Abstract: Regular vertical variations in mineralogy and chemistry indicate that underclay beneath the Herrin (No. 6) coal in southwestern Illinois has undergone in situ alteration. Alteration resulted from the downward movement of hydrogen ions, as indicated by the progressive leaching of acid-sensitive minerals adjacent to the coal. Mineralogical trends observed in the underclay with increasing depth below the coal include: (1) a decrease in the expandability of mixed-layer illite/smectite (I/S); (2) an increase in the amount of ordered I/S with respect to randomly interstratified I/S; (3) an increase in the amount of discrete illite with respect to expandable clays; and (4) an increase in chlorite and calcite. Ordered I/S is the dominant mixed-layer clay where calcite is present, but randomly interstratified I/S dominates where calcite is absent. The pH of the underclay also increases with depth. These trends are consistent with an origin by acid leaching of a preexisting mineral assemblage that included illite, chlorite, and calcite. Other acid-alteration trends may be expected for different precursor minerals and for different leaching intensities and durations.