Native Cd⁺ in sedimentary fluorapatite

A native Cd+ centre (5s(1), S = 1/2) was detected in natural sedimentary fluorapatite using Electron Paramagnetic Resonance (EPR). The Cd+ centre generated by artificial X-ray irradiation of the same sample has an EPR spectrum similar to the native Cd+ centre but differs in its thermal stability. The artificial Cd+ centre exhibits fast thermal decay, i.e., t(1/2) = 380 30 h at room temperature (RT), although the native centre is stable at RT. Comparison with Cd+ centres obtained by X-ray irradiation of synthetic Cd-doped fluorapatite samples indicates that both the native and artificial Cd+ centres correspond to Cd+ ions substituting for Ca2+ in the apatite structure. Comparison with other artificial Cd+ centres already reported in various minerals suggests that the Cd+ centre in the studied fluorapatite sample mainly occupies the Ca(1) site with coordination number [9]. The apparent stability of the native Cd+ centre is not yet explained but could be related to charge compensation.