It crystallizes in the triclinic system and develops only small prism or thin to thick tabular crystals, usually occurs as flaky or spheroidal aggregates and efflorescent coatings.

Johannite is a strong radioactive mineral with a calculated activity of 87,501,143 Bq/g (to the comparison: natural potassium: 31.2 Bq/g).

Johannite was first described in 1830 by Wilhelm Karl Ritter von Haidinger.

Localities include Argentina, Czech Republic, France, Gabon, Germany, Greece, Italy, Switzerland, United Kingdom and the United States.

Type locality is the “Elias Mine” in Jáchymov (Czech Republic).