During an extensive survey of brickmaking clays by the differential thermal analysis method (carried out from 1947 onwards) it was observed that the clays of the Keuper Marl gave a series of very similar thermal curves which could not be explained by the presence of known clay minerals. Dr D. M. C. MacEwan of Rothamsted Experimental Station kindly carried out an X-ray examination of one of the samples for us, and he has reported the results of this examination, and the examination of a Keuper Marl sample from another source, to the last meeting of this Group. He postulated the existence of a “hydrated chloritic material”—sometimes exhibiting an expanding lattice stabilised by glycerol.

Since then the Keuper Marls have been further studied at the Building Research Station both by thermal analysis and X-ray methods and this paper is an interim report of this work. MacEwan's findings have been confirmed and some additional information is presented.