Clarence S. Ross

The clay minerals and their identity

ABSTRACT A study of the clay minerals is in progress and while not complete is yielding information about some of the fundamental minerals of sedimentary materials and the conditions that favor their formation. There are several kaolin minerals, but kaolinite is the only one that is likely to be found in sedimentary beds. Clays of the heidellite, montmorillonite, or other types are, however, more widespread in soils and shales than kaolinite. Calculations based on the assumption that kaolin is the typical clay-forming mineral are commonly in error. The relationship between geologic processes and the formation of different types of clays is outlined; and also the way in which geologic conditions may be reflected by base exchange.

Natroalunite in Upper Cretaceous Sedimentary Rocks, North-Central Texas

ABSTRACT Natroalunite, the dominantly sodic member of the alunite group of minerals, occurs sporadically over wide areas in north-central Texas and so constitutes a significant occurrence in sedimentary rocks of a mineral more commonly resulting from alteration of non-sedimentary volcanic materials. It occurs typically as thin lenses of nodules in friable sandstone at or near the Eagle Ford-Woodbine disconformity of the Upper Cretaceous. Microscopic and X-ray studies indicate that typical natroalunite has only traces of impurities. The natroalunite is exceedingly fine-grained but retains a replica of a radial plumose structure which is reflected by the cauliflower-like outer form. Thus the nodules give evidence of having been derived by transformation from some material with spherulitic structures. The Texas material is dominantly natroalunite with minor K2O, but it also contains essential calcium, strontium, barium, magnesium, and rare earths, whose presence is correlated with P2O5, proxying SO4. A deficiency of bases is believed to be compensated by oxonium H3O+ ions. These variables give a complex chemical formula, but one conforming to the isostructural group of minerals typified by alunite. X-ray powder diffraction data agree well with those of earlier studies of natroalunite. The natroalunite is believed to have developed under oxidizing conditions, with pyrite as the source of the SO4. There is no direct evidence of the character of the antecedent material from which the natroalunite was derived, but experiment suggest derivation from aluminite, Al2(SO4) (OH)4.7H2O.