A clay occurring in an altered vein in a granite at Ballater, Aberdeenshire, has been examined by optical, X-ray, chemical, and thermal methods. The clay mineral is classified as illite although its thermal data are different from those of any illite so far described. As distinct from the usual illite thermograms, which show a peak in the region 500–600° C., this mineral shows two peaks: a broad one at 550– 600° C. and a sharp one at 713° C. A small amount of montmorillonite is found as a contaminant, but this is insufficient to account for the 713° C. peak, while the area of this peak increases as the montmorillonite content decreases. This peak therefore is assigned to illite.