The source of the purple zircons in the sedimentary rocks of Scotland

Abstract

In a former communication 1 I directed attention to the prevalence in many of the sedimentary rocks of Scotland of a particular type of zircon crystals, characterised by a purple colour of varying intensity and often exhibiting rounded or ovoid forms. Not only was their outline found in very many instances to be rounded to such a degree—and this much more frequently than in the case of the accompanying colourless zircons—that they appeared not infrequently as all but perfect spheres, but their surfaces were often seen to be finely polished, suggesting the operation of wind action as the agency by which the rounding had been accomplished. Where the individual grains had suffered less attrition they occasionally showed a well-defined colourless outer zone. Their centres were at times so deeply coloured as to be practically opaque in transmitted light. Some of them showed fine zoning with or without a dark central spot from which irregular fissures radiated to the periphery. Often the colouring was uniform throughout, in which case they might or might not show the presence of different kinds of inclusions. Their characters were so uniform as to suggest a common origin, and the object of the present paper is to attempt to trace that origin. Where during geological times there have been so many sources from which zircons have been contributed to the sedimentary rocks this at first sight might seem a particularly hopeless task. The zircons found in sedimentary rocks have for the most part been originally derived