Sodium fluorosilicate (Na₂SiF₆) is a white crystalline solid, slightly soluble in water. It decomposes into silicic acid and sodium fluoride upon reaction with strong alkalis. Excess alkali produces silicates, which are commonly used to suppress silicate minerals such as quartz, feldspar, and serpentine. When flotation with oleic acid, it can suppress garnet, monazite, and tourmaline. When amines are used as collectors, small amounts of sodium fluorosilicate can activate quartz, feldspar, and tantalum-columbite, while larger amounts suppress them. In the flotation of sulfide ores, sodium fluorosilicate can activate pyrite that has been suppressed by lime. It can also act as a depressant for apatite.
The hydrated silica produced after the hydrolysis of sodium fluorosilicate has a suppressive effect on silicate gangue minerals, similar to that of water glass. Its suppressive power against quartz is stronger than that of water glass, second only to sodium hexametaphosphate. The F- ions released after the hydrolysis of sodium fluorosilicate precipitate Ca²⁺, which inhibits the activity of pyrite, thereby activating the pyrite that has been inhibited by lime.