Electron paramagnetic resonance of single crystals of sanidine was studied at frequencies of the X band (∼9.5 GHz) between 4.2 K and 295 K and the Q band (∼35 GHz) at 295 K. Five broad resonance lines at geff ≈ 7.9, geff ≈ 4.3, and geff ≈ 3.7 (X band) and at geff ≈ 2.04 and geff ≈ 2.003 (Q band) were identified. These five g values refer to the orientation B parallel to c*. The lines at geff ≈ 4.3 and geff ≈ 3.7 result from transitions ms = ±3/2, those at geff ≈ 7.9 from the transition ms = 5/2 → 3/2, and those at geff ≈ 2.045 and geff ≈ 2.003 to transitions ms = ± 1/2. These five lines are due to Fe3+ at the tetrahedral positions T1 and T2 of sanidine. The lines geff ≈ 4.3 and geff ≈ 2.003 were assigned to the position T1, the lines geff ≈ 3.7 and geff ≈ 2.045 to the position T2. The parameters λ = E/D were estimated: λ ≈ 0.32 for T1 and λ ≈ 0.22 for T2. An additional, very weak resonance line at geff ≈ 6 (B parallel to a) in the Q band was attributed to Fe3+O3OH substitutions at T1 and/or T2, in agreement with a value λ ≈ 0 indicating pseudotrigonal symmetry.

Fe3+ ions in sanidine are disordered over the T1 and T2 positions but show some preference for T1. The kinetics for Fe3+ ↔ Al3+ as well as Fe3+ ↔ Si4+ exchange appear to be slow, possibly slower than the exchange of Al and Si between these positions.

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