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Petrology of Tertiary Bentonites of Texas
Herman E. Roberson
On the basis of X-ray diffraction analyses and thin-section studies, bentonites from the Jackson and Gueydan Groups (late Eocene and Oligocene?, respectively) of Texas can be differentiated. Bentonites from the Caddell Formation and lower part of the Manning Formation are generally composed of well-crystallized Ca++ montmorillonites with very few impurities. Upper Manning and Cataholua bentonites are generally composed of poorly crystallized montmorillonite, and often contain a substantial amount of quartz, cristobalite, feldspar, and volcanic shards.
Bentonites from the Jackson Group and Gueydan Group in east and southwest Texas contain more quartz, feldspar, and clay minerals in addition to montmorillonite, than do those of central Texas.
Bentonites formed in place are generally composed of well-crystallized montmorillonite, whereas re-deposited bentonitic material is usually composed of poorly crystallized montmorillonite.
Potassium saturation with subsequent treatments reveals that the bentonites are composed of complex mixtures of different types of montmorillonite particles. There is apparently some substitution of Al3+ for Si4+ in the tetrahedral layer of most of the particles, and some of the particles are characterized by substantial amounts of aluminum in the tetrahedral layer.
Some of the bentonites from the Caddell Formation and lower part of the Manning Formation in central Texas are though to be lacustrine deposits.
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