Mineralogy of Ordovician K-Bentonites in Kentucky
Preliminary results of investigations of Trenton-Black River (Ordovician) K-bentonites from well cores in northern Kentucky and from outcrops along the Kentucky River illustrate the variations in some physical and chemical properties. It is hoped that some features may eventually be useful in stratigraphic correlation, considering that the very nature of these altered ash beds makes them ideal time-lines.
That these beds are derived from volcanic ash is evidenced by significant amounts of biotite, magnetite, leucoxene, apatite, and zircon.
The following means of investigation have been employed: X-ray diffraction, DTA, cation exchange, heavy mineral studies, and insoluble residue studies of adjacent limestones. X-ray analyses of untreated and glycolated clay samples indicate a mixed-layer, dioctahedral, illite-montmorillonite type structure with an I-M ratio varying from 4:1 to 3:2. The proportion of expandable layers in many samples is higher than that reported for Ordovician K-bentonites elsewhere. No evidence of chlorite is detected.
DTA data show a primary hydroxyl peak near 725°C suggesting a basic montmorillonite-type of hydroxyl bonding. Treatment with 1M solutions of various cations produces a slight expansion of the expandable layers with Ca2+, and contraction with Na+, K+, and NH4+ giving some indication of original environmental composition. The evidence supports suggestions by earlier authors that the clays are basically montmorillonites which have undergone partial contraction due to the adsorption of potassium in the interlayer positions.
While euhedral zircons are most likely non-detrital, recent work on zircons in igneous rocks suggests the same may also be true for some of the sub-rounded and fractured species.
Insoluble residues of limestones contain abundant chert and mixed-layer illite-montmorillonite clay directly adjacent to the ash beds, but farther away illite and chlorite dominate and are probably detrital in origin.
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