Abstract: Endellite is being formed south of Stanford, Kentucky, in an acid environment (pH 3.0–3.7; acidity at source, pH 1). Parent materials are principally a weathered residuum, derived presumably from the Brassfield Limestone that lies between the illitic, Devonian New Albany (Chattanooga) Shale and illitic, Ordovician Richmond Shale, and added Si and Al dissolved from the New Albany Shale. The source of the acid is the weathering iron sulfide in the New Albany Shale.
Apparent standard free energies of formation, calculated from aqueous solution data measured from the shales, endellite, and residual clays, indicate that essentially equilibrium conditions developed between the endellite, the illite of the New Albany Shale, and the permeating solution. Illites yield a range of values in Ff, owing presumably to variation in chemical composition and physical disorder in the crystals, which also may be polymorphic; hence, illites likewise vary in susceptibility to alteration and/or conversion of kaolin minerals.
It is suggested that Indiana endellite (“indianaite”) was formed at the Mississippian-Pennsylvanian unconformity in an environment similar to that (Ordovician-Devonian) yielding the Stanford, Kentucky, endellite.