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Formula: Na2(Si3Al2)O10.2H2O
Tectosilicate (framework silicate), zeolite group,
feldspathoid
Crystal System: Orthorhombic
Specific gravity: 2.2 to 2.26 measured, 2.25 calculated
Hardness: 5 to 5½
Streak: White
Colour: Colourless, white, yellow
Solubility: Moderately soluble in hydrochloric acid
Common impurities: Ca,K
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 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).
At the Copper Falls Mine, Copper Falls, Keweenaw county, Michigan, USA, mineralisation occurs primarily in hydrothermal veins
cutting preexisting lavas and as amygdules in the Ashbed flow.
This mine is one of the better localities in the Keweenaw Peninsula for natrolite crystals, some showing reddish
hematite and sometimes copper inclusions. The
reddish colour is rare elsewhere in the district
(MinRec 54.1.112).
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). It sometimes has weak cream-white fluorescence under short wave UV
(R&M 97.5.443).
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