AN ELECTRON MICROSCOPY INVESTIGATION OF THE HETEROGENEITY OF MINERALOGY OF THE MILLBRIG K-BENTONITE LOCALITY AT SHAKERTOWN, KENTUCKY

An exposure at Shakertown, Kentucky of the Middle-Ordovician Millbrig K-bentonite is one of the important reference sections for this regional stratigraphic horizon. This unit at this location has been debated as representing a single volcanic event or multiple events. Furthermore the unit is being investigations for it’s potential as an applied material such as in catalyst supports. Understanding the variability of the mineralogy of this unit places constraints on both mode of origin and application potential. The clay mineralogy is dominated by R1 illite-smectite and varies little stratigraphically with minor amounts of goethite observable in transmission electron microscopy near the bottom contact. Quartz, potassium feldspar (sanidine), apatite, zircon, rare earth phosphate and iron titanium oxide are common grain minerals. Scanning electron microscopy indicates that a wide range of textures in grains exist including evidence for mechanical rounding and authigenesis. Delicate textures on K-feldspars exist suggesting epitaxial growth phases. Limited melt inclusion data from previous work provides a reference for alteration, and indicates a single geochemical trend however data is available only from a single horizon. Discrete irregular particles of galena that commonly are 10 to 15 micormeters in diameter were observed occasionally and pose some concern for applied environmental work. Several grains of euhedral pyrite commonly 20 to 100 micrometers in diameter were also observed which likely would have a negative impact on catalytic applications owing to oxidation of sulfur. Alteration (oxidation) of pyrite is evident form SEM imaging of pitting and pyrite oxidation is interpreted to be the source of goethite near the lower contact as observed in transmission electron microscopy. Results suggest that reworking occurred and multiple phases may be represented by the Millbrig at Shakertown, however there are clear arguments for single or dominant discrete phases of eruption. From the perspective of applied mineralogy the unit is fairly homogeneous and although galena and pyrite are minerals of concern, mineral separation methods should be able to purify the illite-smectite and potential exists for applied materials use.