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The AAPG/Datapages Combined Publications Database
Journal of Sedimentary Research (SEPM)
Abstract
Allophane and Na-Rich Alunite from Kaolinitic Nodules in Shale
W. D. Keller, (2) Richard J. Gentile, (3) A. L. Reesman (2)
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 Ff0 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.
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