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Formula: CaAl2Si4O12.4H2O
Tectosilicate (framework silicate), zeolite group
Crystal System: Monoclinic
Specific gravity: 2.23 to 2.41 measured, 2.25 calculated
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 the Marron Volcanics of the Olalla Area South-Central British Columbia, Canada, pinkish laumontite is common at the
Yellow Lake road cuts and occurs as prisms to 1.5 cm long in veinlets and vesicles in several areas along the Yellow Lake
cliffs. Veinlets of solid laumontite and calcite up to several centimeters
wide have been found, with long, thin cavities containing laumontite crystals to 7 mm and green
fluorite
(R&M 96.6.523).
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 Glen Brittle, Minginish, Isle of Skye, Eilean á Chèo, Highland, Scotland, UK, vesicles are abundant in
basaltic lava, mostly filled with
mordenite-quartz intergrowths,
with other minerals present only in very subordinate amounts. In a few of the open cavities the central terminated
quartz crystals are overgrown by scattered small white to colourless bladed
crystals of laumontite, up to 5 mm long with a characteristic pearly lustre
(JRS 23.86-90).
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).
The Cliff Mine, Phoenix, Keweenaw county, Michigan, USA, is situated at the base of a roughly 70-metre
basalt cliff. A curious feature of the impressive thickness of the
greenstone flow here is that it contains zones of “pegmatoid”: areas
where
slow cooling in the core of the lava flow allowed for large feldspar crystals
exceeding 1 cm to grow. Such features are normally only observed in intrusive igneous rocks and are almost unheard of
in basalt flows.
The Cliff mine primarily exploited rich copper mineralisation in the Cliff
fissure (vein). Although mineralised with copper to some extent along its
entire length, the part of the vein just below the greenstone flow
carried the richest copper mineralisation by far. A significant amount of the
copper recovered at the Cliff mine came from amygdaloids in the tops of 13
basalt flows which were cut by the Cliff vein. The discovery and mining
of this vein proved that the veins were the source of the large masses of float
copper that were already well known, and proved that the
primary ore mineral in the district was native
copper, not sulphides, as had been suspected earlier.
Laumontite occurs at the Cliff mine as pinkish aggregates and single elongated crystals, both in amygdules and
in the vein. Common associates include copper,
prehnite and analcime
(MinRec 54.1.25-49).
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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