Tincalconite is a mineral closely related to borax, and is a secondary mineral that forms if borax is allowed to dry. The crystal shape of borax does not change during the drying process. Its formula is Na2B4O5(OH)4 - 3H2O or Na2B4O7 - 5H2O.

While most tincalconite is created by man through exposing borax to dry air, there are natural occurrences of tincalconite, like in Searles Lake , California.