The desirable blue color of lazurite (lapis lazuli) and synthetic ultramarine blue is primarily due to the presence of S₃⁻ radicals. In natural lazurite, the color depends on the ratio of S₃⁻: S₂⁻ since S₂⁻ is a yellowish color. Continuous wave and pulsed EPR spectra and the relaxation times of S₃⁻ are compared for solution state S₃⁻(degassed DMSO), three sources of synthetic ultramarine blue samples, and for samples of lapis lazuli obtained from Afghanistan, India, Chile, and Colorado, USA.
For the lapis lazuli, strong temperature dependence of relaxation rates and very short phase memory times were attributed to spin-spin interactions, to a local mode mechanism, and to the motion of the radical in the sodalite cages. The mineral samples contained other components with large spectral contributions from Mn²⁺. Features in the spin-echo detected spectra attributed to forbidden Mn²⁺ transitions were confirmed by comparison with Mn²⁺ spectra in CaO powder. The high spin concentration synthetic ultramarine blue spectra exhibited field-dependent spin-echo phase behavior.
For the solution-state S₃⁻ in degassed DMSO, longer relaxation times were observed, likely due to the lower concentration of S₃⁻ radicals. The temperature dependence (4K−50K) of T_m and T₁ was also studied.

Deborah Mitchell

University of Denver

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