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, the most common of these, is a hydrous calcium and aluminium silicate, (Ca,Na)2−3Al3(Al,Si)2Si13O36·12H2O.

The mineral is usually colourless or white, but may be orange, brown, yellow, brick-red, or green due to inclusions of celadonite.

The two minerals may, however, be readily distinguished by the fact that in heulandite the acute positive bisectrix of the optic axes emerges perpendicular to the cleavage.

[6] Heulandite occurs with stilbite and other zeolites in the amygdaloidal cavities of basaltic volcanic rocks, and occasionally in gneiss and hydrothermal veins.

[7] Good specimens have been found in the basalts of Berufjörður, near Djúpivogur, in Iceland, the Faroe Islands and the Deccan Traps of the Sahyadri Mountains of Maharashtra near Mumbai.