 (SO4)2.12H2O, where A was a monovalent metallic ion (NH4+, K+, Rb+, Cs+, or Tl+), B was a trivalent metallic ion (Al3+, Cr3+, or Fe3+) and S was usually sulfur, but could also be selenium or tellurium.Because the alum crystals were largely isomorphous when the heavy atoms were changed out, they could be phased by isomorphous replacement.Fourier analysis was used to find the heavy atom positions.