stilbite

n.
A white or yellow lustrous zeolite mineral, essentially (Ca,Na)₂Al₂Si₇O₁₈·7H₂O.

[French, from Greek stilbos, shining. See stilbene.]

A mineral belonging to the zeolite family of silicates. It crystallizes in sheaflike aggregates of thin tabular crystals. There is perfect cleavage parallel to the side pinacoid, and here the mineral has a pearly luster; elsewhere the luster is vitreous. The color is usually white but may be brown, red, or yellow. Hardness is 3½–4 on Mohs scale; specific gravity is 2.1–2.2. See also Zeolite.

Stilbite is found in Iceland, India, Scotland, Nova Scotia, and in the United States at Bergen Hill, New Jersey, and the Lake Superior copper district in Michigan.

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(Ca,Na)
Monoclinic -- prismatic

Environment

Typical zeolite associations and very common.

Crystal description

In tabular crystals, commonly intergrown to give an orthorhombic symmetry, and often in bundles spreading slightly at either end to give the impression of wheat sheaves or bow ties Sometimes in dull-surfaced 1 in. (2-4 cm) rounded knobs with radiating structures.

Physical properties

Yellow, brown, reddish, white. Luster glassy, pearly on cleavage face; hardness 3Ɖ-4; specific gravity 2.1-2.2; fracture irregular; cleavage 1 perfect. Brittle; transparent (small crystals) to translucent.

Composition

Hydrous calcium, sodium, aluminum silicate (1.4% Na ₂ O, 7.7% CaO, 16.3% Al ₂ O ₃ , 57.4% SiO ₂ , 17.2% H ₂ O).

Tests

On fusion swells and writhes like heulandite, but the protuberances are more wormlike.

Distinguishing characteristics

The larger, sheaflike crystals are sufficiently typical to distinguish stilbite from the other common zeolites. In fusing, the worms are more splintery than in heulandite. Apophyllite boils and melts to droplets with less swelling.

Occurrence

Excellent specimens come from the Paterson, New Jersey, district and from Nova Scotia around the Bay of Fundy. Bright orange crystals have been found at Great Notch, New Jersey; Victoria, Australia; and Kilpatrick, Scotland (at the latter, 1Ɖ in. [4 cm] long). There are numerous other localities for stilbite; it may be encountered in many types of occurrences, including ore veins and late pegmatite stages. The crystals of Iceland, of Rio Grande do Sul, Brazil, and Poonah, India, are notable. Probably the most common of the zeolites to occur in atypical zeolite surroundings (seams in granite and the like).

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