Leucite-Bearing Ternary Joins and Systems

This is probably the most fundamental system related to the genesis of highly undersaturated alkalic rocks. Bowen (1937) considered that after fractional crystallization of a primary basalt magma the residual liquid will be highly enriched with respect to alkali, alumina, and silica and its bulk composition should lie in the nepheline-kalsilite-SiO2 system. He thus called the system “petrogeny’s residua system”, which was investigated by Schairer and Bowen (1935) and Schairer (1950). Figure 7.1 shows that the join albite-sanidine divides the system into a silica-saturated and a silica-deficient portion. There are two temperature minima, one on the boundary between the tridymite and feldspar fields (G, Fig. 7.1) known as the granite minimum, and another one which lies in the silica-undersaturated region (M, Fig. 7.1), known as the nepheline syenite minimum. While the silica-rich portion of the system is important to understand the genesis of granitic rocks, the discussion here is mainly concerned with the silica-poor region of the system. Figure 7.1 shows that there is a large field of leucitess⋆ which extends even beyond the albite-sanidine join. The field of nepheliness covers a considerable portion of the system between the alkali feldspar and nepheline-kalsilite joins. The field of nepheliness is separated from that of orthorhombic kalsilitess at about 65% KAlSiO4 when the former does not contain excess SiO2, and at 80% when nepheline contains about 16% excess silica (Fig. 7.1). The course of crystallization of a melt can be best understood by considering liquids of three different compositions.