Laumontite |Ca4(H2O)18| [Al8Si16O48]
     
Morphology:  
  Monoclinic, 2/m
Single crystals are pseudo-tetragonal prisms terminated by a pinacoid.
Crystal sizes range from a few millimeters to several centimeters.
Common forms: {110} and {-201}
 
Physical properties:
  Cleavage:  {110} perfect
Hardness: 5.5
Density:  2.20 - 2.26 g/cm3
Luster:  vitreous to chalky
Streak:  white 		Laumontite prisms (up to 2 cm long) in hydrothermally altered basalt in the conduit system of the 2.6 million year old Koolau volcano, Kailua, northeast Oahu, Hawaii.
   
Optical properties:
  Color: white to gray, pink, yellowish,
brownish, golden brown
colorless in thin section
Biaxial (-)
α = 1.510-1.514, β = 1.518-1.522,
γ = 1.522-1.525, δ = 0.010-0.012
2Vx = 23-47°
Z ^ c = 8-10° (in acute β angle) , Y = b,
O.A.P. = (010)
Dispersion: r < v, distinct, weakly inclined
(Partially dehydrated laumontite:
   α = 1.502 -1.507, β = 1.512-1.516,
   γ = 1.514-1.518.  δ = 0.011- 0.012,
   2Vx = 26-44°)
     
Crystallography:  
  Unit cell: 
   a = 14.587 Å, b = 12.877 Å, c = 7.613 Å, β = 111.159°
Z = 1
Space group: C2/m
Partially dehydrated laumontite (leonhardite):
   unit cell:  a = 14.714 Å, b = 13.132 Å, c = 7.531 Å, β = 111.23° (Wuest and Armbruster, 2000)
 
  Thin section views of partial replacement of albitized plagioclase by laumontite in lower Triassic sandstone of the Fairplace Formation, Hokonui Hills, New Zealand. Top left plain light, top right, crossed polars; lower left, crossed polars with 1° Red compensator. Scale bar 0.1 mm. The blue areas are plagioclase, the yellow is length slow laumontite, which always shows this relationship with plagioclase, meaning that its replacement is oriented to the plagioclase structure (Coombs and Boles, pc.). Sample is in the Otago University Geology collection # OU 30015; the chemical analysis #4 on p. 354 of Boles (1974). Images courtesy of J. R. Boles.
   
Name:  
  Laumontite was first described by Haüy (1801) and named “zéolithe efflorescente.” The mineral was considered a distinct species by Werner (Jameson, 1805), who changed the name to “lomonite”, honoring Gillet de Laumont, who collected the material studied by Haüy. Haüy (1809) changed the spelling to “laumonite”, and finally the name laumontite was suggested by von Leonhard (1821). Leonhardite, recently discredited as a species (Coombs et al., 1997), refers to partially dehydrated laumontite with approximately 14 H2O per unit cell.
       
Crystal structure:  
  Fully hydrated laumontite has the simplified formula |Ca4(H2O)18| [Al8Si16O48] (Armbruster and Kohler, 1992; Artioli and Ståhl, 1993; Ståhl and Artioli, 1993). Before these studies, it was assumed that fully hydrated laumontite contained 16 H2O pfu (e.g. Coombs, 1952; Pipping, 1966; Gottardi and Galli, 1985). When laumontite is exposed to low humidity, it partially dehydrates at room temperature to a variety named “leonhardite” (Blum, 1843; Delffs, 1843) with the simplified formula|Ca4(H2O)14| [Al8Si16O48] (e.g. Coombs, 1952; Pipping, 1966; Armbruster and Kohler, 1992; Artioli et al., 1989). This dehydration can be reversed by soaking the sample in H2O at room temperature (e.g. Coombs, 1952; Armbruster and Kohler, 1992) and observed with the polarizing microscope: extinction angle varies directly with degree of hydration (Coombs, 1952).
 

Various structural studies of laumontite with different degrees of hydration (between 10.8 and 18 H2O pfu) show that the framework topology remains unchanged and can be described in C2/m symmetry (Artioli et al., 1989; Armbruster and Kohler, 1992; Artioli and Ståhl, 1993; Ståhl and Artioli, 1993). The framework exhibits two different types of four-membered rings, those where SiO4 and AlO4 alternate and those formed only by SiO4 tetrahedra. Large channels run parallel to the c-axis (figure here and LAU) confined by ten-membered rings (aperture 4.0 x 5.3 Å). Ca occupies a four-fold site on a mirror within the c-extended channels and is coordinated by three H2O molecules and four oxygens, belonging to AlO4 tetrahedra. See the center of the figure.






  Room-temperature unit-cell parameters for fully hydrated laumontite with 18 H2O are a = 14.863, b = 13.169, c = 7.537 Å, β = 110.18°, Z = 1 (Ståhl and Artioli, 1993), whereas the 14 H2O variant has a = 14.75, b = 13.07, c = 7.60 Å, β = 112.7°, Z = 1 (Pipping, 1966). Coombs (1952) found no piezoelectric effect for laumontite and leonhardite but a strong pyroelectric effect. Thus, it may be concluded that the structure lacks a center of symmetry and the space group is either C2 or Cm. Ståhl and Artioli (1993) discussed the possibility of a locally ordered H2O superstructure leading to the lowering of symmetry. The simplest model would lead to doubling of the a-axis with space group P2. Armbruster and Kohler (1992) and Ståhl et al. (1996) investigated the dehydration of laumontite. The complicated clustering of H2O and accompanying phase transitions in laumontite with varying degrees of hydration were studied by Gabuda and Kozlova (1995) using NMR 1H and 27Al spectroscopy between 200 and 390 K.
  Fersman (1909) introduced the term “primary leonhardite” for the composition |Na1.24K1.59Ca2.55(H2O)14| [Al8.18Fe3+0.03Si15.86O48] later confirmed by Pipping (1966). This “primary leonhardite,” with more than five channel cations pfu, neither dehydrates nor rehydrates at room temperature. The excess of channel cations compared with ordinary laumontite indicates that “primary leonhardite” has additional non-framework cation sites (Baur et al., 1997; Stolz and Armbruster, 1997) occupied by H2O molecules in fully hydrated laumontite. This also explains why “primary leonhardite” cannot be further hydrated and why it shows no indication of weathering as usual for exposed laumontite. The species name “leonhardite” was recently discredited as a mineral species (Coombs et al., 1997; Wuest and Armbruster, 2000), because “leonhardite” is just a partially dehydrated variety of laumontite.
   
Chemical composition:
  Laumontite composition deviates little from the formula |Ca4(H2O)18| [Al8Si16O48] both in the Si,Al content of the framework and the cation content of the channels. Most analyses show small amounts of Na and K, while Mg and Sr replacement for Ca is minor. Rarely Fe3+ replaces tetrahedral Al, giving crystals an orange color. However, the red color of some laumontite crystals is more likely to be a result of hematite inclusions. Because of the ordered structure, TSi (the fraction of Si in tetrahedral sites) varies only slightly from 0.667.
  The largest compositional variation is in the water content, because laumontite at least partially dehydrates in low humidity air. Typical laumontite analyses yield 14 to 16 weight % H2O, which corresponds to 13 to 17 H2O molecules per unit cell. Fully hydrated laumontite with 18 H2O pfu, (Armbruster and Kohler, 1992; Artioli and Ståhl, 1993; Ståhl and Artioli, 1993) contains 16.9 weight % H2O. Even though laumontite easily dehydrates with a slight framework collapse, immersion in water permits full rehydration, without any change in the appearance of the crystals.
  The most alkali-rich laumontite, “primary leonhardite,” contains 1.24 Na and 1.59 K pfu  (Fersman, 1909). Physical properties, such as refractive index, differ from those of the usual partially dehydrated laumontite (Kiseleva et al., 1996).
       
Occurrences:
  Most laumontite occurs as an alteration product of rocks that had a dominant plagioclase and/or basaltic glass component and have been exposed to slightly elevated temperatures (50° to 250°C) in the presence of water. Such rocks include sandstone from arc-source terrains, basaltic lava flows, and granodiorite and granodioritic gneiss. Some laumontite occurs as a replacement of earlier calcic zeolites, such as heulandite and stilbite, and is a key mineral of the zeolite facies of very low-grade metamorphic rocks. The following summary is adapted from a more complete section in Deer et al., 2004)
  Diagenesis and burial metamorphism of sediment and sedimentary rocks
    Reaction between the volcanic component of various kinds of sediment with interstitial water may produce authigenic zeolite minerals. In thick accumulations of volcaniclastic sediment, laumontite is a common alteration product deep in the section. Laumontite replaces early formed heulandite and is associated with albitized plagioclase in andesitic volcaniclastic sediment of fore- and back-arc basins, persisting into burial metamorphism conditions.
  Deep marine sediment
    Authigenic zeolites in deep marine sediment consist largely of phillipsite and clinoptilolite. However, Sands and Drever (1978) report laumontite in sediment from the drill core taken from DSDP Site 33 on the Bellinghausen Abyssal Plain a little west of the straits between Antarctica and South America. Laumontite occurs in four horizons between burial depths of 167 and 697 m. Although the laumontite may have crystallized from particularly Ca-rich stratal solutions, Kastner and Stonecipher (1978) raise the possibility that it is detrital, as all of the sediment above 698 m depth has a terrestrial source.
  Diagenesis and burial metamorphism of marine sediment from arc-source terrains
    In a classic paper Coombs (1954) demonstrated the importance of zeolite minerals as products of diagenesis and early stages of metamorphism of volcanic sedimentary rocks. In the Taringatura Hills, New Zealand, the Southland Syncline exposes about 10 km of Triassic sediment with a large component of volcanic detritus. In the absence of intrusive rocks the progressive changes in alteration minerals with depth apparently occur in response to the temperature and pressure increase from burial. It was Coombs’ study of this section that led to the concept of burial metamorphism and the establishment of the zeolite facies (Coombs et al., 1959). Laumontite occurs in the lower half of this section associated with albite or albitized plagioclase.
    Later work by Boles (1974) and Boles and Coombs (1977) on a similar section of the Southland Syncline exposed in the Hokonui Hills showed that several other factors may play a role in various phase appearances, such as ionic activity in stratal waters, P(CO2), incomplete reactions, and the relationship between P(total) and P(fluid). Boles and Coombs (1975) estimate the average geothermal gradient in the Southland syncline to be about 25°/km, based on the depth where analcime + quartz reacted to form albite, which occurs near 190°C (Liou, 1971).
    Laumontite is typically associated with albite, calcite, chlorite, quartz, and clay minerals in zeolite facies assemblages, which occur in many areas of volcanogenic sediment accumulation. Some examples are, the Permian Broughton Sandstone, Kiama District, New South Wales (Raam, 1968); Cretaceous sedimentary rocks of western Alaska (Hoare et al., 1964); Triassic and Jurassic andesitic sedimentary rocks of central Oregon (Dickinson, 1962; Brown and Thayer, 1963); the late Mesozoic arkosic sandstone of the Great Valley sequence of central California (Dickinson et al., 1969); Miocene arkosic sandstone units of the central Coast Ranges, California (Madsen and Murata, 1970; Murata and Whiteley, 1973); lower Cretaceous sediments of south-eastern Alexander Island, Antarctica (Horne, 1968); Cretaceous andesitic sediments west of Santiago, Chile (Levi, 1969); Cretaceous andesitic pyroclastic rocks and lava flows of east-central Puerto Rico (Otálora, 1964; Jolly, 1970); Lower Jurassic red beds in the Malka River basin in the North Caucasus, Russia (Rengarten, 1950); lower Triassic beds in southern Tanzania (Wopfner et al. 1991); Eocene and Oligocene andesitic graywacke of the Taveyanne Formation, especially in the Thones syncline, France and Switzerland (Sawatzki, 1975). The apparent depths at which laumontite appears in many of these areas is shallower than that in the Southland Syncline, New Zealand, suggesting higher geothermal gradients in some cases from nearby intrusions.
    In Japan, the pervasive zeolitic alteration of Neogene pyroclastic rocks was reviewed by Utada (1971), Iijima and Utada (1972), and Utada (2001). They classify these occurrences in four genetic types: burial diagenesis, submarine burial hydrothermal diagenesis, thermal effects of intrusive masses, and hydrothermal. Among the first three genetic types Utada (1971, 2001) recognized four zoning types based on the local geothermal gradient -- L (low), M, H and VH (very high, or the caldera-type zeolitization). The effects of burial diagenesis are most readily assessed in oil and gas fields. Miocene sedimentary rocks in the oil and gas fields of the Akita and Niigata Districts, northwestern Honshu Island, Japan, contain abundant rhyolite and dacite tuffs. Boreholes, as deep as 5 km, reveal zoning in the occurrence of diagenetic zeolites (Iijima and Utada, 1972; Iijima, 1995). Correlation of depth and borehole temperatures between three wells suggests active diagenesis. The most important variable is temperature; much less important is depth (Pload). Laumontite occurs between about 110° and 140°C over depths of 3 to 5 km, overlapping the analcime-albite transition. Limits of zeolite occurrence in other oilfields may be affected by saline pore fluids and hydrothermal heating (Iijima, 1995).
    Other direct measurements of the conditions of laumontite crystallization during diagenesis come from studies of California oilfields and nearby surface exposures of the same formations. Galloway (1979) reviewed the diagenesis of sandstone in convergent plate-margin basins in the Pacific Northwest (USA and Canada). Sandstone beds that have reached the laumontite stage of diagenesis have limited permeability and, therefore, limited reservoir potential. McCulloh et al. (1978) and McCulloh and Stewart (1979) investigated the occurrences of diagenetic laumontite in actively subsiding sedimentary basins in southern San Joaquin Valley and the Los Angeles Basin, California, USA. Laumontite was found to have crystallized from about 60°C at 1,100 m to 193°C at 6100 m. The 60°C temperature is the apparent low temperature of laumontite occurrences.
  Very low-grade metamorphism, zeolite facies
    The thermal aureole surrounding the central intrusive quartz diorite in the Tanzawa Mountains, central Japan, is divided into five zones (Seki et al., 1969a). The outer most zone has stable stilbite, and the next inward one, laumontite. These are followed by the pumpellyite-prehnite-chlorite zone. The original rocks affected by the metamorphism were basaltic to andesitic breccias and tuffs, totaling about 3000 m in thickness.
 

 

 The upper Triassic Karmutsen Group, Vancouver Island, British Columbia, consists of about 6000 m of basaltic pillow lava, pillow breccia, aquagene tuff, and massive amygdaloidal basalt flows. This section was first subjected to burial metamorphism and later to thermal metamorphism by the intrusion of the Jurassic Coast Range Batholith. Progressive metamorphism from the zeolite facies to the prehnite-pumpellyite facies was investigated by Cho et al. (1986). Based on petrologic and mineralogic evidence two reactions were proposed that limit the upper range of laumontite stability:
      laumontite + prehnite + chlorite →  pumpellyite + quartz + H2O
laumontite + pumpellyite →  epidote + chlorite + quartz + H2O.
    Although the temperature at which these reactions occur is affected by the iron content of prehnite and pumpellyite, graphical analysis suggests that the zeolite to prehnite-pumpellyite facies transition occurred at P = 1.1 ±0.5 kb and T = 190 ±30°C.
    Very low-grade metamorphism of the Eocene to Oligocene Taveyanne greywacke in the Helvetic nappes on the northern and western flanks of the Alps of France and Switzerland has produced extensive laumontite and albitized plagioclase (Rahn et al., 1994; Martini and Vuagnat, 1965; Martini, 1968). High grades typically contain prehnite and pumpellyite.
    Middle Triassic volcaniclastics are exposed in the Dinarides, a belt of mountains along the eastern margin of the Adriatic Sea running through Slovenia, Croatia,  Bosnia-Herzogovina, Montenegro, and western Serbia (Obradovic, 1979). The development of zeolite alteration products may be the result of burial diagenesis and very low grade metamorphism. There are five zones of alteration consisting of the following assemblages, (1) glass plus smectite, (2) clinoptilolite, (3) clinoptilolite-mordenite-analcime-laumontite, (4) analcime-laumontite, and (5) albite, similar to those of the Green Tuff region of Japan.
    Liou (1979) showed that the basaltic rocks of the fragmented East Taiwan Ophiolite were subjected to “ocean floor” metamorphism, hydrothermal metamorphism near a midocean ridge. Associated with prehnite and pumpellyite are the zeolites, analcime, natrolite, thomsonite, stilbite, heulandite, and laumontite. Liou (1979) estimates that metamorphism occurred at T = 150-250°C and at depths of 0.6 to 1.6 km.
    Aguirre et al. (1978), Offler et al. (1980), Aguirre and Offler (1985), and Levi et al. (1989) show that the rocks in basins along the Andes of Peru and Chile have not been deformed and that grade increases with depth from zeolite to greenschist facies. In the Santiago area of Chile Levi et al. (1989) divide the zeolite facies into a shallow mordenite subfacies, which generally overlies the laumontite subfacies. The affected rocks are Mesozoic marine and continental lavas and volcaniclastic rocks of mafic to intermediate composition. These are overlain by Cenozoic subareal andesitic flows and clastic debris. Interestingly the mineralogic breaks tend to be at unconformities.
  Diagenesis of basalt flows
 

 

 The numerous worldwide localities of laumontite occurring in cavities of basalt are described by Tschernich (1992). Common associated minerals are prehnite, quartz, scolecite, stilbite, heulandite, chabazite, yugawaralite, and calcite. Very few occurrences have been investigated with respect to the temperature and pressure of formation. Keith and Staples (1985) used δ18O to estimate the temperature of 60-70°C for formation of zeolites, including laumontite, in the Siletz River volcanics, Oregon, USA. Wise and Moller (1990) used fluid inclusions in calcite crystals associated with laumontite from the Khandivali Quarry, near Mumbai, India. The temperature 190°C combined with the estimated maximum depth of 2000 m indicates a high geothermal gradient.
  Laumontite as a product of hydrothermal alteration
    Laumontite is a common product of reaction between hot water and permeable rock with a calcic component. Environments that produce laumontite range from shattered rock near fault zones with low water to rock ratios to active geothermal systems with much higher ratios. The formation of laumontite depends as much on the chemical environment as it does on temperature.
  Active geothermal systems
    Laumontite is known from most of the geothermal areas of the world, and occurs at intermediate depths in steam wells. At Wairakei, New Zealand (Coombs et al., 1959) and Katayama, Japan (Seki et al., 1969b) the zeolite zones are mordenite near the top, laumontite at intermediate depths, and wairakite near the bottom of wells. The transition between mordenite and laumontite zones occur at about 100 m and 80°C, and between the laumontite and wairakite zones, at 115 to 255 m and 75° to 175°C at Katayama. At Wairakei the laumontite to wairakite transition is at 180 to 330 m and 190° to 224°C.
    Because the Yellowstone geothermal areas are in rhyolitic, pyroclastic host rocks, laumontite has been found in only three drill cores (Bargar et al., 1981). It occurs in the lower section of all cores, in Y-3 from about 140° to 196°C, in Y-13 at about 160°C, and in Y-2 at 200°C. The Ca for laumontite and other Ca-zeolites must have come from the circulating ground waters.
    The distribution of zeolites in the geothermal areas of Iceland has been documented by Kristmannsdóttir and Tómasson (1978) and Kristmannsdóttir (1982). Zeolites are common in the drill core taken from low-temperature geothermal areas in basalt bordering active volcanic zones, in which temperatures reach about 190°C. In the Reykjavík area laumontite appears at depths greater than about 900 m and at temperatures above 90°C. Zeolites are rare in the high temperatures geothermal areas, where temperatures exceed 300°C.
    McCulloh et al. (1981) report the apparent active precipitation of laumontite from hot spring waters in the western Transverse Range, California, USA. Discharging water is 89°C, has a pH of 7.74, and 1200 mg/liter total dissolved solids. Thermodynamic calculations based on water chemistry indicate that the spring water is saturated with respect to laumontite. There is a possibility that the laumontite may have come from the surrounding country rock.
  Fossil hydrothermal systems
    Laumontite is rare in the hydrothermal alteration aureoles associated with ore deposits, largely because those fluids are commonly acidic. However, a few examples are well documented, such as the breccia pipe deposit of the Tribag Mine, Ontario (Armbrust, 1969). Where laumontite does occur, it is an indicator of low copper concentrations.
    Crystals of laumontite up to 15 cm in length were found in a breccia zone in granodiorite near the scheelite ore body of the Pine Creek Mine, Bishop, California,USA. Gray et al. (1968) suggest that the breccia was altered by low temperature, ascending hydrothermal solutions.
    Westercamp (1981) found that the distribution of zeolites and other amygdule minerals in the lavas of Martinique, French West Indies, is not related to regional burial. Instead they are in a zonal distribution around roots of deeply eroded volcanoes. Thomsonite and analcime characterize zone II, which is well outside the innermost, laumontite-rich zone IV. The interpretation here is the zeolite zonation represents isotherms of hydrothermal systems centered in the areas of volcanic conduits.
    The presence of yugawaralite and wairakite in the Abanico Hill area of central Chile indicate an abrupt change from the burial metamorphic effects (heulandite- and laumontite-bearing assemblages) in an Upper Cretaceous and Tertiary sequence (Vergara et al., 1993). Zoning toward a graben indicated by yugawaralite±laumonite and then wairakite±epidote is evidence for a fossil “geothermal-type” alteration overprint on the regional zeolitic burial metamorphism.
  Late stage, deuteric alteration
    Laumontite occurs in certain rocks as a late stage deuteric alteration product of feldspars or other Ca-silicates. An example is in the pocket zone of pegmatite dikes, for example, the Himalaya dike, San Diego County, California (Foord, 1977) and the dike at Jensen Quarry, Riverside County, California (DeVito and Ordway, 1984). In both of these localities laumontite occurs in druses in partially dissolved feldspar. It has also been found in cavities in two pegmatite dikes in the granodiorite stock of Monte Capanne, Elba, Italy (Orlandi and Scortecci, 1985).
    An early discovery in the drill hole into the San Andreas fault zone at Cajon Pass, Southern California, is the pervasive occurrence of laumontite and stilbite in fractured granodiorite and gneiss. Both minerals replace plagioclase and fill fractures and microfractures throughout the fault zone (James and Silver, 1988; Vincent and Ehlig, 1988). The stilbite to laumontite transition occurs at about 2100 m and 94°C. This mineralization is attributed to ground water circulation in fractured and frictionally heated rocks within the fault zone.
       
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