Mössbauer, Thermomagnetic, and X-Ray Study of Cation Ordering and High-Temperature Decomposition in Biotite

V. I. Bagin, T. S. Gendler, L. G. Dainyak and R. N. Kuz'min
Institute of Geophysics, Academy of Sciences, Moscow, U.S.S.R.
Geological Institute, Academy of Sciences, Moscow, U.S.S.R.
Moscow State University, Moscow, U.S.S.R.

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.

Key Words: Biotite • Ferrispinel • Mössbauer spectra • Oxybiotite • Thermal decomposition • Thermomagnetism

Clays and Clay Minerals; June 1980 v. 28; no. 3; p. 188-196; DOI: 10.1346/CCMN.1980.0280304
© 1980, The Clay Minerals Society
Clay Minerals Society (www.clays.org)