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 SiO 2 :Al 2 O 3 ratio in the allophane is 1.29:1, representing typical enrichment in allophane of Al 2 O 3 , relative to the SiO 2 :Al 2 O 3 ratio in kaolinite. The calculated apparent standard free energy of formation of this allophane is -900.14 + or - 47 kcal per formula weight that is based upon the kaolinite formula. Its stability in aquenous environment is approximately the same as halloysite; the calculated F f0 of halloysite being -899.4 + or - 1.0, and of endellite, as Al 2 Si 2 O 5 (OH) 4 , -901.7 + or - 1.0, and kaolinite, -904.0 + or - 1.0. Other chemical analyses are documented. The direction of reaction, enrichment of A1 2 O 3 , 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.

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