FLEHMIG, W. & GEHLKEN, P.-L. 1988):
Chemical variations in the octahedral composition of Paleozoic illites and their genetic significance:
An infrared spectroscopic study.

Chemical changes in the illite/mica composition are expressed more sensitively and distinctively by infrared spectra than by various X-ray methods. A newly developed method based on measuring frequency shifts and extinction ratios of selected absorption bands enables a rapid and exact determination of chemical composition of unknown samples.
The examination of illites from several hundred Rhenohercynian and Ardennan pelitic rocks shows a systematic variation in the Mg+Fetot content with geological time. This is based neither on different source rocks nor on diagenetic and metamorphic processes but may reflect certain tectonic and volcanic cycles in the geological past.