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Room-temperature, single-crystal W-band (~94 GHz) electron paramagnetic resonance (EPR) spectra of a flux-grown fluorapatite (AP30-0) containing 0.8(1) ppm Gd and 0.9 ppm Mn disclose the presence of two even-isotope Gd 3+  centers (electron spin number S = 7/2 and nuclear spin number I = 0) and a  55 Mn 2+  center (S = I = 5/2). The relative abundance of the two Gd 3+  centers (corresponding to recently established centers “a” and “b” containing Gd 3+  ions at the Ca2 and Ca1 sites, respectively) in AP30-0 has been estimated to be 0.20, indicating that “a” in this sample arises from the presence of ~0.2 ppm Gd at the Ca2 site. In addition, high-resolution W-band spectra of this sample at ~120 and ~77 K disclose the  155 Gd and  157 Gd spectra of “a,” in which these isotopes (I = 3/ 2) are only ~0.02 ppm in abundance. To the best of our knowledge, this is the first-ever demonstration of structural characterization of sub-ppm-level trace elements in minerals and their synthetic analogs. Moreover, the fact that the Gd 3+  and  55 Mn 2+  centers in AP30-0 are detected despite the multiplicity of lines arising from their complex fine structures, hyperfine structures, and magneticsite splittings, suggests that the W-band EPR technique is potentially capable of characterizing trace elements with a single unpaired electron (S = 1/2) and zero nuclear spin (I = 0) at even lower concentrations.

american mineralogist

Electron paramagnetic resonance spectroscopic study of synthetic fluorapatite: Part III. Structural characterization of sub-ppm-level Gd and Mn in minerals at W-band frequency