HEULANDITE (miniature)

Heulandite refers to a series of tecto-silicate minerals within the zeolite group. Originally classified as a distinct mineral species until 1997, it was redefined by the International Mineralogical Association to encompass a series, with specific species identified as Heulandite-Ca, Heulandite-Na, Heulandite-K, Heulandite-Sr, and Heulandite-Ba (the latter described in 2002). Heulandite-Ca, the most prevalent variant, is a hydrous calcium and aluminum silicate represented by the formula (Ca,Na)2−3Al3(Al,Si)2Si13O36·12H2O. Typically, small quantities of sodium and potassium substitute for calcium, while strontium replaces calcium in the heulandite-Sr variant. The designation of the species is based on the predominant element, although they are visually indistinguishable, and the term heulandite is still applied in cases where specific testing has not been conducted.