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

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).


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).


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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