What is Zunyite?
Zunyite is a rare cubic aluminium silicate from intense hydrothermal alteration, recognised by tetrahedral crystals and a chemically unusual combination of hydroxyl, fluorine and chlorine.
Mineralogy and structure
Reference formula: Al13Si5O20(OH,F)18Cl. Zunyite is cubic with the idealised formula Al13Si5O20(OH,F)18Cl. The hydroxyl-fluorine balance varies, so the formula is best read as an end-member expression; the Guide uses a natural COD representative model. Zunyite is classified with silicates and has cubic crystal symmetry. Typical hardness is 7–7 on the Mohs scale and specific gravity is approximately 2.8–2.9.
Colour, lustre and habit
Typical colours include colourless, white, grey, flesh-red. The usual tetrahedral, granular, massive habit and vitreous lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Zunyite forms in or is associated with high-alumina hydrothermal alteration and advanced argillic zones around volcanic and ore-forming systems. Important documented localities include the Zuni Mine in Colorado, Iran, Chile, Turkey and other advanced-argillic districts.
A sharp cubic crystal from an altered volcano
Zunyite is a premium conversation piece for collectors of alteration minerals: a rare cubic form and an unusual halogen-bearing formula make the specimen look simple while its chemistry is anything but.
Collecting and care
Hard but brittle: protect tetrahedral points, avoid acids and keep locality information with the specimen. Do not infer fluorine content from colour.
