Kernite

Also called  Rasorite,   borate mineral, hydrated sodium borate (Na2B4O7×4H2O), that was formerly the chief source of borax (q.v.). It forms very large crystals, often 60 to 90 centimetres (2 to 3 feet) thick; the largest observed measured 240 by 90 cm. The crystals are colourless and transparent but are usually covered by a surface film of opaque white tincalconite. Kernite is associated with other borate minerals as