Natrolite, Scolecite & Thomsonite Composition, Crystallization & Structure

Natrolite

Composition. Na2Al2Si3O. A zeolite.

Crystallization. Orthorhombic. Crystals usually slender prismatic, often acicular. Prism zone vertically striated. Some times terminated by low pyramid. Crystals often appear to be tetragonal in symmetry. Sometimes in cruciform twins.

Structure. Usually in radiating crystals groups (see Fig. C, pl. II); also fibrous, massive, granular or compact.

Physical Properties. Perfect prismatic cleavage. H. = 5-5.5. G.= 2.25. vitreous luster. Coloress or white. Sometimes tinted yellow to red. Transparent to translucent.

Test. Easily fusible (2.5) to a clear, transparent glass giving a yellow (sodium) flame. Water in C. T. Soluble in hydrochloric acid and gelatinises upon evaporation. Recognised chiefly by its radiating crystals.

Occurrence. A mineral of secondary origin , found lining amygdaloidal cavities in basalt , etc. Associated with other zeolites, calcite, etc. Notable localities for its occurrence are Assign and Salesel Bohemia; Puy-de- Dome, France; Val di Fassa, Trentino, Italy; in various places in Nova Scotia; Bergen Hill, etc., New Jersey. 

Scolecite
A Zeolite with composition CaAl₂Si₃O10.₃H₂O. Monoclinic. In slender prismatic, twinned crystals. In radiating groups. Sometimes fibrous. Prismatic cleavage. H. = 6-.5.5. G. = 2.16-2.4. Vitreous luster; silky when fibrous. Colorless or white. Transparent to almost opaque. Fuses at 2.5 to a voluminous frothy slag. Gelatinizes in acids. Water in C. T. A mineral of secondary origin, found lining cavities in basic igneous rocks, associated with other zeolites, ete.

Thomsonite
A zeolite, having the composition (Na₂Ca)Al₂(Si0₄)2.21/2H₂0. Orthorhombic but distinct crystals rare. Commonly columnar with radiated structure. Perfect pinacoidal cleavage. H. = 5-5.5. G. = 2.3-2.4. Vitreous luster. Colorless, white, gray. Transparent to translucent. Fuses with intumescence at 2-2.5. Soluble and gelatinizes in acids. Much water in C. T. Occurs in arnygdaloidal cavities in basalt, etc., associated with other zeolites.