The Pendennis minette has an exceptionally high K₂O/Na₂O ratio and is strongly peralkaline. Its constituent minerals, which are described in detail, are sanidine, biotite, two carbonates (dolomite and ferroan magnesite), two amphiboles (riebeckite and a new variety, potassium magnesio-arfvedsonite), and quartz. The major and trace dement composition of the minette indicates close affinities with the lamproitic lavas.