Gonnardite - Encyclopedia

    Class : Silicates
    Subclass : Tectosilicates
    Crystal system : Tetragonal
    Chemistry : Na2CaAl4Si6O20 7H2O
    Rarity : Rare

Gonardite is a rare zeolite which is found in the cavities of basalts, leucite tephrites, and more rarely in syenitic pegmatites and skarns. It was named in honor of the French mineralogist Ferdinand Gonnard, Professor at the University of Lyon who described the mineral in 1871 under the name mesolite. The crystals are prismatic to fibrous, frequently grouped in spherolites not exceeding 3 cm in diameter. The association with thomsonite is frequent, it sometimes crystallizing epitaxially on gonnardite cores. Intergrowths of gonnardite on natrolite are also common. It is colorless, white, yellowish white or salmon pink.

Main photo : Gonnardite from La Chaux de Bergonne, Gignat, Puy-de-Dôme, France © Yves Masson

Gonnardite from Poudrette Quarry, Mont St-Hilaire, Canada © Stephan Wolfsried
Gonnardite from Vevja Quarry, Tvedalen, Norway © Matteo Chinellato
Gonnardite from Cerro Moreno Volcano, Almagro, Spain © José Rafael González López
Gonnardite from Hayata, Chinzei-machi, Japan © Volker Betz

Gonnardite in the World

Gonnardite sites are not rare. The basalts of the Eifel, in Germany (Zilsdorf, Brenk...) gave centimeter-sized aggregates considered to be the best samples in the world. Gonardite is also known in various contexts in the United States (skarn from Magnet Cove, Arkansas ; basalt from Honolulu, Hawaii ; Table Mountain, Colorado...), at Mont St-Hilaire (Quebec), in volcanic rocks from several Italian localities (Cabo di Bove, Osilo, Aci Trezza, etc.) and in the syenitic pegmatites of Langesundsfjord in Norway.

Gonnardite in France

In France, gonnardite has been known since 1891 in the Chaux de Bergonne basalt, near Gignat (Puy-de-Dôme). It is also reported at Coste Belle near Espalion (Aveyron), Piton des Neiges (Réunion), the Kerguelen Islands and Tahiti.

Twinning

No twin known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.


Hardness : 4 to 5
Density : 2.25 to 2.36
Fracture : Irregular
Streak : White

TP : Translucent to transparent
RI : 1.514 to 1.520
Birefringence : 0.006
Optical character : Biaxial +
Pleochroism : None
Fluorescence : Sometimes blue

Solubility : -

Magnetism : NoneRadioactivity : None

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