Kaolinite From Warsaw Geodes, Keokuk Region, Iowa

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Proceedings of the Iowa Academy of Science, Vol: 70, Issue: 1, Page: 261-279

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Hayes, John B.
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White, compacted masses of kaolinite are common in the famous geodes from the upper mudstone unit of the lower Warsaw Formation around Keokuk, Iowa. X-ray diffraction studies of the fine silt-size, pseudhexagonal grains show the kaolinite to be exceptionally well crystallized and free from impurities. Stacking faults are produced by grinding the kaolinite, which have a marked effect on X-ray patterns. The relationship of kaolinite to other mineral euhedra in geode cavities indicates that kaolinite formed early in geode history. Several lines of evidence suggest that the kaolinite originated from the fine-grained silicate residue remaining after acid dissolution of early diagenetic calcareous concretions, the progenitors of geodes. The kaolinite formed in an acid environment into which aluminum was introduced to react with the layer silicate "wreckage" from the concretions.