Characterization of the Waukesha Illite: A mixed-polytype illite in the Clay Mineral Society repository

摘要

The Waukesha Illite is an excellent example of the illites found in argillaceous rocks, typical for Paleozoic shales that have undergone significant burial diagenesis during their geologic history. It consists of a mixture of detrital 2M₁, interpreted to be a residuum of karstification within Silurian carbonates, and diagenetic 1M and 1M_d illite. The chemistry and the age of the illite polytypes are different. Extrapolating to 100%, the 1M and 1M_d polytypes have an apparent diagenetic age between 295 and 325 Ma. The chemistry of the 1M polytype could not be determined because of its low abundance. The approximate chemical composition of the 1M_d polytype is 0.67 K, 3.6 Si, and 1.9 Al per half unit cell. The 2M₁ polytype has an apparent detrital age between 440 and 520 Ma, and an approximate chemical composition per half unit cell of 0.78 K, 3.4 Si, and 2.1 Al, all within our margin of error.