Heulandite

Heulandite
Heulandite, Lonavala Quarry, Pune District, Maharashtra, India. Size of individual crystals: 3-5cm.
General
Category	Zeolites (tectosilicates)
Formula (repeating unit)	(Ca,Na)2-3Al3(Al,Si)2Si13O36·12H2O
Strunz classification	9.GE.05
Crystal system	Monoclinic More than one space group
Crystal class	Prismatic (2/m) (same H-M symbol)
Identification
Colour	colourless, yellow, green, white, pale pink
Crystal habit	tabular, parallel aggregates
Cleavage	perfect basal
Mohs scale hardness	3.5-4
Lustre	pearly, vitreous
Streak	white
Diaphaneity	transparent to translucent
References

Heulandite is the name of a series of tecto-silicate minerals of the zeolite 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, the most common of these, is a hydrous calcium and aluminium silicate, (Ca,Na)_2-3Al₃(Al,Si)₂Si₁₃O₃₆·12H₂O. Small amounts of sodium and potassium 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.

Crystals are monoclinic. They may have a characteristic coffin-shaped habit, but may also form simple rhombic prisms. Frequently, a crust of fine crystals will form with only the ends of the rhombs visible, making the crystals look like wedges. They have a perfect cleavage parallel to the plane of symmetry, on which the lustre is markedly pearly; on other faces the lustre is of the vitreous type. The mineral is usually colourless or white, but may be orange, brown, yellow, brick-red, or green due to inclusions of celadonite. It varies from transparent to translucent. Isomorphous with heulandite is the strontium and barium zeolite brewsterite.

The Mohs hardness is 3-4, and the specific gravity 2.2. Heulandite is similar to stilbite. 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.