The field of nepheline ss covers a considerable portion of the system between the alkali feldspar and nepheline-kalsilite joins. Figure 7.1 shows that there is a large field of leucite ss⋆ which extends even beyond the albite-sanidine join. 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. 7.1), known as the nepheline syenite minimum. 7.1) known as the granite minimum, and another one which lies in the silica-undersaturated region (M, Fig. There are two temperature minima, one on the boundary between the tridymite and feldspar fields (G, Fig. Figure 7.1 shows that the join albite-sanidine divides the system into a silica-saturated and a silica-deficient portion. ![]() He thus called the system “petrogeny’s residua system”, which was investigated by Schairer and Bowen (1935) and Schairer (1950). 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-SiO 2 system. ![]() This is probably the most fundamental system related to the genesis of highly undersaturated alkalic rocks.
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