Leucite-sodic plagioclase and melilite-plagioclase do not coexist in natural rocks. However, leucite and calcium-rich plagioclase may remain in equilibrium in alkaline silica-deficient rocks. The join leucite-akermanite-albite studied at atmospheric pressure is characterized by two four-phase points: one point occurs at leucite
31akermanite
47albite
22 and 1275 °C, where leucite
ss, melilite
ss and diopside
ss coexist in equilibrium with liquid, and the other point occurs at leucite
38akermanite
22albite
40 and 1250 °C, where diopside
ss, alkali feldspar, leucite
ss and liquid are in equilibrium. In the case of the join leucite-akermanite-albite
50anorthite
50 investigated at atmospheric pressure, there are two four-phase points: one point occurs at leucite
14akermanite
48(albite
50anorthite
50)
38 and 1260 °C, where melilite
ss, leucite
ss, diopside
ss and liquid coexist in equilibrium, and the other point occurs at leucite
15akermantite
35(albite
50anorthite
50)
50 and 1240 °C, where diopside
ss, leucite
ss and calcium-rich plagioclase are in equilibrium with liquid.
When the join leucite-akermanite-albite
50anorthite
50 was studied at 1 GPa (in H
2O-saturated conditions) to determine the effect of pressure, it was noted that the field of melilite
ss completely disappears, and this join is characterized by a single four-phase point occurring at leucite
43akermanite
22(albite
50anorthite
50)
35 and 1060 °C, where diopside
ss, calcium-rich plagioclase, and leucite
ss are in equilibrium with liquid. The present study indicates that: 1) leucite and sodic plagioclase do not coexist in equilibrium. When leucite is in equilibrium with feldspar, it is alkali feldspar having the compositional range, An
3-4Ab
50-53Or
45-47. 2) In the presence of leucite, akermanite and albite react to form diopside
ss, nepheline
ss, wollastonite
ss, and alkali feldspar. 3) In the join leucite-akermanite-albite
50anorthite
50, plagioclase coexisting with leucite has the compositional range, An
60-66Ab
17-26Or
12-17. 4) In the join leucite-akermanite-albite
50anorthite
50 studied at 1 GPa (in H
2O-saturated conditions), melilite is completely eliminated.
View full abstract