X-ray powder diffraction studies have been made of the structure changes which occur on dehydrating the minerals diaspore and goethite, and also the synthetically-prepared “δ-Fe2O3.” For diaspore and goethite the changes are interpreted in terms of superstructures referred to a hexagonal, anion lattice sub-unit which is common to both the α-type oxyhydroxides and the anhydrous corundum or hematite structures. The ferromagnetic δ oxide, obtained by rapid oxidation of ferrous hydroxide, is shown to be a true oxyhydroxide of the form FeOOH. It transforms into goethite on heating at about 150°C but the change is incomplete due to a tendency to lose water directly to the atmosphere. An approximate structure analysis has been made, using the X-ray powder data for two relatively well-crystallized δ oxide preparations. It is concluded that the ferric ions occupy the interstices of the anion lattice in such a way that approximately 80 per cent. of the iron is randomly distributed in octahedral sites, while the remainder is similarly placed in tetrahedral sites. This cation distribution enables both the transformation to goethite and the magnetic properties of the oxide to be satisfactorily explained.

The transformations in the iron oxide-hydroxide system have been interpreted in a rational crystallochemical manner, some being characterized as topotactic and some as non-topotactic. Crystallographic measurements have been made on the more reactive or metastable phases, particularly on the “green rusts.” The oxyhydroxides β-FeOOH and δ-FeOOH have been examined more fully, but data on all the phases have been checked. New data on transformations, such as FeCO3 → FeO and Fe(OH)2 → FeO are reported.

The apparatus described enables investigations to be carried out in any desired gas or vapour at pressures not far removed from atmospheric. Two different specimen-holder blocks have been developed: a nickel one for use with inert gases (e.g., nitrogen) and high sensitivity of recording, and a ceramic one for use with active gases and vapours (e.g., oxygen or water vapour) and rather lower sensitivities. Evacuation of the system to pressures of less than 0·05 mm Hg is also possible. The temperature control system consists of a commercial anticipatory temperature controller and a motorized autotransformer; the differential thermal curve is recorded photographically. Some results are given to illustrate the high sensitivity of the apparatus, its application to highlyorganic soil-clays and to the study of combustion reactions.

A genetic classification of clay rocks is given in which five main types are recognized. In this classification minerals of the kaolin group formed directly from hydrothermal solutions, along with sulphides, sulphates, and pneumatogenic minerals, are separated from the same minerals formed by the hydrothermal-metasomatic action of gases and vapours on aluminosilicate rocks. Some data on the occurrence of these two types in Azerbaijan are recorded. The origin of the bentonitic clays (“gilabi”) of Azerbaljan is also discussed; the geology of the deposits indicates cycles of volcanic activity and long-range transport of volcanic ash.

A report is given of the decisions reached at a meeting on the classification and nomenclature of clay minerals held under the auspices of Comité International pour l’Etude des Argiles at Brussels in July 1958. Tables of recently-proposed classification systems are included and some comments made on problems requiring agreement.

Only recently has it been possible to perform quantitative mineralogical analysis of clays and to establish the crystallochemical and morphological characteristics of the different clay minerals. The classification and nomenclature of these minerals is complicated by the existence of mixed-layer structures which are essentially epitaxial assemblages.