Note 15 — Chapter II · THE SILICATES
Reading edition
The symbol (Ca,Fe) here indicates that the iron and calcium occur in variable proportions, the variation however occurring in such a way that the equivalent of the two taken together is always the same, i.e. the iron can replace the calcium, or vice versa, atom by atom. The recognition of this possibility of ‘Vicarious Replacement’ between similar elements first brought order into the confused field of mineral chemistry, and allowed a systematic classification of minerals according to chemical composition to be made. Iron and calcium, or, according to the more convenient nomenclature of the mineralogists, lime and ferrous oxide, are here vicarious constituents.
The symbol Ce here stands for elements of the cerium group, which are never found singly.