natrolite

n.
A mineral in the zeolite family with composition Na₂Al₂Si₃O₁₀·2H₂O.

[NATRO(N) + -LITE.]

A fibrous or needlelike mineral belonging to the zeolite family of silicates. Most commonly it is found in radiating fibrous aggregates. The hardness is 5–5½ on Mohs scale, and the specific gravity is 2.25. The mineral is white or colorless with a vitreous luster that inclines to pearly in fibrous varieties. The chemical composition is Na₂(Al₂Si₃O₁₀) · 2H₂O, but some potassium is usually present substituting for sodium.

Natrolite is a secondary mineral found lining cavities in basaltic rocks. Its outstanding locality in the United States is at Bergen Hill, New Jersey. See also Zeolite.

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Na
Orthorhombic -- Rhombic pyramidal

Environment

Typical zeolite associations.

Crystal description

Slender needles make sprays of long, prismatic, often very slender, square needle crystals, ending in what resembles the low four-faced pyramid expected on a tetragonal crystal. Also white radiating nodules, compact masses in nepheline syenite class rocks, and in gemmy singles, sometimes immense (Russia).

Physical properties

Colorless or white. Luster glassy; hardness 5-5Ɖ; specific gravity 2.2; fracture uneven across the prism; cleavage good prismatic. Very brittle (needles fall off on every inversion; don't touch); transparent to translucent; often fluorescent orange.

Composition

Hydrous sodium aluminum silicate (16.3% NaO, 26.8% Al ₂ O ₃ , 47.4% SiO ₂ , 9.5% H ₂ O).

Tests

Melts rather quietly to a bubbly but colorless glass. Heated needles and glass fluoresce greenish white or blue. Partially melted crystals fluoresce brightest.

Distinguishing characteristics

Its ready fusibility distinguishes it from most acicular crystals, and the crystal form is characteristic among the zeolites. The hardness distinguishes it from prismatic gypsum; its fluorescence is a help when it is a rock constituent, as in Magnet Cove, Arkansas.

Natrolite is one of three minerals very similar in habit. Scolecite, a calcium equivalent (Ca[Al ₂ Si ₃ ]O ₁₀ ·3H ₂ O), forms comparable sprays and in many zeolite occurrences of the volcanic type substitutes for natrolite. It fuses with wormlike extrusions; hence the name, which is derived from the Greek for "worm." Mesolite is intermediate in composition, having both calcium and sodium (Na ₂ Ca ₂ (Al ₆ Si ₉ )O ₃₀ ·8H ₂ O). The sodium-bearing pair are structurally monoclinic in symmetry, but with no inclination of the front axis. Usually twinned on the front face, they appear to be as orthorhombic (and tetragonal) as natrolite. As a rule (one known exception), mesolite crystals are very slender. Scolecite, on the other hand, tends to be sturdier than natrolite in filling its volcanic pockets.

Occurrence

Natrolite can form part of the groundmass of igneous rocks and can occur in large individual crystals in pegmatitic phases of such rock, as at St. Hilaire, Quebec, and in the Kola Peninsula, Russia. More often it forms crystal sprays in vesicles in lavas. Scolecite and mesolite occur in the same way and are not usually rock constituents; they are slightly less frequent. Natrolite is found in New Jersey traprock quarries, on Table Mountain, Golden, Colorado, and in Oregon traprock areas. Large crystals to 8 in. (12 cm) in length have been found mainly as loose and sometimes doubly terminated singles in Bound Brook, New Jersey. Crystals 1-2 in. (2.5-5 cm) long were found in California in an asbestos quarry near Coalinga.

Many old European localities such as Ustí nad Labem (Aussig), Bohemia; County Antrim, Ireland; and the Faroe Is. are famous. Very large crystals, far bigger than those of Bound Brook, New Jersey, Montana, or Coalinga, California, have been found in Russia.

Tight veins of white natrolite cut other rocks. In San Benito Co., California, such natrolite is noteworthy because it fills seams in serpentine walls that are lined with neptunite and benitoite; these can be exposed by dissolving away the natrolite with hydrochloric acid.

Scolecite has turned out to be relatively abundant in many important localities, such as Theigerhorn and Berufjördhur, Iceland; Poona, India; and Santa Catarina, Brazil. In each of these deposits it forms long sprays, those of Brazil and India being as much as 6 in. (15 cm) long.

Mesolite is usually in thinner needles, but in the building of a dam at Skookumchuck, Washington, crystals of a size comparable to the normal ones of scolecite were found in what appears to have been a unique occurrence. Mesolite is common in the Oregon-Washington traprock, but is usually hairlike in dimension.

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The noun has one meaning:

Meaning #1: a group of minerals of the zeolite family consisting of a hydrous silicate of sodium and aluminum

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