Borax is a sodium salt of tetraboric acid. Fine powdered colemanite ( Ca2B6O11 ). Is boiled with sodium carbonate ( washing soda ) solution. The insoluble calcium carbonate is filtered off ( CaCO3 ). Then it is concentrated and cooled to get the crystals of borax. The sodium metaborate is treated with CO2 gas which converts into borax.
Ca2B6O11 + 2Na2CO3 ———> Na2B4O7 + 2NaBO2 + 2CaCO3
Colemanite Borax Sodium calcium
2NaBO2 + CO2 ———> Na2B4O7 + Na2CO3