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

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

No fakes recorded for this mineral species.