The occurrence characteristics and formation conditions of spodumene and petalite in granitic pegmatite

Spodumene (Spd) and petalite (Ptl) are important lithium-aluminosilicate minerals in Li-rich granitic pegmatite. It is of great significance to find out the formation conditions and evolution process of spodumene and petalite for further indicating the formation mechanism of granite-pegmatite type lithium deposit. Spodumene and petalite are usually occurred in the intermediate zone or core of pegmatites, but formed in different temperature and pressure conditions. The former one is formed at high temperature and low pressure, whereas the latter one is stable at relatively low temperature and high pressure, and the two can be converted with the change of temperature and pressure conditions. When the temperature decreases, the petalite can be converted into spodumene and quartz (Qtz) intergrowths (SQI), or when the pressure increases, the spodumene and quartz intergrowths (SQI) can be converted into petalite. The P-T trajectory of the formation and evolution of Li-rich pegmatite can be revealed by accurately determining the temperature and pressure conditions of the mutual conversion between spodumene and petalite. Based on recent research results on spodumene and petalite, this paper summarizes the production characteristics and formation conditions, and concludes that: (1) In the Li-rich pegmatite system, the crystallization of lithium-aluminosilicate minerals is mainly controlled by temperature and pressure conditions, and less relevant with the melt composition. The stable temperature and pressure range of spodumene and petalite is 400 similar to 680 degrees C and 160 similar to 400MPa; (2) Under the superposition of exsolution alkaline fluid, the hydrothermal alteration process of the late spodumene occurred albitization/potash feldspathization, muscovitization and lithium chloritization successively; (3) The higher the temperature, the less the aluminum saturation index (AST), and the more H2O content may increase the solubility of lithium in the melt; (4) The crystallization experiments of spodumene and petalite indicate the rapid crystallization process of pegmatite, and the formation of its texture is influenced by the degree of cooling. High temperature and pressure experiments show that 5% H2O content (unsaturated condition) and 50 similar to 130 degrees C undercooling degree range are the best conditions developing the pegmatitic texture; (5) The content of CO2 and Li in magma plays a key role in regulating pH, reducing liquidus and melt viscosity, and increasing hydration cluster molecules in the crystallization system of Li-rich pegmatite. In the future, the application of in-situ high temperature and high pressure experimental techniques (such as hydrothermal diamond anvil cell) will provide more direct and reliable experimental basis for researching the metallogenic mechanism of granitic pegmatite rare metal deposits.