Formula: Na(AlSi2O6).H2O
Tectosilicate (framework silicate),
zeolite group, feldspathoid, forms series with pollucite and with wairakite (ZW).
Specific gravity: 2.24 to 2.29
Hardness: 5 to 5½
Streak: White
Colour: White, colourless, gray, pink, greenish, yellowish; colourless in thin section.
Solubility: Moderately soluble in hydrochloric acid
Environments:
Igneous environments
Pegmatites
Sedimentary environments
Basaltic cavities
Analcime occurs as a primary mineral in some
igneous rocks; it is the only
zeolite that crystallises directly from molten rock. In pegmatites and miarolytic cavities cesium-rich
analcime forms from 250° to 150°C. Analcime phenocrysts crystallise in deep basaltic magma chambers at temperatures between 600 and 640°C, and at
pressures from 5 to 13 kilobars. Rapid vertical transport to the surface is required for analcime to survive the decrease in temperature and pressure.
A slow decrease in temperature and pressure would allow the analcime to react with the basaltic magma to produce
albite
(ZW).
Analcime is also the product of hydrothermal action in the
filling of basalt cavities, where analcime phenocrysts crystallise in deep
basaltic magma chambers at temperatures between 600oC and 640oC, and pressure between 5 and
13 kbar, in associations with
prehnite, calcite and
zeolites such as chabazite,
thomsonite and stilbite
(DHZ 4 p346).
Geothermal wells have been drilled through a thick series of basalt flows in
western Iceland, where it was found that
analcime crystallised at temperatures from to 300oC at depths between 72m and 1600m
(ZW).
Analcime may occur in mafic rocks such
as aegirine-analcime-nepheline
syenite.
In saline alkaline lakes, analcime forms near the centre of the lake, where the water is highly saline with pH 9 to 10 (strongly alkaline), and in
these conditions analcime, K-feldspar, trona and borosilicates
are dominant. Analcime forms at a higher pH (more alkaline) by dissolving phillipsite,
erionite, chabazite, or silica-rich
heulandite that crystallised when the pH was lower (less strongly alkaline)
(ZW).
In deep sea sediments phillipsite and heulandite alter
to analcime over a period of millions of years
(ZW).
In lake beds, analcime may be altered
from pyroclastics or clay minerals, or it may be a
primary precipitate. It is authigenic (formed in place) in
sandstone and
siltstone.
In Western Tasmania, Australia, analcime occurs as a late stage primary mineral with
gonnardite-natrolite and as an
alteration product of feldspar. It also occurs in
basaltic cavities
( AJM 10.2.62-63 ).
In the vicinity of Meshkinshahr, Ardabil Province, Iran, crystals of analcime are found embedded in small amygdules
in potassium-rich basalt, associated with
chabazite, mesolite and/or
thomsonite. The analcime originated hydrothermally
(R&M 92.6.541).
At Mount Kahoven, Semnan Province, Iran, orange coloured crystals of analcime are found on perched on
vesicular basalt
(R&M 92.6.542).
In the alkaline rocks of the Kola peninsula, Russia, analcime is associated with
aegirine and may be either primary
or secondary after sodalite
or aegirine
(DHZ 4 p345).
At Palabora, Limpopo, South Africa, analcime is an early phase in the deepest parts of the dyke fracture zone. Crystal cores may have
originally been wairakite overgrown by analcime, then later
hydrothermal alteration converted wairakite to
laumontite. A later generation of analcime occurs on
fluorapophyllite
(R&M 92.5.433).
At Croft quarry, Leicestershire, England, UK, analcime occurs in tonalite, with
laumontite, calcite,
galena and marcasite. Analcime and
calcite may form pseudomorphs
after laumontite
(RES p185 to 190).
At the Bearpaw Mountains, Montana, USA, analcime occurs in cavities in igneous rock, associated with
axinite, prehnite and
datolite
(DHZ 4 p345).
Alteration
analcime to jadeite and H2O
Na(AlSi2O6).H2O → NaAlSi2O6 + H2O
At high temperature analcime changes to jadeite
(ZW)
analcime and quartz to
albite and H2O
Na(AlSi2O6).H2O + SiO2 ⇌ Na(AlSi3O8) +
H2O
High temperature favours the forward reaction
(JVW p 144, ZW).
nepheline and H4SiO4 (silicic acid) to analcime and H2O
NaAlSiO4 + H4SiO4 ⇌ Na(AlSi2O6).H2O
+ H2O
(http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.613.9474&rep=rep1&type=pdf)
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