Natrolite


Natrolite		\|Na₂(H₂O)₂\| [Al₂Si₃O₁₀]

Morphology:			Image courtesy of Olaf Medenbach
	Orthorhombic mm2 single crystals are pseudo-tetragonal prisms terminated by a pyramid sizes range from a few millimeters to several centimeters Common forms: {110} and {111}

Physical properties:
	Cleavage: {110} perfect Hardness: 5.5 Density: 2.20 - 2.26 g/cm³ Luster: vitreous Streak: white

Optical properties:
	Color: colorless to gray, bluish, yellowish colorless in thin section Biaxial (+) α = 1.473 - 1.483, β = 1.476 - 1.486, γ = 1.485 - 1.496, δ = 0.012 2V_z = 58 - 73° X = a, Y = b, Z = c Dispersion: r < v, weak
			Thin section view of natrolite in hydrothermal portion of the Coyote Peak alkalic, ultramafic diatreme, Humboldt County, California Crossed polars. Width of view, 5 mm.

Crystallography:
	Unit cell: a = 18.2928 Å b = 18.6383 Å c = 6.5848 Å (Neuhoff et al., 2002) Z = 8 Space group: Fdd2

Name:
	The earliest names for the natrolite group, which included natrolite, mesolite, scolecite, and thomsonite, was some form of fibrous zeolite, such as Faserzeolithe of A.G. Werner and mesotype of Haüy (1801). Klaproth (1803) proposed the name natrolite, referring to its composition, for the mineral from Hohentweil, Hegau, Bade-Württemberg, Germany.
	A tetragonal natrolite, first found at Ilimaussaq, Greenland (Krogh Andersen et al., 1969), was later named tetranatrolite by Chen and Chao (1980) for similar material from Mont Saint-Hilaire, Quebec, Canada. Gonnardite has a variable composition, both in the framework and in the channels. Both tetranatrolite and gonnardite have the natrolite framework, but are disordered. All known samples form a continuous compositional series from Na-rich tetranatrolite to gonnardite, which has as much as 35% of the Na replaced by Ca. In 1998 the International Mineralogical Association, Commission on New Minerals, Nomenclature and Classification voted to abandon (discredit) the mineral name tetranatrolite, and voted to retain gonnardite to apply to all compositions with the disordered natrolite structure. Paranatrolite was named by Chao (1980) for a form of natrolite with the composition \|Na₂(H₂O)₃\| [Al₂Si₃O₁₀]. It appears to be an overhydrated form of gonnardite, and has been assigned doubtful status (Coombs et al., 1997), based on their Rule 4, which does not recognize states of partial hydration or overhydration as sufficient grounds for separate zeolite species.

Crystal structure:
	The structure of natrolite was postulated by Pauling (1930) and was soon confirmed by X‑ray diffraction methods (Taylor et al., 1933). Meier (1960) also confirmed the structure and refined the atomic positions. The framework of the natrolite and the other fibrous zeolites consist of linked T₅O₁₀ chains (NAT). In natrolite the chains have the composition Al₂Si₃O₁₀, and where highly ordered, the repeat distance along the chain, the c-axis, is 6.6 Å.
	Because the T₅O₁₀ chain has four tetrahedra to link with other chains, the topological symmetry of the framework is tetragonal. In the case of the natrolite arrangement, each chain is translated by c/4, in order to avoid Al-O-Al linkages. This type of linkage of the chains, whether ordered or not, yields the topology of the natrolite framework.
	The space between four linked chains forms a channel parallel to the chain lengths. Bonding non-framework cations causes a partial collapse of the channel through rotation of about 24° of each chain. This reduces the crystal symmetry
	to the orthorhombic space group Fdd2 for natrolite. Because the chains are polar, and in the natrolite framework all chains are oriented in the same direction, the structure is non-centrosymmetric. This aspect of the structure explains the weak pyroelectric effect of natrolite with a single polar axis, the c-axis (Hey, 1932).
	In natrolite Na cations (yellow in the figure) are symmetrically located along a screw-diad in the center of each channel (see the bottom edge of the figure). Each cation is coordinated with four framework oxygen anions on one side of the oval shaped channel and to the oxygen of two water molecules (Artioli et al., 1984). The water molecules (blue) are located near the “window” between the two chains forming the flat side of the channel. The protons of the water molecules are each bonded to a framework oxygen, and the water oxygen is bonded to the Na cation. In contrast to many other zeolites the cations and water molecules are tightly held in fixed positions, which affect the cation exchange and dehydration energy.

	Following the discovery of tetragonal natrolite (Krogh Andersen et al., 1969), which was hypothesized to have a disordered framework (Pabst, 1971), the refinement of the structure of several other samples of natrolite showed them to be partially disordered (Alberti and Vezzalini, 1981). One way the degree of ordering is estimated uses the difference between the b and a unit cell dimensions. Natrolite with the highest degree of ordering (b - a) is 0.346 Å (Neuhoff et al., 2002) and is zero for tetragonal natrolite. Alberti et al. (1995) reviewed the ordering in thirteen refined crystal structures. Neuhoff et al. (2002) investigated 4 natrolite samples with ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, finding one sample (San Benito County, California, USA) that is perfectly ordered.
	Lee et al. (2002) showed that at pressures of 0.8 to 1.5 GPa natrolite has an abrupt volume expansion of about 2.5% without altering the framework topology. The anomalous swelling is due to sorption of water from the pressure-transmission fluid giving rise to a "superhydrated" phase of natrolite. The approximate formula of this phase is Na₂(H₂O)₄[Al₂Si₃O₁₀], which contains hydrogen-bonded helical water nanotubes along the channels.

Chemical composition:
	Critical reviews of the chemical compositions of various members of the natrolite group have been published by Hey (1932), Foster (1965a and 1965b), Alberti et al. (1982), Ross et al. (1992), and Deer et al. (2004).
	Natrolite compositions are all near the pure sodium end member. Ca varies from 0.0 to 0.43 atoms per unit cell of 80 framework oxygen anions, and Sr, Ba, Mg, and K do not exceed 0.03. T_Si (the fraction of Si in all tetrahedral sites) averages slightly higher than 0.600, but the amount is, in many cases, within analytical error. The framework structure cannot be highly ordered if T_Si is lower than 0.600. For example, the natrolite from Palagonia, Sicily, Italy, with the highest amount of Ca (0.43) and lowest T_Si (0.591) is one of two samples most disordered without having tetragonal symmetry (Alberti et al., 1995). Another consequence of ordered structures is intermediate compositions between those of natrolite, mesolite, and scolecite do not occur. For example, where mesolite occurs as an epitaxial overgrowth on natrolite, both minerals have the usual compositions (Gunter et al., 1993).

Occurrences:
	Natrolite is a common zeolite, occurring worldwide in a variety of settings. The most common is in cavities and veins cutting altered basaltic rocks, such as lavas or shallow intrusives, including ophiolite sequences. There are rare occurrences as diagenetic alteration products in sedimentary rocks. An especially important occurrence of natrolite is the late stage product of autometasomatism or deuteric alteration of alkaline intrusions, like nepheline syenite. The following summary is adapted from Deer et al., 2004).
	Diagenesis of sediment and sedimentary rocks
	Natrolite rarely occurs as an authigenic mineral in sedimentary rocks. Such occurrences tend to involve unusual chemical compositions of the host rocks or interstitial waters.
	Hydrologically closed systems
		Three lithofacies associated with a Pleistocene lake comprise the exposures in Olduvai Gorge, Tanzania (Hay, 1966, 1970). These are 1) lake beds of tuff and non-tuffaceous claystone, 2) lake-margin deposits of zeolitic limestone, “zeolitite”, and claystone, and 3) alluvial beds of claystone and tuffs and mudflows deposits. Twenty to 40 m of alluvium overlie the lake-margin beds, and consist of claystone and sandstone with some tuff and mudflows, all from nearby volcanic highlands and exposed crystalline basement. All rock types show alteration to zeolites at least to some degree. Nearly all claystones have been chemically modified, mostly oxidized reddish-brown and partially altered to analcime with natrolite being a minor constituent. Alteration of the claystone to analcime and natrolite probably occurs from reaction with pore waters made especially alkaline by evapotranspiration (Hay, 1970).
		Natrolite was found as a very minor constituent in drill core from the Green River Formation in Garfield County, Colorado, USA (Pabst, 1971).
	Marine sediment from arc-source terrains
		Jurassic to Cretaceous of the Eugenia Formation exposed on the western Vizcaino Peninsula, Baja California Sur, Mexico, contain authigenic natrolite and mesolite, heulandite-Na, and analcime (Barnes et al., 1984). These zeolites replace plagioclase and glassy vitric fragments, as well as form cement, in the heterogenous marine volcaniclastic rocks.
		Bass et al. (1973) report natrolite in basalt within deep sea sediments in the central Pacific Ocean.
	Diagenesis and very low grade metamorphism of basalt and other kinds of lava flows
		Cavity and vein filling in altered basaltic lavas is the setting for most occurrences of natrolite. The occurrences in Iceland and Ireland provide relationships that indicate the general temperatures and depths of formation of fibrous zeolites. In eastern Iceland the lowest exposures are rich in zeolites and define a zone from below sea level to about 800 m, which is overlain by the analcime zone (Walker, 1960b). Compared with geothermal areas in Iceland this zone forms in temperature range 70° to 90°C (Kristmannsdóttir and Tómasson, 1978).
		Some notable occurrences of natrolite in altered basalt are in breccia zones and cavities of Lower Jurassic basalt, pillow basalt, and diabase exposed by quarries at Bergen Hill, Hudson County; and at Prospect Park, Paterson, and Bound Brook, Passaic County, New Jersey, USA (Sassen, 1973, 1978); in large cavities in highly altered Tertiary basalt at the New and Old Springfield quarries, Lane Co., Oregon, USA (Tschnerich, 1992); widespread in Tertiary olivine basalt of eastern County Antrim, Northern Ireland, with exceptional crystals occurring at White Head and the Magheramorene Quarry, Larne (Walker, 1960a, 1962); in vesicular Tertiary, porphyritic basalt at Puy de Marman, north of Veyre, Puy de Dôme, Auvergne, France; the type locality in veins cutting phonolite at Hohentwiel, Hegau, Baden-Württemburg (Ramdohr, 1978) and basalt in the Höwenegg quarry, Germany; in middle Tertiary submarine basalt exposed along the Pahau River, Culverden, Canterbury, South Island, New Zealand (Mason, 1946); in Pliocene and Pleistocene basalt at Cairns Bay, Flinders, and Bundoora, Victoria, Australia (Birch, 1989). Many other localities are listed and described by Tschernich (1992).
		There are some occurrences of the natrolite in cavities of lavas that are part of a sequence that exhibits the effects of progressive metamorphism. For example, natrolite and thomsonite occur in vesicles in some pillow lavas in the metamorphosed Horokani ophiolite of the Kamuikotan Zone, Hokkaido, Japan (Ishizuka, 1985). Natrolite and thomsonite also occur in vesicles of dikes within the Takitimu Group, Southland, New Zealand (Houghton, 1982).
	Deuteric to hydrothermal alteration
		Natrolite, along with gonnardite in some cases, crystallizes in miarolitic cavities and fractures in nepheline syenite, nepheline phonolite, and syenite pegmatite dikes. Both may crystallize from late alkaline fluids or by reaction of fluids with nepheline or analcime in a deuteric alteration process.
		At Mont Saint-Hilaire, Québec, natrolite is an abundant constituent, especially of pegmatitic veins and miarolitic cavities, but occurs in most rock types (Horváth and Gault, 1990). Some cavities are large enough to produce crystals as large as 15 cm long and 2 cm wide. Commonly the larger natrolite crystals are epitaxially overgrown by several millimeters of “paranatrolite”, overhydrated natrolite, which upon exposure to the atmosphere dehydrates to gonnardite (formerly “tetranatrolite”). In many instances natrolite crystals grow on analcime. These relations indicate that this natrolite crystallized from volatile-rich fluids developed late in the crystallization of the syenite body, probably at temperatures below 200°C (Senderov and Khitarov, 1971).
		Similar parageneses of natrolite and gonnardite (“tetranatrolite” and “paranatrolite”) have been described for the Lovozero and Khibiny Massifs, Kola Peninsula, Russia (Labuntzov, 1927 and Pekov, 2000). Natrolite is very abundant, especially in late hydrothermal alteration zones, occurring as fine needles to giant prisms 40 by 15 cm. In some pegmatite bodies it is the dominant mineral, comprising whole zones with volumes up to several hundred cubic meters. Some veins within sodalite-hackmanite aggregates are filled with chalcedony-like natrolite in masses up to 20-30 cm thick. Natrolite pseudomorphs of primary pegmatite minerals, such as nepheline and ussingite, are common.
		Natrolite with edingtonite occurs in miarolitic cavities and veins in syenite of the Ice River alkaline complex, Yoho and Yootenay National Parks, British Columbia, Canada (Grice and Gault, 1981).
		There are several examples of natrolite forming by metasomatism of xenoliths or wall rocks associated with syenitic intrusions. At Magnet Cove, Arkansas, USA, natrolite with thomsonite and gonnardite replaces nepheline in ijolite xenoliths in garnet-pseudoleucite syenite (Flohr and Ross, 1989). A jacupirangite intrusion was similarly altered by fluids from nearby late phases of intrusion (Ross et al., 1992).
		As the altering fluids extend into the wall rocks, natrolite forms the major portion of veins or is a component of dikes. Veins several centimeters in width fill gashes in metabasaltic tectonic blocks in the New Idria serpentine mass, San Benito County, California, USA (Louderback, 1907; Wise and Gill, 1977). Natrolite crystals up to 4.5 cm encase benitoite, neptunite, and other earlier formed minerals. The source for the alkaline fluids may have been a syenitic intrusion, exposed 1.5 km away. In southwest Pakistan natrolite fills fractures in a dolerite dike within serpentinized harzburgite, near Hazarganji, Khudar District (Gnos et al., 1999). Huge crystals, up a meter in length, occur in aplite dikes at the Johnston asbestos mine, near Thetford, Quebec (Poitevin, 1938).
		Panzner (1987) lists natrolite as occurring in several mines in Mexico, presumably in epithermal veins. These localities include the Noche Buena mine, Zatatecas; Charchas, San Luis Potosi; and the San Felipe mine, Morelos.

References:
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