Natrolite

tectosilicate

basalt cavities

Formula: Na2(Si3Al2)O10.2H2O
Tectosilicate (framework silicate), zeolite group
Specific gravity: 2.2 to 2.4
Hardness: 5 to 5½
Streak: White
Colour: Colourless, white, yellow
Solubility: Moderately soluble in hydrochloric acid
Environments:

Plutonic igneous environments
Pegmatites
Basaltic cavities

Natrolite is characteristically found lining cavities in basalt associated with other zeolites and calcite; it is one of the later zeolites to crystallise. It is also found in syenite and nepheline syenite.
Geothermal wells have been drilled through a thick series of basalt flows in western Iceland, where it was found that natrolite crystallised at temperatures from 70oC to 100oC at depths between 450m and 1200m (ZW).
It may also be an alteration product of nepheline, sodalite or plagioclase. (DHZ4 p374).

At Abbotsford and Bundoora, inner Melbourne, Victoria, Australia, gonnardite-natrolite occurs in cavities in basalt associated with chabazite and calcite or with phillipsite and thomsonite (AJM 20.1.44-46).

At Kragerø, Telemark, Norway, natrolite occurs on joint surfaces in a quartz-rich gneiss, associated with stilbite, heulandite and laumontite (DHZ4 p375).

In the basalts of northern Ireland, UK, natrolite typically occurs in cavities associated with analcime, often together with chabazite and calcite. (DHZ4 p374).

Common impurities: Ca,K

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