Summary: In addition to the normal type of chromite alteration, an antipathetic form of alteration has been observed in the ultramafic zone of the Panton Sill, Western Australia. The chromites display margins of lower reflectivity related to a marked enrichment in MgO and Al₂O₃ and depletion in Cr₂O₃, Fe₂O₃, and FeO, such that a spinellid of composition lying between the end members MgAl₂O₄ and FeA₂O₄ is formed. The alteration is related to the formation of secondary spinel in the groundmass during the hornblendization of primary silicates under metasomatic conditions accompanied by low oxygen activity.