adjacent to the Merensky Reef. Chemical analyses of such rocks have Cl concentrations from 0.08 to 0.16 ± 0.02 wt%. Analyses of comparable fresh rocks, Merensky pyroxenite and also iron-rich ultramafic discordant pipes, give values of less than 0.04 wt% Cl. The mineral hibbingite, Fe 2 Cl(OH) 3 , has been tentatively identified, on the basis of its petrographic features and qualitative electron-microprobe analysis, as veins 0.1 mm thick that cut, and have sharp contacts with, serpentine that occurs in olivine grains. These textural relationships show that this alteration occurred below ±500°C. There is no evidence for any other whole-rock geochemical changes, except Cl.