E. N. Gramenitskii, T. I. Shchekina, Ya. O. Alfer’eva
Dec 20, 2008
Citations
0
Influential Citations
4
Citations
Journal
Moscow University Geology Bulletin
Abstract
Experimental data allow modeling the behavior of the named elements during formation of fluorine- saturated leucocratic rocks of silicic and alkaline compositions. The distribution of alkaline and alkaliearth elements is discussed at equilibrium between the silica-alumina melt with fluoride phases (crystalline and liquid) and with feldspar. Cryolite crystals form during saturation of silica-alumina melt of normal alkalinity with fluorine. Continuous solid solution of sodium-potassium cryolite is stable at 800°C. The equilibrium between melt and crystals continues up to the maximum molar fraction of 0.1 lithium end member in cryolite, at which two fluoride phases (crystalline and liquid) coexist with the silica-alumina melt of fixed composition. Separation of salt melts during late differentiation stages of granite and alkaline rocks is a regular process continuing the natural evolution of ore-magmatic systems. At equilibrium of two liquid phases, the silica phase is relatively enriched in potassium, and the fluoride phase is substantially enriched in sodium. This detected effect is the only currently possible mechanism for the occurrence of the potassium differentiation trends of granite melts. All effects related to crystallization cause enrichment in sodium. In other cases (with Ca, Sr, Mg, Rb, and Cs), separation of the second liquid phase acts in the same direction and enhances the action of crystallization. Comparison between partition coefficients allows derivation of the following affinity rows of alkaline elements for fluoride melt: Li > Na > K > Rb≈Cs and Mg > Ca > Sr > Ba. Hence, the known rule for joining strong bases with strong acids and weak bases with weak acids is fulfilled.