EPR OF [PB-PB]3+ MIXED-VALENCE PAIRS IN AMAZONITE-TYPE MICROCLINE

Using electron paramagnetic resonance (EPR) at 9.2 GHz between 5 and 295 K, [Pb-Pb]3+ pairs, unknown in natural minerals, were studied in single crystals of amazonite of different colors and localities. The EPR data indicated two nonequivalent Pb ions, A and B, at adjacent K positions in the microcline structure. The calculated Hamiltonian parameters for the [Pb(A)-Pb(B)]3+ dimeric center are g11 = 1.80 +/- 0.03, g22 = 1.56 +/- 0.03, g33 = 1.36 +/- 0.03, and A11A = 790 +/- 10, A22A = 1575 +/- 10, A33A = 1730 +/- 10 x 10(-4) T, and A11B = 695 +/- 10, A22B = 1270 +/- 10, A33B = 1530 +/- 10 x 10(-4) T. Estimated electron spin density coefficients c(s)2 and c(pz)2 of both Pb ions A and B are (A)c(s)2 = 0.04, (A)c(pz)2 = 0.44, (B)c(s)2 = 0.05, and (B)c(pz)2 = 0.31. Stable [Pb-Pb]3+ dimeric centers can be formed only in ordered feldspar and only if one of the Pb2+ ions is charge compensated by Al,Si exchange at adjacent T1m positions. If the second Pb2+ ion is also compensated, no stable [Pb-Pb]3+ centers can arise. Heating at 543 K for 10 h caused Pb diffusion, and about 70% of Pb pairs were destroyed, whereas the color, EPR spectrum, and optical absorption (OA) band at 630 nm became unobservable. Subsequent irradiation can restore about 30% of the EPR spectrum, the OA band, and the blue color. Heating above 1073 K caused diffusion of the remaining Pb (about 30%), and the EPR spectrum, OA band, and color were destroyed irreversibly. The calculated activation energy for Pb diffusion for light blue, blue, and green amazonite in the temperature range 673-773 K is 12, 14.5, and 21 kcal/mol, respectively. Irradiation-induced, stable [Pb-Pb]3+ pairs causing the typical blue-green color were found only in amazonite-type microcline. In other similarly colored potassium feldspar and sodium feldspar, such centers are not known. Therefore, the name ''amazonite'' should be limited to classical, ordered microcline of blue-green color with [Pb-Pb]3+ pairs as the chromophore.