Gonnardite


Gonnardite		\|(Na,Ca_0.5)_8-10(H₂O)₁₂\| [Al_8+xSi_12-xO₄₀], x = 0 - 2

Morphology:
	Tetragonal: point group -42m. Prismatic to fibrous crystals in radiating spherules to 3 cm, commonly massive

Physical properties:
	Hardness: 5. Density = 2.25 - 2.36 gm/cm³ Luster: silky. Streak: white

Optical properties:
	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).

Crystallography:
	Unit cell: a 13.21(1), c 6.622 Å, Z = 1. Space Group: I-42d . (Mazzi et al. 1986).

Name:
	Gonnardite was named by Lacroix (1896) for material found at Chaux de Bergonne, Gignat, Puy-de-Dome, France, and named after Ferdinand Gonnard, who earlier described the material as mesole (now thomsonite). 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 highly disordered. 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.

Crystal 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, Quebec (Evans et al. 2000) show that the space group symmetry is I-42d. 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. Gonnardite exhibits a transition to high-hydrated paranatrolite at elevated humidity (Seryotkin et al. 2016).

Chemical composition:
	Gonnardite compositions vary broadly in both the framework composition and channel cations. Si varies from 0.634 in the most Na-rich sample to 0.506, and Ca/(Ca+Na) varies from near zero to 0.345. "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.

Occurrences:
	The earliest growth of many natrolite clusters appears to be gonnardite, and as growth slows the crystal becomes ordered. Gonnardite clusters occur 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-Dome, 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 Kloch, 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, Quebec, natrolite is an abundant constituent, especially of pegmatitic veins and miarolitic cavities, but occurs in most rock types (Horvath 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 formed 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).

References:
	Alberti, A., Pongiluppi, D., Vezzalini, G. (1982) The crystal chemistry of natrolite, mesolite and scolecite. Neues Jahrb. Miner. Monatsh. 1982, 231-248. Artioli, G. and Galli, E. (1999) Gonnardite: re-examination of holotype material and discreditation of tetranatrolite. Am. Mineral. 84, 1445-1450. Chao, G.Y. (1980) Paranatrolite, a new zeolite from Mont St-Hilaire, Quebec. Can. Mineral. 18, 85-88. Chen, T.T. and Chao, G.Y. (1980) Tetranatrolite from Mont St-Hilaire, Quebec. Can. Mineral. 18, 77-84. Evans, H.T.Jr., Konnert, J.A., and Ross, M. (2000) The crystal structure of tetranatrolite from Mont Saint-Hilaire, Quebec, and its chemical and structural relationship to paranatrolite and gonnardite. Am. Mineral. 85, 1808-1815. Flohr, M.J.K. and Ross, M. (1989) Alkaline igneous rocks of Magnet Cove, Arkansas: metasomatized ijolite xenoliths from Diamond Jo quarry. Am. Mineral. 74, 113-121. Harada, K., Iwamoto, S., and Kihara, K. (1967) Erionite, phillipsite and gonnardite in the amygdales of altered basalts from Maze, Niigata Pref. Japan. Am. Mineral. 52, 1785-1794. Horvath, L. and Gault, R.A. (1990) The mineralogy of Mont Saint-Hilaire, Quebec. Mineral. Rec. 21, 284-359. Khomyakov, A.P., Cherepivskaya, G.E., and Mikheeva, M.G. (1986) First finds of paranatrolite in USSR. Dokl. Akad, Nauk. 288, 214-217. Krogh Andersen, E., Dano, M., and Petersen, O.V. (1969) A tetragonal natrolite. Meddr. om Gronlands 181, 20 pp. [M.A. 74-1451]. Labuntzov, A.N. (1927) The zeolites from Khibinsky and Lovozersky Mtns., Russian Lapland. Trav. Musee Miner. Acad. Sci. USSR 2, 91-100. Lacroix, A. (1896) Sur la gonnardite. Bull. Soc. fr. Mineral. 19, 426-429. Mazzi, F., Larsen, A.O., Gottardi, G., and Galli, E. (1986) Gonnardite has the tetrahedral framework of natrolite: experimental proof with a sample from Norway. Neues Jahrb. Miner. Monatsh. 1986, 219-228. Meixner, H., Hey, M.H., and Moss, A.A. (1956) Some new occurrences of gonnardite. Min. Mag. 31, 265-271. Mikheeva, M.G., Pushcharovskii, D.Yu., Khomyakov, A.P., and Yamnova, N.A. (1986) Crystal structure of tetranatrolite. Sov. Phys. Crystallogr. 31, 254-257. Passaglia, E., Tagliavini, M.A. and Boscardin, M. (1992) Garronite, gonnardite and other zeolites from Fara Vicentina, Vicena (Italy). Neues Jahrb. Miner. Monatsh. 1992, 107-111. Pekov, I.V. 2000. Lovozero Massif: History, Pegmatites, Minerals. Ocean Pictures Ltd., Moscow, Russia. 484 pp. Pongiluppi, D. (1976) Offretite, garronite and other zeolites from "Central Massif", France. Bull. Soc. fr. Minaral. Cristallogr. 99, 322-327. Ross, M., Flohr, M.J.K., and Ross, D.R. (1992) Crystalline solution series and order-disorder within the natrolite mineral group. Am. Mineral. 77, 685-703. 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. Seryotkin, Y.V., Dement’ev, S.N. and Ancharov, A.I. (2016) The influence of extraframework cations on the behavior of K-exchanged gonnardite at high pressure. J. Struct. Chem., 57, pp.1386-1391. Updated: June 2025.