Hand Specimen identification: Heulandite may be difficult to distinguish from other zeolites, but crystal habit, perfect one direction of cleavage, and luster aid identification.
Chemical Composition : CaAl2Si7O18.6H2O
Hardness : 31 2 to 4
Cleavage/Fracture : one perfect (010)/subcon-
Luster/Transparency : vitreous/transparent to
Color : white, variable
Streak : white
Optical Properties : Biaxial 1+ 2, a = 1.49, b = 1.50, g = 1.50, d = 0.005, 2V = 35°.
Crystallography : Monoclinic, a = 17.73, b = 17.82, c = 7.43, b = 116.3°, Z = 4; space group Cm; point group m.
Habit : Crystals are typically platy with a diamond or modi-fied diamond shape.
Structure and composition : The structure of heulandite is similar to that of other zeolites, except that tetrahedra are linked in 6-member rings that align to give a more planar structure than most others. End member calcic heulandite, called mordenite, is rare. Considerable solid solution towards clinoptiloite, 1Na,K2Si7Al2O18 . 6H2O, is typical.
Occurrence and associations : Heulandite is one of the more common zeolites. It is a secondary mineral, found with calcite and with other zeolites, that forms in cracks or on cavity walls in mafic igneous rocks, and in some metamorphic rocks.
Related Minerals : Other zeolites, including clinoptiloite, 1Na,K2Si Al
O18 6H2O, and stilbite, CaAl2Si7O18 7H2O, have structures similar to heulandites