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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
The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia,
are skarn deposits at the contact between
granodiorite and volcanic rocks.
Laumontite is a primary silicate that occurs as pink
to salmon coloured aggregates in altered dacitic
tuff at the Two Mile deposit. It is mostly associated with
quartz
(AJM 22.1.35).
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).
At the Potter-Cramer mine, Vulture Mining District, Maricopa county, Arizona, USA, laumontite has been observed
as squarish white crystals with internal hollow spaces filled with small yellow
mimetite crystals
(R&M 96.1.32).
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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