A study of accessory minerals in the Cape York iron meteorite has been carried out with the electron microprobe. Phases analyzed include chromite, sphalerite, two closely related potassium-bearing sulfides, silica, and native copper. In addition, four phosphate minerals were found: One chemically similar to buchwaldite, but rather rich in iron, one corresponding to terrestrial maricite, and two more, which seem to be new minerals on the basis of microprobe analysis. Evidence from the composition of accessory minerals as well as texture elucidates some details of the fractional crystallization of the group III AB parent melt. A modification of the fractional crystallization model is therefore proposed. We suggest that sulfide nodules have originated as droplets of an immiscible sulfur-rich liquid that continuously segregated from the parent melt during crystallization of the metal. The modified model also explains the Ni/Cr anticorrelation and the high abundance of sulfur in group III AB irons which could not be satisfactorily explained by previous models.