The pseudobinary system NaAlSiO4 (nepheline, carnegieite)—Ca2MgSi2O7 (akermanite) has a pseudoeutectic point at 1210° ± 5°C and Ne67Ak33. Below the subsolidus temperatures forsterite appears and coexists with nepheline and melilite.
The pseudoternary system CaMgSi2O6 (diopside)—Ca2MgSi2O7 (akermanite)— NaAlSiO4 (nepheline, carnegieite), a join in the system NaAlSiO4-CaO-MgO-SiO2, has two piercing points: (1) One, at 1212° ± 5°C and Di55Ak6Ne39, shows liquid coexisting with forsterite, melilite, and diopside, the phase assemblage corresponding to olivine melilitite; (2) the other, at 1169° ± 5°C and Di38Ak3Ne59, shows forsterite, nepheline, and melilite coexisting with liquid, and this phase combination is found in nepheline melilitite. These two points are univariant and the crystallization moves towards the invariant point within the system NaAlSiO4-CaO-MgO-SiO2. At this point, about 1135°C, forsterite, diopside. melilite, and nepheline coexist with liquid, and this phase assemblage is the same as in olivine-melilite nephelinite.
Diopside, akermanite, nepheline, and carnegieite in the system diopside-akermanite-nepheline are solid solutions of varying compositions. The nepheline-carnegieite inversion, takes place over a range extending from 1250°C to 1208°C. The birefringence of melilite solid solution decreases with increasing content of NaAlSiO4. It is likely that sodium melilite (NaCaAlSi2O7) is incorporated into akermanite as a solid solution.