Analcime

minerals

prehnite

laumontite

chabazite

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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 association 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.

Analcime crystallises in the isometric system, point group 4/m 3 2/m, and crystals usually have the form {211} (trapezohedron) Mindat Image

Localities

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 Copthill Quarry, Stanhope, County Durham, England, UK, crystals of analcime on dolerite have been found (JRS 21.6).

At Cambokeels Mine, Westgate, Stanhope, County Durham, England, UK, analcime crystals typically up to 2 mm across occurred coating surfaces of dolerite. In many specimens the analcime was accompanied, and locally overgrown by, small crystals of apophyllite, and in a few specimens was also accompanied by crystals of calcite up to 2 mm across (JRS 21.6).

At the High Force Quarry, Forest and Frith, County Durham, England, UK, analcime crystals up to 1 mm across were found in samples as crusts up to 12 cm across coating joint surfaces of dolerite pegmatite. In some examples the analcime was accompanied by a few crystals of chabazite up to 2 mm across. Exactly similar small analcime crystals were also noted in situ in narrow stilbite - bearing veins in slightly chloritised dolerite pegmatite exposed in the quarry (JRS 21.6).

At Croft quarry, Croft, Leicestershire, England, UK, analcime has been found in a zeolite-filled fissure in tonalite, together with chabazite. The first generation of analcime occurs as crystals to 1mm associated with quartz. The second generation analcime crystals were numerous and larger, up to 13 mm across, associated with calcite, marcasite, chalcopyrite or galena. It is suggested that the analcime was produced by the action of water on feldspar:

plagioclase + water → analcime + laumontite + quartz

Both generations of analcime may enclose, or be coated with, hematite. Analcime and calcite may form pseudomorphs after laumontite (JRS 20.6-9, RES p185 to 190).

At Enderby Warren Quarry, Enderby, Blaby, Leicestershire, England, UK, analcime occurs in quartz-diorite and tonalite associated with laumontite (JRS 20.9).

At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, analcime has been identified in the central portion of lepidolite veins cutting pollucite masses. After pollucite has formed no more cesium is available, thus only analcime can form. The Emmons pegmatite is an example of a highly evolved boron-lithium-cesium-tantalum enriched pegmatite (R&M 94.6.503).

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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