Perthite-like exsolution in the nepheline-kalsilite system

Forelesninger holdt ved Advanced Study Institute of Feldspar - Oslo 1962
Lectures held at Advanced Study Institute of Feldspar - Oslo 1962

The analogy between the alkali feldspars and the feldspathoids of composition (Na,K)AlSiO4 would suggest that, within a certain range of the alkali ratio, an exsolution into a sodium-rich and a potassium-rich component takes place in the NaAlSiO4-KAlSiO4 system. In fact, the existence of a relatively large gap in solid solubility between nepheline and kalsilite has been demonstrated by Tuttle and Smith (1958) using synthetic materials. The phase relationships presented by these authors are very complicated, especially in the potassium-rich part of the phase diagram. Due to loss of alkalies at high temperatures, some of the charges used may have been off the strict NaAlSiO4 - KA1SiO4 system. Therefore, as was mentioned by these authors, the phase diagram presented may be considered only tentative. The existence of the large solvus limiting the two-phase field is, however, definite.