Natrolite

Hand Specimen identification: The many different zeolites have similar occurrences and may be hard to distinguish. Natrolite typically occurs as white radiating needles or as radial aggre-gates. Its perfect prismatic cleavage and uneven frac-ture and association also aid identification. Natrolite may be confused with aragonite or pectolite.

Chemical Composition : Na2Al2Si3O10.2H2O

Hardness : 5 to 51 2

Specific Gravity2.23

Cleavage/Fracture : perfect prismatic {110}/uneven

Luster/Transparency : vitreous/transparent to
translucent

Color : colorless, gray

Streak : white

Optical Properties : Natrolite may be difficult to distinguish from other zeolites. It has two perfect cleavages, is of parallel extinction, is length slow, and has a moderate 2V. Biaxial 1+ 2, a = 1.48, b = 1.48, g = 1.49, d = 0.012, 2V = 38° to 62°.

Crystallography : Orthorhombic, a = 18.30, b = 18.63, c = 6.60; space group Fd2d; point group mm2.

Habit : Acicular crystals, often radiating, are typical. Crystals may show vertical striations. Radiating rounded masses are also common. Less commonly, it is fi-brous, massive, or granular.

Structure and composition : Natrolite and all zeolites are framework structures built of AlO4and SiO4 tetrahedra. The tetrahedra are linked to form chains, cages, rings, or loops. In contrast with some other framework silicates, the zeolite structure contains many large holes and channels, which hold weakly bonded H2O. Slightly smaller holes contain Na, Ca, or K. In natrolite, scolecite, and some other zeolites, the tetrahedral framework has a strong linear fabric parallel to the c-axis. Crystal habit is therefore fibrous or acicular. Natrolite always contains minor amounts of Ca and K and has variable H2O content.

Occurrence and associations : Natrolite and most other zeolites are secondary minerals that form in cracks or on cavity walls in mafic igneous rocks. It is found with calcite and other zeolites.

Varieties :

Related Minerals : Scolecite, CaAl2Si3O10 .3H2O, and thomsonite, NaCa2 Al5Si5O20 6H2O, are closely related to natrolite.

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