Zero-Field EPR of Fe3+ in Quartz

The zero-field EPR transition frequencies of Fe3+ in synthetic brown quartz (SBQ) and natural amethyst (NA) have been measured directly. The values obtained, 7113.2 ± 0.2 and 8812.5 ± 0.2 MHz for SBQ and 24 157.5 ± 0.5 and 35 365 ± 2 MHz for NA, are in excellent agreement with those calculated from the spin Hamiltonians derived from ordinary high-field EPR spectroscopy.