FELDSPAR CHEMISTRY FROM THE MILLBRIG K-BENTONITE AT SHAKERTWON, KY

Authigenic K-feldspars are characterized by Or contents near end member composition with only trace amounts of Ab or An content. Trace amounts of Ba and Sr occur in the authigenic feldspars. Strontium bearing feldspars occur in the lower 14-cm of the Millbrig while barium containing feldspars occur throughout the bed. This separation in chemistry may indicate multiple generations of diagenetic feldspar growth in the Millbrig. The primary volcanic sanidine compositions range from Or60 to Or89 with an average composition of Or75. Trace amounts of Sr (0.006 to 0.167 wt % SrO), and Ba (0.025 to 1.38 wt % BaO), were detected in most sanidine while Fe (0.020 to 0.271 wt % FeO) and Mg (0.005 to .045 wt% MgO) were detected only in a few sanidines. Both Or content and Ba concentration of sanidines are split at the base of the Millbrig and converge at the top of the unit. This phenomenon is interpreted as a result of phase separation close to and perhaps during the eruption of the magma and not the result of mixing of different ash from separate eruptions. Although the sanidine data set points to a single genetically related volcanic event, this conclusion cannot be substantiated without detailed studies of the Millbrig from different localities.