Abstract
Ions with charges of four or more, and also some trivalent ions, form complexes with oxygen and hydroxyl in the magma. It is necessary to study the properties of the complex as a whole rather than the properties of the individual ion, in order to predict its behaviour. In most cases the complexes are not readily accepted into silicate minerals, and hence become enriched in residual magmas, eventually reaching a concentration which is sufficient to precipitate them in the form of primary minerals. This often occurs at the pegmatitic stage of differentiation. The influence of volatiles (in the form of (OH) −1 and F −1 ions) on trace-element behaviour is discussed in terms of the constitution of silicate melts. A parent magma must possess a high initial content of volatiles if rare elements are to be concentrated in pegmatites derived from such a magma, The effect of the volatiles is to increase the solubility of accessory minerals and therefore to prevent early crystallization and separation of rare elements in the form of accessory minerals. Ni 2, V 3, Cr 3, and Sc 3 are usually concentrated in early crystallates from a magma. However, in the presence of a large percentage of volatiles these ions may form complexes and become concentrated to some extent in residual magmas.
Published Version
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