EXPERIMENTAL-STUDY OF LI-RICH GRANITIC PEGMATITES .2. SPODUMENE + ALBITE + QUARTZ EQUILIBRIUM

Spodumene is one of the principal sources of Li and the most important Li mineral in pegmatities. Experiments have been carried out at 450-degrees-C and 600-degrees-C at 4 kbar and 750-degrees-C at 1.5 kbar to study the equilibrium between spodumene, albite, quartz, and the coexisting hydrothermal chloride solution. Solid-solution formation in spodumene as well as in albite is very much restricted. When spodumene coexists with albite and quartz, the alkali composition of the fluid is buffered. The value of the ratio Na/(Na + Li) of the fluid has been determined to be 0.550 +/- 0.020 at 450-degrees-C, and 0.485 +/- 0.025 at 600-degrees-C, conditions at which the stable form of spodumene is alpha spodumene. At 750-degrees-C, 1.5 kbar, the stable form is the high-temperature polymorph beta spodumene. In the presence of an excess of quartz, the assemblage albite + beta spodumene partially melts from Ab30-Sp70 to Ab75-Sp25. The value of the ratio Na/(Na + Li) in the fluid buffered by coexisting albite and spodumene, with or without melt, is constant and has been determined to be 0.510 +/- 0.020 at 750-degrees-C.