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Electron spin resonance and optical spectra of sodalite Na8(Al6Si6O24)Cl2 have led to the proposition that colloidal metal clusters are responsible for the blue coloration of this compound. F-centres generated in sodalite by ionizing radiations give rise to a pink coloration. Annihilation of the F-centres by visible light is suggested as the mechanism by which the colloids are formed. A deep blue coloration is acquired by the sodalites after a number of colour-bleach cycles.

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