Zippeites: Chemical Characterization and Powder X-ray Diffraction Studies of Synthetic and Natural Samples

The zippeites are a group of uranyl sulfate minerals containing a mono- or divalent cation, defined based on a similar crystal structure. Although progress has been made toward understanding the crystal structure and chemistry of these minerals by studying synthetic analogues, very little work has been done with natural samples due to their small grain size and tendency to grow as mixtures with other minerals. This study compared natural samples to synthetic crystals using X-ray diffraction and electron microprobe techniques while conducting the first comprehensive study of the crystal chemistry of these minerals. Although wavelength-dispersive spectroscopy was difficult, largely due to the quality of the samples, evidence was found for solid solution between the divalent cation members, between the monovalent cation members, and even between both groups. It was also determined that the synthetic samples used for crystal structure determinations are reasonable analogues for the natural minerals.