Natroalunite occurs typically here as thin lenses of nodules in friable sandstone at or near the Eagle Ford-Woodbine disconformity of the Upper Cretaceous. Typical natroalunite has only traces of impurities; it is exceedingly fine-grained but retains a replica of radial plumose structure which is reflected by the cauliflower-like outer form. Thus, the nodules were derived by transformation from material with spherulitic structures. Minor K 2 O is present, and essential Ca, Sr, Ba, Mg, and rare earths. A deficiency of bases is believed to be compensated for by oxonium H 3 O (super +) ions. The natroalunite is believed to have developed under oxidizing conditions, with pyrite as the source of the SO 4 . Experiments suggest its derivation to be from aluminite.