A hydrated borate mineral with chemical composition Na₂B₄O₆(OH)₂·3H₂O. It is monoclinic with symmetry 2/m. It occurs only very rarely in crystals but is found most commonly in coarse, cleavable masses (see illus.) and aggregates. It has two perfect cleavages, (001) and (100), at about 71° to each other, which together produce cleavage fragments that are elongated parallel to the b crystallographic axis. It is colorless to white; colorless and transparent specimens tend to become chalky white on exposure to air with the formation of tincalconite, Na₂B₄O₅(OH)₄·3H₂O. The structure of kernite, with space group P 2/a, contains infinite chains of composition (B₄O₆(OH)₂)^2- which run parallel to the b crystallographic axis. The chains are linked to each other by bonding to Na⁺ and by hydroxyl-oxygen bonds.