Natrolite is a mineral species belonging to the zeolite group. It is a hydrated sodium and aluminium silicate with the formula (Na,Al)₂SiOio-2H,O and containing sodium (NaCl, 16.3%). It was named natrolite by M. H. Klaproth in 1803. Needle stone or needle-zeolite are other names, alluding to the common acicular habit of the crystals, which are often very slender and are aggregated in divergent tufts. Larger crystals have the form of a square prism terminated by a low pyramid, the prism angle being nearly a right angle (88 451/2). The crystals are tetragonal in appearance, though actually orthorhombic. There are perfect cleavages parallel to the faces of the prism. The mineral also often occurs in compact fibrous aggregates, the fibers having a divergent or radial arrangement (hence the name radiolite for one variety). From other fibrous zeolites natrolite is readily distinguished by its optical characters.

Between crossed nicols the fibers extinguish parallel to their length, and they do not show an optic figure in convergent polarized light. Natrolite is usually white or colorless, but sometimes reddish or yellowish. The luster is vitreous, or in finely fibrous specimens sometimes silky. The specific gravity is 2.2, and the hardness is 5 1/2. The mineral is readily fusible, melting in a candle-flame to which it imparts a yellow color owing to the presence of sodium. It is decomposed by hydrochloric acid with separation of gelatinous silica.

Natrolite occurs with other zeolites in the amygdaloidal cavities of basic igneous rocks. The best specimens are the diverging groups of white prismatic crystals found in compact basalt at the Puy-deMarman, Puy-de Dome, France. The largest crystals are those from Brevig in Norway. The walls of cavities in the basalt of the Giants Causeway, in Co. Antrim, are frequently encrusted with slender needles of natrolite, and similar material is found abundantly in the volcanic rocks (basalt and phonolite) of Salesel, Aussig and several other places in the north of Bohemia.

Several varieties of natrolite have been distinguished by special names. Fargite is a red natrolite from Glenfarg in Perthshire. Bergmannite or Spreustein is an impure variety which has resulted by the alteration of other minerals, chiefly sodalite, in the augitesyenite of southern Norway.

This article incorporates text from the public domain 1911 Encyclopædia Britannica.