Potassic Rocks and the Lamproite Clan

Abstract

Sørenson (1974, p.7), in a review of the varied definitions proposed for alkaline rocks, concludes that the term has been used by so many petrologists in so many different ways and usually so vaguely that “it is hard to know what is covered by the term.” Alkalinity implies enrichment in the alkali elements, sodium and potassium, but this in turn can be related to either the silica or alumina content of the rock. There is no general agreement among petrologists as to the amounts of Na₂O and K₂O that need to be present for a rock be termed alkaline, and commonly the question is carefully avoided by many petrologists in discussions of alkaline rocks.

Many protests have been made against the use of long compound names but unless they become ridiculously cumbrous they are thoroughly justified in the interests of clearness.

Arthur Holmes(920)