On the atomic arrangement and variability of the members of the montmorillonite group (With Plate V.)

Abstract

According to their atomic arrangement the minerals found in clays can be divided into the two-layer and the three-layer structures. The members of the first group have alternating silica and alumina layers, e.g. kaolinite; the members of the second group have one alumina (or magnesium or iron) layer between two silica layers, e.g. pyrophyllite. The clay minerals can also be classified into two groups according to their power to diffract X-rays. The first group gives good powder photographs with twenty to forty well-developed lines if copper or iron radiations are used, whereas the second group gives poor diagrams with only a few lines, usually under fifteen, some of which are wide and indistinct. Exposure times for the second group are about two to three times as large as those for the first group. The poor diffracting power for X-rays may be due either to a lack of regularity (‘roughness’) of the lattice, or to an extremely small size of the single crystallites. It is possible that both effects are aetive to some extent, but the lack of regularity of the lattice is probably the main reason. Neither the microscopic evidence nor sedimentation data based on Stokes's law show these minerals to have particles smaller than 10-6 cm., though it is possible that the optical effects such as double refraction are due to aggregation, and it is clear that the equivalent radius is an unsatisfactory measure for plate~shaped particles. Both classifications are shown on p. 141.