The Origin of Missouri Fire Clays

Abstract

The origin of Missouri fire clays is interpreted from the testimony of the clays in terms of the geologic setting in which they were deposited and developed. Hence, the approach to the interpretation is fundamentally that of sedimentary petrology, rather than economic geology. The clays, as sedimentary rocks, are considered to be the products (mineralogic and geologic) resulting from the responses of their source materials to the energetics of the physical and chemical environments which were impressed upon them during the time interval of their deposition and diagenesis.

The fire clays are inferred to have a sedimentary origin; it is believed that their present-day characteristics were essentially developed in the time intervals of the Cheltenham to pre-Fort Scott (Pennsylvanian) age. The different varieties of fire clay are interpreted as different lithofacies within a continuous Cheltenham deposit. The high-alumina facies (diaspore-burley and boehmite) appears to have been formed by severe leaching on a relatively stable land area, whereas the kaolinitic-illitic facies (semi-plastic to semi-flint clay) seem to characterize products formed near the margin of a slowly sinking depositional region. Both processes of destruction and construction (“katamorphism” and “anamorphism,” as used by Van Hise, 1904) appear to have operated in the genesis of the clay minerals.

Evidence points toward a negative Eh and an approximately neutral to slightly acid pH during the formation of the clays. The presence of diaspore and boehmite and the absence of gibbsite in the high-alumina clay are interpreted as indications of leaching of clay minerals under waterlogged conditions. Diaspore may inherit part of the crystal structure and energy of the kaolinite from which it was derived.