The common pale-blue color of celestite bleaches at about 200°C and reappears upon X-ray irradiation. A thermoluminescence maximum for blue celestite also occurs around 200°C. The thermal stability of the blue color (as measured by the bleaching time at 190°C) was found to be proportional to potassium content. Electron spin resonance data indicate the presence of SO4, SO3, SO2, and O in blue celestite crystals. These paramagnetic hole centers presumably arise by the action of ionizing radiation on SO42-, which releases energetic electrons that can decompose other SO42- groups. The centers SO2, SO2, and O absorb in the visible, producing the blue color, while SO4 absorbs in the ultraviolet. The color-producing centers are stabilized in the celestite lattice by the presence of trace components, primarily K.+ substituting for Sr2+.

The color in two specimens of orange celestite was correlated to the presence of copper, probably as Cu+, in the order of 50 ppm.

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