Abstract: Although the wide range of minerals resulting from alteration of tuffaceous rocks under hydrothermal and weathering conditions is well known, the extent of variation in composition of discrete bentonite beds subjected only to normal diagenesis and compaction is perhaps less well known.
Three bentonite samples from the Atoka Formation of Pennsylvanian age are predominantly mixed-layer illite-montmorillonite with subordinate kaolinite. Most bentonites in the Fierre Shale of Late Cretaceous age are entirely montmorillonite, but a few contain some kaolinite; one is predominantly kaolinite; some also contain appreciable interlayer Al(OH)3 or Mg(OH)2 forming a mixed-layer montmorillonite-chlorite clay; several are composed of considerable clinoptilolite, and one is largely phillipsite. Bentonite beds in the Carmel Formation of Middle and Late Jurassic age differ among themselves in composition; in a few, illite is the dominant clay; in others, mixed-layer chlorite-montmorillonite-illite in different proportions is the dominant clay; kaolinite is common and makes up nearly half of one sample. A rhyolite tuff near Denver, Colorado, has been altered to halloysite. Reasons for variation in the composition of these bentonites are only partly understood.