Illite occurring in decomposed granite at Ballater, Aberdeenshire

The illite in some Old Red Sandstone soils and sediments

An illite separated from an Upper Old Red Sandstone sediment at Denholm Hill Quarry, Roxburghshire, when examined by optical, X-ray, and chemical methods, appeared to be closely similar to Fithian illite. Its composition was , where M+ for the Ca-saturated sample was [(H3O)0·58Ca0·10Na0·06K1·12]. The differential thermal curve showed two peaks: one at 550° C. and the other at 600° C. (i.e. somewhat intermediate between the Fithian and Ballater illites). The dehydration curve was also intermediate. Electron-microscope examination showed rather better crystallization than in Fithian illite. The possible reasons for variation of the illite peak temperature are discussed.