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 $A11A=790±10,A22A=1575±10,A33A=1730±10×10−4⁢T$⁠, and $A11B=695±10,A22B=1270±10,A33B=1530±10×10−4⁢T$⁠. Estimated electron spin density coefficients $Cs2$ and $Cpz2$ of both Pb ions A and B are $ACS2=0.04,ACpz2=0.44,BCs2=0.05$⁠, and $BCpz2=0.31$⁠. 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 anse.

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.