Crystal system Orthorhombic
Transparency Transparent to translucent
Luster Vitreous to dull
Fracture Uneven to Conchoidal
Cleavage Perfect prismatic
Specific Gravity 2.23
Hardness 5 to 5.5
Optical Character Biaxial + ; Double Refractive
Refractive index 1.480-1.493
Birefringence 0.013
Chemical Formula Na2Al2Si3O10.2H2O
Comments Attacked by acids
Streak White

Natrolite (NAT-row-lite), a member of the zeolite family, is a sodium aluminosilicate hydrate with the formula Na2Al2Si3O10.2H2O. Its, name is derived from the French natron, meaning "soda", in reference to its sodium content. It ranges in color from colorless to red, yellow, white and gray.

Facetable natrolite crystals are very rare; crystals seldom occur any larger than 1/8 inch in diameter. Natrolite has been cut for collectors, but this mineral is usually found in a white, massive form, consisting of orthorhombic fibrous crystals, which is usually not suitable for cutting.

The optic character, specific gravity and birefringence of natrolite are important in its identification.

The largest natrolite crystals come from San Benito County, California;. Natrolite is found in Nova Scotia, Canada, as well as in Czechoslovakia, the Alps, France, Norway, Scotland, and Germany. Some German specimens are very colorful and are sometimes polished to exhibit attractive circular patterns.

Perfect cleavage parallel to the length of the natrolite crystal makes it difficult to cut. It is fairly heat sensitive, whitening with excessive heat and fusing readily at fairly low heat. Therefore, heating must be done very carefully, and a cold dop is the safest course. Natrolite polishes readily with cerium oxide on lucite and less readily on tin with Linde A. Crown angles of 43° and pavilion angles of 44° are effective in faceting.

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