Heulandite is the name of a series of tecto-silicate minerals group. Prior to 1997, heulandite was recognized as a mineral species, but a reclassification in 1997 by the International Mineralogical Association changed it to a series name, with the mineral species being named: * Heulandite-Ca * Heulandite-Na * Heulandite-K * Heulandite-Sr * Heulandite-Ba (described in 2002).
Heulandite-Ca, the most common of these, is a hydrous calcium and aluminium, (Ca,Na) 2-3Al 3(Al,Si) 2Si 13O 36·12H2O. Small amounts of sodium are usually present replacing part of the calcium. Strontium replaces calcium in the heulandite-Sr variety. The appropriate species name depends on the dominant element. The species are visually indistinguishable, and the series name heulandite is still used whenever testing has not been performed.