Hardness: 5. D = 2.25 - 2.36 gm/cm3
Luster: silky.
Streak: white
Color: Colorless, yellowish to salmon-red; colorless in thin section. Uniaxial ( - or +)
ω 1.480 - 1.506, ε 1.493 - 1.508, δ 0.002 - 0.013.
Na-rich crystals are length slow, Ca-rich can be length slow or fast
Silky gonnardite prisms on analcime, Verja, Tvedalen, Telemark, Norway. Width of cluster 2 mm. (See Mazzi et al., 1986).
Space Group: I42d . (Mazzi et al., 1986).
All known samples form a continuous compositional series from Na-rich tetranatrolite to gonnardite with as much as 35% of the Na replaced by Ca. In 1998 the International Mineralogical Association, Commission on New Minerals and Mineral Names voted to abandon (discredit) the mineral name tetranatrolite, and voted to retain gonnardite to apply to all compositions with the highly disordered natrolite structure.
Gonnardite, as recently defined (Artioli and Galli, 1999), has the framework of natrolite (NAT), but the (Si,Al) occupancy is disordered. The cation contents ranging from nearly all Na cations (tetragonal natrolite) to as much as 35% Ca. Single crystal refinement of tetragonal natrolite from the Khibinsk alkali massif (Mikheeva et al., 1986) and gonnardite from Tvedalen, Norway (Mazzi et al., 1986) and Rietveld refinement of gonnardite from the type area at Gignat, France (Artioli and Galli, 1999) and tetragonal natrolite from Mont Saint-Hilaire, Québec (Evans et al., 2000) show that the space group symmetry is 142d. The framework and channel occupancy of each of these samples is nearly to completely disordered. There are two cation positions (red) in each channel, and both of these are occupied by Na and H2O in both W1 sites (dark blue). Where Ca is present, it occupies one of the two cation (red) sites, and the other will be vacant. H2O molecules occupy W1 and one of the W2 positions (light blue). “Paranatrolite” readily dehydrates to “tetranatrolite”, and therefore, probably has the same structure as gonnardite, but with enough extra occupancy of the W2, to contain third H2O molecule.
“Paranatrolite” is generally an overhydrated form of the Na-rich gonnardite (tetranatrolite), and dehydrates spontaneously when exposed to the atmosphere (Chao, 1980). Khomyakov et al. (1986) report “paranatrolite” from the Khibiny massif, Kola Peninsula, Russia, with 1.76 K atoms per unit cell of 80 framework oxygen anions. This sample is relatively stable when exposed to the atmosphere.The earliest growth of many natrolite clusters appears to be gonnardite, and as growth slows the crystal becomes ordered. Gonnardite clusters occurs in two principal environments, in cavities of altered basaltic lavas and as hydrothermal alteration of crystallization products in syenite.
Diagenesis and very low grade metamorphism of basalt and other kinds of lava flows
Some of the described localities of gonnardite occurring in basalt cavities are Gignat, Puy-de-Dôme, France, where it occurs with garronite, chabazite, and phillipsite in late Pleistocene porphyritic basalt (Pongiluppi, 1976; Artioli and Galli, 1999); with natrolite in cavities of Pliocene nepheline basalt at Klöch, Styria, Austria (Meixner et al., 1956); with thomsonite and natrolite in middle Miocene pillow basalt at Maze, Iwamure district, Niigata Prefecture, Japan (Harada et al., 1967); with phillipsite and chabazite in vesicular basalt at Aci Castello and Aci Trezza, Catania, Sicily (Meixner et al., 1956); in cavities in basalt at Halap Hill, Hungary (Alberti et al., 1982); and with analcime, garronite, phillipsite, and chabazite in middle Oligocene brecciated basalt at Fara Vicentina, Vicenza, Italy (Passaglia et al., 1992).
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. In many cavities 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”). These relations indicate that this natrolite and gonnardite 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; Pekov, 2000).
There are several examples of gonnardite and 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).
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Chao, G.Y. (1980) Paranatrolite, a new zeolite from Mont St-Hilaire, Quebec. Can. Mineral. 18, 85-88.
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Senderov, E.E. and Khitarov, N.I. (1971) Synthesis of thermodynamically stable zeolites in the Na2O-Al2O3-SiO2-H2O. In Molecular Sieve Zeolites - 1. Amer. Chem. Soc., Adv. Chem. Ser. 101, p. 149-154.