Three Central Appalachian boreholes, each representing a distinct coal rank range (high volatile A bituminous, medium through low volatile bituminous, and semianthracite), were analyzed for vitrinite maximum reflectance and illite crystallinity for the coals and shales, respectively. The results reflect differential perfection of illite along its 001 crystallographic axis and its internal elemental arrangement, resulting from variations in ionic composition and strength of interstitial fluids and/or the type and composition of the dominant illite precursors. Despite the differences in the rate of metamorphic reaction in illite and vitrinite, generally, among the Weaver, Flehmig, and Kubler illite crystallinity indices, the Kubler index shows the best correlation with vitrinite maximum reflectance. (C) 2011 Elsevier B.V. All rights reserved.