Magmatic equilibria between feldspathoids and feldspars: Implementation of thermodynamic models and implications for norm calculation and petrographic interpretation

In contrast to thermodynamic models for metamorphic mineral solutions, which often concern quartz-saturated assemblages, the phase equilibria of feldspars, feldspathoids and melilite in the K₂O-Na₂O-CaO-Al₂O₃-SiO₂-H₂O space are much less established. They provide basis for classification and interpretation of alkaline silica-undersaturated rocks and essential constraints for developing normative calculation and classification algorithms. The existing normative schemes (CIPW, Müller, Le Bas, Rittmann and Curie norms) do not provide satisfactory treatment of nepheline, leucite, clinopyroxene and melilite stabilities and equilibria. In many conventional schemes (1) anorthite persists to very low silica activities despite of its instability and incompatibility with melilite; (2) normative larnite is a principal indicator of melilite presence but provides a poor chemical proxy; (3) compatibilities between alkali feldspar, leucite and nepheline solid solutions are incorrectly predicted (e.g., leucite-albite, feldspar-melilite); (4) desilication steps do not uniquely follow decreasing activity of silica. As a consequence, the norms applied to alkaline silica-undersaturated igneous rocks do not provide unique or correct view of mineral assemblages and their chemographic relations. In this contribution we explore phase equilibria and compatibility relations in the system SiO₂-CaAl₂O₄-NaAlSiO₄-KAlSiO₄-H₂O. We have adopted the thermodynamic models for nepheline, leucite and kalsilite solutions (Sack & Ghiorso 1998). The order-disorder models have been converted to the relevant sets of linearly independent end-members including ordered or anti-ordered intermediate members with macroscopic Margules parameters. These models were designed as transferable between Berman, Holland-Powell or other end-member datasets. In the composition space SiO₂-NaAlSiO₄-KAlSiO₄ at P = 1 bar and T = 1000 ^oC, alkali feldspar is compatible with nepheline or leucite, separated by an invariant composition Or₅₁Ab₄₉. Consequently, sodic rocks, with molar Na/(Na+K) > 0.75, contain the anorthoclase + nepheline assemblage, whereas with increasingly potassic character, the rocks contain leucite + nepheline, leucite + tetrakalsilite or leucite + kalsilite; the SiO₂-richer assemblages consist of leucite + sanidine. With decreasing temperature, tetrakalsilite becomes unstable and is replaced by K-bearing nepheline + kalsilite. Furthermore, the invariant feldspar composition separating the nepheline vs. leucite assemblages rapidly migrates towards sanidine (Or₉₀Ab₁₀ at 800 ^oC and 1 bar). As a consequence, decreasing crystallization temperature favors the assemblage of alkali feldspar and nepheline (over leucite) for a wide range of bulk Na/(Na+K) ratios. This explains the rarity of sodic feldspar + leucite assemblages in nature. At feldspar subsolvus temperatures, both albite and orthoclase coexist with nepheline, and leucite becomes Na-poor and eventually breaks down to kalsilite and orthoclase. This is consistent with frequent replacement in nature of leucite by secondary products, in particular analcime. The compatibility relations in the SiO₂-NaAlSiO₄-KAlSiO₄ system are conveniently delineated by several intermediate members, e.g., Or₇₅Ab₂₅ or Ne₆₀Ks₄₀, which are used in a new algorithm for normative classification and interpretation of alkaline undersaturated rocks.

 

References

Sack R.O., Ghiorso M.S., 1998. Contrib. Mineral. Petrol. 130, 256-274.