Chemical and Physical Constraints for Crystallization of Feldspathoids and Melilite in Potassium-Rich Rocks
Leucite appears as a mineral in K-rich lavas, if the SiO2 content of the rock varies between 44 and 56 wt% (Washington 1906). When the SiO2 content is close to 56 wt%, it may appear but then the SiO2/K2O ratio should be below 10; in that case the K2O content of the rock is close to 6 wt%. In silica-deficient rocks (SiO2 < 45 wt%), leucite may appear even though the K2O content is slightly higher than 1 wt%. The K2O/Na2O ratio is always greater than one in leucite-bearing rocks, but the prohibitive tendency of high soda may be eliminated if the SiO2/K2O ratio is low.