Crystals of brookite tabular parallel to the basal plane

Abstract

During the last year, Dr. R. H. Rastall has been engaged on an investigation of the mineral content of certain sandstones of Middle Jurassic age in north-east Yorkshire. These sandstones proved to be unusually rich in the titanium minerals rutile and anatase. In some samples the proportion of the last-named is greater than in any other specimen previously recorded, so far as can be ascertained. Some of the heavy-mineral separations also contained a number of crystals of quite unmistakable brookite, with normal crystallographic development and normal optical properties, namely Bx perpendicular to (100) and intense dispersion ρ > v. Associated with these, however, there were often observed a number of smaller crystals of very similar appearance, whose optical orientation seemed to be entirely different. In particular, no optic picture could be obtained, and the extinction between crossed nicols in parallel light was perfectly sharp, with no suggestion of strong dispersion. Dr. Rastall therefore asked me to carry out some further investigations into the nature of these crystals, which were unlike anything he had previously seen in sedimentary rocks.