What is Johannite?
Johannite is a vivid green hydrated uranyl sulfate, a classic secondary uranium mineral from oxidised veins and historic uranium districts.
Mineralogy and crystal structure
Reference formula: Cu(UO2)2(OH)2(SO4)2·8H2O. Johannite has a triclinic hydrated sulfate structure built around uranyl groups and copper. Its intense green colour is striking, but radioactivity and water sensitivity dominate practical collecting decisions. Johannite is classified with sulfates and has a triclinic crystal system. Typical hardness is 2.5–3 on the Mohs scale and specific gravity is approximately 3.3–3.5.
Colour, lustre and habit
Typical colours include green, emerald-green, yellow-green. The usual tabular, crusty, fibrous habit and vitreous, pearly lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Johannite forms in or is associated with oxidised uranium deposits and mine workings where copper and sulfate are available. Important documented localities include Jáchymov in the Czech Republic, Germany, France, the United States and Australia.
A vivid green specimen for a specialist cabinet
Johannite is memorable for the contrast between emerald-green crusts and the serious scientific story of uranium alteration. It belongs in a carefully documented cabinet collection where colour, locality and safety are presented together.
Green colour, handled responsibly
Green minerals are often given healing or renewal claims in popular crystal culture; those claims are not established facts and are inappropriate for a radioactive specimen. Johannite should be valued for its mineralogical history and displayed responsibly.
Collecting and care
Radioactive and fragile: display in a sealed case, avoid dust and prolonged handling, and follow local radiation-safety rules.
