A description is given of the adaptation of an energy dispersive portable XRF spectrometer (with Si(Li) detector), to allow it to be used simultaneously for the Giessen Gordon XRD mineral identification technique and for XRF chemical analysis. The technique was applied to the determination of the minerals used as black, red and white pigments in the decorations of Minoan pottery. Twenty-six decorated Minoan shards from the Ashmolean Museum, Oxford, were analysed quantitatively for major metals (XRF) and simultaneously for the minerals used as pigments (XRD). The impression obtained from the analysis is consistent. Throughout the entire Bronze Age at Knossos, blacks, brown, red and orange-reds were all derived from ochres -- natural iron compounds. The white pigments analyzed were metakaolinite and huntite. This set of analyses had abundantly confirmed the use of iron earths in Minoan pot decoration, already attested by other scholars, but of more importance is the demonstration of the possibility of using widely available, portable XRF equipment for non-destructive diffraction studies.