OFFRETITE

    Class : Silicates
    Subclass : Tectosilicates
    Crystal system : Hexagonal
    Chemistry : (K,Ca,Mg)3Al5Si13O36 15H2O
    Rarity : Rare

Offretite is a rare mineral from the zeolite group, known to fill cavities in basalts where it accompanies calcite and other zeolites. It was named in honor of Albert Jules Joseph Offret, Professor of Mineralogy at the University of Lyon (France). The crystals are small hexagonal prisms, sometimes cavernous and striated, not exceeding 3 mm, which can form hemispheres with a radiated texture. Offretite is sometimes found in intergrowths with erionite and levynite.

Main photo : Offretite from Zajecí hill, Arnultovice, Czech Republic © Volker Betz

Offretite from Rossberg Quarry, Hesse, Germany © Volker Betz
Offretite of Mont Sémiol, Loire, France © Yannick Vessely
Offretite from Mont Sémiol, Loire, France © Volker Betz
Offretite of Mt Foppa, Lombardy, Italy © Franco Luca Bonino

Offretite in the World

The Queen Creek basalts (Arizona) yielded the largest known crystals (3 mm). This rare zeolite is also reported in basalts of Oregon, Australia and Germany as well as the Canadian Westwold deposit in British Columbia.

Offretite in France

In France, offretite is known in the vacuoles of the basalts of Mont Sémiol (Loire) which contain tiny sub-millimeter crystals.

Twinning

No twins known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.


Hardness : 4 to 4.5
Density : 2.13
Fracture : Irregular
Streak : White

TP : Translucent to transparent
RI : 1.486 to 1.495
Birefringence : 0.003
Optical character : Uniaxial -
Pleochroism : None
Fluorescence : None

Solubility : Acids

Magnetism : NoneRadioactivity : None