Some aspects of one-mineral geological environment indicators are discussed. Factors controlling the composition of biotite in different P, T, and chemical environments are reviewed. On the basis of theoretical considerations and empirical observation it is concluded that biotites in igneous granites may tend to be somewhat richer in iron than biotites from gneisses, but the relation is not strict, and the Fe/Mg or FeO/MgO/Al2O3 ratios in biotite do not provide unequivocal indications of the origin of biotite-bearing rocks
