A Co-bearing nickel-rich iron monoarsenide corresponding in composition and structure: to a Ni-rich member of the orthorhombic FeAs-(Fe,Ni)As solid solution series of synthetic alloys occurs in chromite-niccolite ores from La Gallega, Spain. The name westerveldite is proposed for minerals in this solid solution series. The average composition of Co-bearing nickeloan westerveldite from La Gallega as determined by microprobe analyses is (Fe0.e35, Ni0.400, Co0.015)1.050 As1.000. The orthorhombic unit-cell has a = 3.46Å, b = 5.97Å, c = 5.33Å, all ± 0.01Å. The mineral shows strong reflection pleochroism in brownish and brownish grey colours, strong anisotropism between crossed nicols, a higher polishing hardness and apparently somewhat lower reflectance than maucherite. Westerveldite occurs in chromite-niccolite ores, where adjacent to later serpentine veinlets the niccolite is replaced by intergrowths of maucherite and westerveldite.