What is Offretite?
Offretite is a rare hexagonal zeolite that forms bright crystals and sprays in cavities and altered volcanic rocks.
Mineralogy and crystal structure
Reference formula: (Ca,K2,Na2)Al2Si7O18·6H2O. Offretite is a channelled zeolite with exchangeable cations and water. It is closely related to erionite, so exact identification depends on crystal chemistry and locality rather than habit alone. Offretite is classified with silicates and has a hexagonal crystal system. Typical hardness is 3.5–4 on the Mohs scale and specific gravity is approximately 2.1–2.2.
Colour, lustre and habit
Typical colours include colourless, white, yellow, pink. The usual prismatic, fibrous, radiating habit and vitreous, pearly lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Offretite forms in or is associated with low-temperature hydrothermal alteration of basalt, volcanic tuffs and zeolite cavities. Important documented localities include Mont Saint-Hilaire in Canada, Iceland, Italy, Japan, India and the United States.
A channelled zeolite with a clean hexagonal habit
Offretite turns a volcanic cavity into a miniature filtration system: its open framework hosts water and exchangeable ions while its crystals preserve the chemistry of alteration.
Collecting and care
Protect from abrasion and humidity swings; avoid dust and aggressive cleaning of fibrous aggregates.
