ABSTRACT

Allophane and Na-rich alunite have been developed from kaolinitic nodules in Pennsylvanian-age shale in Bates County, Missouri, by action of supergene sulphate solutions derived from oxidizing iron sulphide. Reversibility of the commonly reported change of allophane to kaolinite is here demonstrated. The SiO₂:Al₂O₃ ratio in the allophane is 1.29:1, representing typical enrichment in allophane of Al₂O₃, relative to the SiO₂:Al₂O₃ ratio in kaolinite. The calculated apparent standard free energy of formation of this allophane is -900.14 ± 47 kcal per formula weight that is based upon the kaolinite formula. Its stability in aquenous environment is approximately the same as halloysite; the calcul ted F_f⁰ of halloysite being -899.4 ± 1.0, and of endellite, as Al₂Si₂O₅(OH)₄, -901.7 ± 1.0, and kaolinite, -904.0 ± 1.0. Other chemical analyses are documented.

The direction of reaction, enrichment of A1₂O₃, and development of alunite are thought to have resulted from intense acid sulphation. A unique set of geologic-geomorphic conditions over a small area accounts for the geochemical situation responsible for this occurrence.