Johannite is a rare uranium sulfate mineral. It crystallizes in the triclinic crystal system with the chemical composition Cu[UO₂(OH)SO₄]₂·8H₂O. 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. Its color is emerald-green to apple-green and its streak is pale green.

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Johannite
General
Category	Sulfate mineral
Formula (repeating unit)	Cu[UO2(OH)SO4]2·8H2O
IMA symbol	Jh[1]
Strunz classification	7.EB.05 (10 ed) VI/D.21-10 (8 ed)
Dana classification	31.8.2.1
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Identification
Color	Emerald-green, apple-green
Crystal habit	Prismatic, tabular, coatings
Twinning	Simple and repeated lamellar twinning
Cleavage	good on {100}
Mohs scale hardness	2 - 2.5
Luster	Vitreous
Streak	Pale green
Diaphaneity	Transparent, Translucent
Specific gravity	3.32, 3.27 (calc.)
Density	3.32
Optical properties	Biaxial (+/-)
Refractive index	nα = 1.572 - 1.577 ; nβ = 1.592 - 1.597 ; nγ = 1.612 - 1.616
Birefringence	δ = 0.040
Pleochroism	strong: x= colorless; y= pale yellow; z= greenish yellow or canary-yellow
2V angle	90°
Other characteristics	Radioactive, Bitter taste
References	[2][3][4]

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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. It was named for Archduke John of Austria (1782–1859), the founder of the Landesmuseum Joanneum (Styria, Austria).

Johannite forms as secondary mineral by oxidation from uraninite as well as different other uranium minerals.

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).

Johannite crystallizes in the triclinic crystal system in the space group P1 with the lattice parameters a = 8.92 Å, b = 9.59 Å, c = 6.84 Å; α = 110°, β = 111.98°, γ = 100.3° and one formula unit per unit cell.