Laumontite

tectosilicate

analcime

Formula: CaAl2Si4O12.4H2O
Tectosilicate (framework silicate), zeolite group
Specific gravity: 2.23 to 2.41
Hardness: 3½ to 4
Streak: White
Colour: White, brown, yellow, pink
Solubility: Moderately soluble in hydrochloric acid
Environments:

Pegmatites
Sedimentary environments
Metamorphic environments
Basaltic cavities

Laumontite is a zeolite facies mineral, formed by the decomposition of analcime. When it is associated with prehnite it forms in the transition zone between the zeolite and greenschist facies (ZW).
Geothermal wells have been drilled through a thick series of basalt flows in western Iceland, where it was found that laumontite crystallised at temperatures from 98oC to 230oC (ZW).
Authigenic (formed in place) laumontite has been reported forming part of the cementing material in sandstone, and it has also been found as a secondary mineral in sandstone (DHZ 4 p406).

At Croft quarry, Leicestershire, England, UK, laumontite is associated with analcime in quartz-diorite (RES p 186,189).

Alteration

laumontite to lawsonite, quartz and H2O
CaAl2Si4O12.4H2O→ CaAl2(Si2O7)(OH)2.H2O + 2SiO2 + 2H2O
In subduction zones, as the pressure rises to above about 1.5 kbar, laumontite alters to lawsonite according to the above reaction (KB p4).

laumontite and calcite to prehnite, quartz, H2O and CO2
CaAl2Si4O12.4H2O + CaCO3 → Ca2Al(Si3Al)O10(OH)2 + SiO2 + 3H2O + CO2
Prehnite and pumpellyite form from calcium zeolites in the presence of calcite, as in the above equation (DHZ 5B p127).

Common impurities: Na,K,Fe

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