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

Plutonic igneous environments
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 the Marron Volcanics of the Olalla Area South-Central British Columbia, Canada, at the White Lake Road site natrolite forms colourless to pale pink to orange needles to 8 mm long, in aggregates to 1.5 cm wide. These are commonly sandwiched between two generations of analcime so that the natrolite is observed as inclusions in the second analcime generation or has a dusting of tiny analcime crystals on it. Some laumontite has overgrown this assemblage before the rest of the cavity was infilled by calcite. Natrolite also occurs here as white radial aggregates to 8 mm across with orange phillipsite (R&M 96.6.524).

At Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Quebec, Canada, cancrinite pseudomorphs after natrolite have been found with rhodochrosite (KL p265). Also at this locality samples of analcime on natrolite pseudomorphs after serandite have been found (KL p267).

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).

The Purple Diopside Mound, Rose Road, Pitcairn, St. Lawrence county, New York, USA, is situated in marble. The development of veins of large crystals probably occurred as a result of fluid penetration from a concurrent intrusion. Many of the minerals of interest to collectors formed during this primary event, with additional species resulting from the subsequent alteration of scapolite. There seems to be little, if any, secondary, late-stage mineralisation present.
Natrolite occurs as translucent white prismatic crystals to 4 cm and also as densely packed radial sprays of acicular crystals to 7 cm in diameter. The natrolite most likely formed as an alteration product of marialite (R&M 96.6.550).

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