Kernite, also known as rasorite is a hydrated sodium borate hydroxide mineral with formula Na2B4O6(OH)2·3H2O. It is a colorless to white mineral crystallizing in the monoclinic crystal system typically occurring as prismatic to acicular crystals or granular masses. It is relatively soft with Mohs hardness of 2.5 to 3 and light with a specific gravity of 1.91. It exhibits perfect cleavage and a brittle fracture.
The mineral was discovered in 1926 in Kern County, California and later named for the county. Kern County was the only known source of kernite for many years but is now mined in Argentina, Spain, and Turkey. Ot occurs in sedimentary evaporite deposits in arid regions.