Potassium: Petrogeny’s Residua System and Ternary Feldspars

The main effect of potassium when present in the small amounts typical of most basalt is to lower temperatures slightly and enter into the feldspar as small amounts of the orthoclase molecule, KAlSi3O8. Potassium is a large alkali ion (1.46 Å radius if we use the compilation of radii by Whittaker and Muntus 1970), considerably larger than sodium (1.10 Å). Because of its large size, it is almost entirely excluded from the common mafic minerals of basalt, and it occurs in only limited amounts in calcic plagioclase. For this reason, K is conserved in residual liquids upon fractional crystallization of most basaltic compositions, because it finds a happier home in the liquid structure than in the structure of the average crystals. This effect is called partitioning. The result of the fractionation process can often be seen in the micropegmatite mesostasis (commonly sanidine + quartz) of some tholeiitic basalts. Although we may rightly say the effect of potassium is minor in such a role, we must nevertheless strive to consider its behaviour in phase diagrams if we wish to follow the question of what basaltic liquids may become as they crystallize. Moreover, despite its small, seemingly trivial abundance in basalts, it would be a grave mistake indeed to treat potassium lightly in any discussion of basalt genesis, for even rather small variations in potassium content appear to be very important indicators of basalt type, source, or even mode of origin. And finally, there is a whole class of strange potassic igneous rocks, whose basic members may include leucite basalt. Although we shall not attempt here to explain why some suites of rocks are potassic, we shall at least explore some of the consequences.