The old value of 0.048 per cent chlorine for the average in the earth's crust is considered too high, and an average of 0.02 per cent appears more probable. The average chlorine content of approximately 100 granitic rocks from North America and other parts of the world was found to be 0.022 per cent; of 20 gabbros, 0.021 per cent; and of 40 basalts and diabases, 0.014 per cent. Chlorine in ultramafic rocks may range from 0.005 to 0.2 per cent or more; alteration (as by autometamorphic processes) may account for the high chlorine contents.
A large fraction of the total chlorine in common igneous rocks occurs in hydroxyl minerals such as biotite and hornblende; presumably chloride ion replaces hydroxyl ion. These minerals may contain several tenths of a per cent of chlorine. In basic rocks containing much phosphorus (>1 per cent P2O5), apatite may account for much or most of the chlorine, but otherwise there is no close relation between phosphorus and chlorine. Some chlorine is also present as chloride in liquid (aqueous) inclusions, and the chlorine in such anhydrous minerals as the feldspars is probably present largely in this form.
Chlorine does not show any marked trend in the differentiation of the common igneous rocks. Since it is an element of well-developed atmophilic and hydrophilic characteristics, a general lack of correlation between chlorine and the major and minor constituents of rocks is not unexpected; an appreciable, perhaps large, fraction of the chlorine in a magma escapes on crystallization.
The ratio Cl/H2O+ shows a general increase from subsilicic rocks (basalts and gabbros) to silicic. This trend may be attributed to enrichment of Cl compared to H2O in the residual liquid as differentiation proceeds to more silicic products.
The general geochemistry of chlorine is discussed. Few elements have siderophilic, lithophilic, hydrophilic, and atmophilic properties all developed in comparable degree.