The order-disorder of Fe2+ and Fe3+ in natural iron-rich biotite and its significance for petrology

The occupancy ratios of Fe2+ and Fe3+ in octahedral sites M1 and M2 of iron-rich biotites are determined, by means of measuring their compositions, Moessbauer spectra. A new parameter OR is suggested to describe the ordering of iron in iron-rich biotites. OR is an intense variable which depends on the equilibrium temperature. The functional relation between OR and the equilibrium temperature is established by using the idealized model of binary solid solution. The higher OR values represent the higher ordering of iron ions in the octahedral sites of iron-rich biotite while the lower equilibrium temperature keeps the structural states. It is convenient and effective to utilize OR of iron-rich biotite to explain petrogenetic evolution of granite complex and its cooling history with petrological studies