Abstract: To explain the large values of the quadrupole splittings and line widths in the Mössbauer spectrum of oxybiotite, a formalized analysis of the distribution of effective charges in the anion network was conducted using an idealized model of the biotite octahedral layer. Based on this analysis, a qualitative physical model has been proposed, according to which the Mössbauer spectrum consists of a superposition of six quadrupole doublets. The proposed model explains the appearance of residual magnetization well below the biotite decomposition temperature. Annealing the biotite at 1070°K leads to the formation of nuclei of the magnetic phase in chains consisting of R3+ and vacancies. This phase is the decomposition product of biotite at 1370°K, i.e., a ferrispinel having Fe2-xMgAlxO4 (x = 0.5–0.6) composition.