The oxides of iron, aluminium, and manganese
THE oxides (including the hydroxides and hydrous oxides) of iron and aluminium are, along with silica, the most common accessory minerals in clays; manganese oxides, on the other hand, occur more sporadically. Nevertheless, it is appropriate here to consider these oxides together, since there are many fundamental resemblances not only in composition and structure, but also in occurrence and origin. The crystalline oxides and hydroxides of iron and aluminium are well-defined and distinctive in character but those of manganese are more poorly-defined and the validity of certain reputed species is as yet by no means certain: in consequence, only well-defined species can be considered. All three metals also apparently form oxides with too low a degree of order to diffract X-rays or even electrons: these will be termed amorphous. Structural relationships between many of these oxides have been discussed by Rooksby (1961) and thermal characteristics by Mackenzie (1957).