Powders of twelve alkali halides have been coloured in an electrodeless discharge. Due to the simultaneous production and bleaching of F centres, F aggregate centres could be produced in an adequate concentration. From their study it has been concluded that powders behave as deformed single crystals. It could also be shown from this study that the interaction between colour centres and dislocations is lattice dependent and varies regularly as cation changes from Li+to Rb+, and anion changes from F−to I−.