Fornacite is a rare lead, copper chromate arsenate hydroxide mineral with the formula: Pb2Cu(CrO4)(AsO4)(OH). It is a monoclinic-prismatic mineral containing arsenic, chromium, copper, hydrogen, lead, and oxygen. It is a rare mineral in the oxidized zones of some hydrothermal base-metal deposits.
It was first described in 1915 and named after Lucien Lewis Forneau (1867–1930) the governor of the French Congo. Its type locality is in Reneville, Congo Republic.
- Category: Arsenate mineral
- Formula: Pb2Cu(CrO4)(AsO4)(OH)
- Crystal system: Monoclinic
- Crystal class: Prismatic (2/m) (same H-M symbol)
It forms a series with the phosphate mineral vauquelinite. It forms variably green to yellow, translucent to transparent crystals in the monoclinic – prismatic crystal system. It has a Mohs hardness of 2.3 and a specific gravity of 6.27.
- Color: Deep olive-green
- Crystal habit: Aggregates of steep pyramidal to bladed, rounded crystals
- Fracture: Irregular/uneven, conchoidal, sub-conchoidal
- Tenacity: Brittle
- Mohs scale hardness: 2 – 3
- Luster: Resinous, waxy, greasy
- Streak: Olive green
- Diaphaneity: Transparent
- Density: 6.27 g/cm
- Optical properties: Biaxial (+)
It occurs in the oxidized zone of ore deposits and is associated with dioptase, wulfenite, hemihedrite, phoenicochroite, duftite, mimetite, shattuckite, chrysocolla, hemimorphite, willemite, and fluorite.
Association: Dioptase, wulfenite, hemihedrite, phoenicochroite, duftite, mimetite, shattuckite, chrysocolla, hemimorphite, willemite, fluorite.