GISMONDINE

    Class : Silicates
    Subclass : Tectosilicates
    Crystal system : Monoclinic
    Chemistry : Ca2Al4Si4O16 9H2O
    Rarity : Uncommon

Gismondine (or gismondite) belongs to the group of zeolites. It occurs as filling of cavities in olivine basalts, sometimes in nepheline basalts or leucite tephrites. It was named in honor of the Italian mineralogist Carlo Giuseppe Gismondi who studied the mineral. Gismondite occurs in bipyramidal, pseudoquadratic crystals which are in reality twins by interpenetration, of 2 cm at most. Millimetric crystals can form rosette aggregates or radiated spherolites up to 15 mm in diameter. Epitaxies with phillipsite are common. Gismondite is white, gray, bluish or reddish in color. Like most zeolites, its density is low (2.2) because of the abundance of water in its structure.

Main photo : Gismondine from Graulay Quarry, Germany © Volker Betz

Gismondine from Vesuvius, Italy © Italo Campostrini
Gismondine from Löhley, Eifel, Germany © Jürgen Greiner
Gismondine epitaxy on phillipsite from Dobrná, Czech Republic © Petr Fuchs
Twinned gismondine from Caspar Quarry, Ettringen, Germany © Volker Heck

Gismondine in the World

Beautiful samples of gismondite come from the Italian basalts of Lazio : Capo di Bove and Montalto di Castro in particular. Gismondite is relatively common in the German olivine basalts of the Eifel, Hesse and Saxony, and in those of Hawaii, Iceland and Northern Ireland (Ballyclare). it is also reported in atypical contexts : the alpine clefts of the Gorner glacier, near Zermatt (Switzerland) and the hydrothermal vein of Concepcion del Oro (Zacatecas, Mexico).

Gismondine in France

In France, gismondite is best known in the basalts of Ardèche in St-Agrève and in those of Montbrison in Phialay (Loire).

Twinning

Twins are common for this mineral species, particularly on {101} and form crosses.

Fakes and treatments

No fakes recorded for this mineral species.


Hardness : 4.5
Density : 2.20 to 2.26
Fracture : Conchoidal
Streak : White

TP : Translucent to transparent
RI : 1.525 to 1.550
Birefringence : 0,016
Optical character : Biaxial -
Pleochroism : None
Fluorescence : None

Solubility : -


Magnetism : None
Radioactivity : None