SUMMARY, 'Blue John' and other purple fluorites have been studied by a wide range of techniques. Hydrocarbons are not the cause of the colour of these fluorites. No significant difference in the concentrations of 73 elements between the purple and colourless zones of Blue John has been detected.The spectroscopic and thermal bleaching behaviour of the purple zones is consistent with the presence of 'colloidal' calcium as colouring agent, and coloration of fluorites with calcium vapour gives material showing similar properties. The distribution of colour suggests trapping of the calcium in lattice defects along {I x I} (cleavage) planes through {ooi} (growth) zones. The 'colloids' could be produced by aggregation of calcium atoms liberated by irradiation from sources found in the geological environment of Blue John. The banding could arise from variations in the concentration of lattice defects caused by variations in growth rates of the fluorite, MACKENZIE AND GREEN (I97 0 have recently described a study of the cause of coloration of 'Blue John', the bluish-purple banded fluorite from Castleton, Derbyshire, and similar fluorites from Ashover, Derbyshire, and Weardale, Co. Durham, and conclude that the colour is due to the presence of 'colloidal' (dispersed) calcium. The present authors have independently studied Blue John and similar coloured fluorites from other localities using a wide range of experimental techniques, and present additional evidence that supports colloidal calcium as the cause of the colour.Materials. The basic material used in our studies was intensely blue-purple fluorite, containing bands of colourless material, from Blue John Cavern, Castleton, Derbyshire (entrance at National Grid Ref. SK I32832). Whenever possible, experiments on purple material were repeated under identical conditions with material picked from a colourless band immediately adjacent to the purple sample.Other materials studied for purposes of comparison are detailed in Table I. No fluorite samples from Weardale of this type of deep purple colour have been seen by the authors. As Calas (I972) points out, more than one cause of colour of blue to purple fluorites exists. Relatively pale mauve to purple, as well as green and yellow fluorites, all with a strong blue fluorescence, are common in Weardale and elsewhere in the northern Pennines, the colours and fluorescence being ascribed to the presence of rare earths, in particular europium, gadolinium and samarium (Dunham, I95Z; Bill, Sierro, and Lacroix, I967). The Blue John-'antozonite' type of purple colour is normally much more intense than these rare earth colours, and is easily seen irt thin section. The only fluorite sample of those studied that displays fluorescence is 9
