Epistilbite - Encyclopedia

    Class : Silicates
    Subclass : Tectosilicates
    Crystal system : Monoclinic
    Chemistry : CaAl2Si6O16 5H2O
    Rarity : Rare

Epistilbite is a relatively rare species of the zeolite group. It is found mainly in the vacuoles of basalts, sometimes in the cracks of gneisses and exceptionally in beryl pegmatites. Its name is taken from the Greek epi (near) and stilbite because of the resemblance between these two minerals. As with many zeolites, the crystals are elongated prismatic, frequently twinned, grouped in spherolites or sheaves ; the largest do not exceed 3 cm. Epistilbite is colorless or variable in color : white, pinkish, sometimes salmon pink or yellow, exceptionally light green.


Main photo : Epistilbite from Jalgaon, Maharashtra, India © Dan Weinrich

Epistilbite from Puu o Ehu Quarry, Kailua, Oahu Island, Hawaii, USA © Matteo Chinellato
Pink epistilbite from Nashik, Maharashtra, India © Knut Eldjarn
Epistilbite on amethyst from Osilo, Sardinia, Italy © Gianfranco Ciccolini
Twinned epistilbite from Wolf Point, Washington, USA © Donald Howard (SEM Image)

Epistilbite in the World

Epistilbite is known from more than 25 localities, the most beautiful crystals being exceptional 3 cm tabular individuals and radiated bundles discovered in the cavities of the Teigarhorn basalts (Iceland). The famous quarries of Khandivali near Bombay (India), Poona and Nasik, opened in the Dekkan basalts, have yielded superb geodic crystals measuring 3 cm, sometimes salmon pink. Riffe Lake (Washington) provided good 2.5 cm crystals which are the best American specimens. In Europe, the most beautiful epistilbites were discovered at San Piero in Campo (Island of Elba, Italy) and Osilo (Sardinia) on amethyst.

Epistilbite in France

In France, epistilbite is only reported in the Kerguelen Islands.

Twinning

Twins are known on {100}, sometimes cyclical and on {110} crosswise.

Fakes and treatments

No fake listed for this mineral species.


Hardness : 4
Density : 2.22 to 2.28
Fracture : Irregular
Streak : White

TP : Translucent to transparent
RI : 1.502 to 1.512
Birefringence : 0.010
Optical character : Biaxial -
Pleochroism : None
Fluorescence : None

Solubility : Hydrochloric acid


Magnetism : None
Radioactivity : None

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