Analcime

tectosilicate

feldspathoid

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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