Laumontite

minerals

lawsonite

zeolite

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
Common impurities: Na,K,Fe
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).

Localities

At Dashkezan, Azerbaijan, laumontite occurs with limonite (FM).

At Malad, Mumbai district, Maharashtra, India, prehnite pseudomorphs after laumontite have been found (KL p243).

At Croft quarry, Croft, Blaby, Leicestershire, England, UK, laumontite is associated with analcime in quartz-diorite. Laumontite crystals occur commonly in the form of rosettes and groups of long prisms, associated with calcite. They also occur as equant prisms intergrown with calcite rhombs. A specimen has been recovered from a vein of laumontite with minor analcime lining cavities filled with calcite. Both analcime and calcite have formed epimorphs after a prismatic mineral, most probably laumontite. Laumontite generally precedes analcime in the paragenetic sequence. (RES p 186,189, JRS 20.18-19).

At Huncote Quarry, Huncote, Blaby, Leicestershire, England, UK, laumontite was probably the most common zeolite, occurring as veins up to 7 mm wide, and occasionally as thin films on joint surfaces. The material is always crystalline, forming rosettes up to 38 mm in length (JRS 20.19).

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).

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