Summary: Natural Al-Fe-Mn epidotes are divided into Al-Fe and Al-Fe-Mn series.
In the Al-Fe series, a, b, and c increase steadily over the range 0 to 0·9 Fe3+ per formula unit, and more slowly at higher Fe contents. Index variation diagrams show the reverse trend, with indices increasing moderately fast over the range 0 to 0·85 Fe, and faster at higher Fe contents.
In the Al-Fe-Mn series, b increases almost linearly over the range of (Fe, Mn) contents, with Mn causing a larger increase than Fe, whilst a and c increase over the range 0 to 0·9 (Fe, Mn), and then decrease. This curious behaviour is accounted for by the application of crystal field theory. The indices increase linearly over the range 0·8 to 1·5 (Fe, Mn), the optic axial angle depending on (Fe, Mn) content and the ratio Mn/(Fe, Mn).
Thermal and mechanical properties (conductivity, thermal expansion, specific heat, compressibility, and hardness), electromagnetic properties (dielectric constants, conductivity, magnetic susceptibility), and thermodynamic data (S) are also summarized, and estimates given where experimental data are not available.