Trioctahedral ferromagnesium micas from the Archaean amphibolites of the Kola super-deep borehole (KSDB-3) complex have been compared with those from analogous Archaean surface rocks in an integrated study of their chemical compositions and structural states (using wet chemistry, microprobe, X-ray, Mössbauer and infrared methods). Reports on both the experimental procedure and the crystal chemistry of the trioctahedral micas are given. This has enabled the revision of the assignment of individual infrared (IR) absorption bands for the hydroxyl ion (νOH−), and demonstrated the possibility of determining the content of Mg, Fe2+, R3+ cations and vacancies in the octahedral sheet to within a few hundredths of atomic units on the basis of the infrared and Mössbauer data. Proof was found of the presence of molecular water replacing K+ in the interlayer, which results in anomalous variations in the cell parameters, as the mica structure expands in response to the ingress of H2O. Variations in the degree of non-stoichiometry of the micas is related to the presence of structural H2O and of octahedral M1 vacancies. We found that samples recovered from depth within the borehole display lower degrees of octahedral order than those found in the analogous surface rocks.