gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1979.043.326.06 The Blue Coloration in Banded Fluorite (Blue John) from Castleton, Derbyshire, England Galwey A. K. 1 Jones K. A. 2 Reed R. 3 Department of Chemistry University, Belfast BT7 1NN, Northern Ireland Department of Geology University, Belfast BT7 1NN, Northern Ireland The Electron Microscopy Laboratory, The Queen's University, Belfast BT7 1NN, Northern Ireland Dollimore D. Department of Chemistry and Applied Chemistry, University of Salford, Salford, M5 4WT, Lancashire, England 6 1979 43 326 243 250 Copyright © 1979, The Mineralogical Society 1979 Summary

From the microscopic examination of lightly etched, cleaved (111) surfaces of banded blue fluorite (Blue John, from Castleton, Derbyshire) it is concluded that the coloured lamellae are not necessarily or exclusively associated with lattice-line imperfections. Elemental analyses, by electron dispersive methods, of fresh cleavage (111) surfaces in the immediate vicinity of small inclusions (c. 10 µm diameter) possessing associated coloured haloes detected no appreciable concentrations of impurities. From the evidence available, it is suggested that the zones of blue colour consist of colloidal calcium resulting from radiation damage caused by the intermittent deposition of radioactive material on the surfaces of fluorite during crystal development. The dispersion of colloidal calcium produced is particularly stable as a consequence of the close correspondence between lattice spacings in calcium fluoride and in calcium metal.