Sky blue crystals of the rare arsenate mineral geminite from the TL in Cap Garonne (Cap Garonne Mine, Le Pradet, Var, Provence-Alpes-Cote d'Azur, France). Ex Vandenbroucke Museum collection from Waregem, Belgium — Photo: David Hospital · CC BY-SA 4.0
Geminite
Geminite is an IMA-recognized mineral species in the arsenate class, with the chemical formula Cu²⁺(AsO₃OH)·H₂O.
It crystallizes in the triclinic crystal system (space group P-1).
Key properties: Mohs hardness 3–3.5; specific gravity 3.7; Very pale green streak; Vitreous luster; optically biaxial.
IMA symbol Gem, IMA status: approved, named in 1988.
From an arsenic-bearing oxidized Pb–Cu deposit (Cap Garonne mine, France); on specimens from mine dumps at a gold-bearing arsenic sulfide deposit (Salsigne mine, France).
It is found associated with Tennantite, covellite, chalcanthite, lavendulan, pushcharovskite, antlerite, brochantite, quartz (Cap Garonne mine, France); lindackerite, yvonite.
The name comes from the Latin gemini, for twins, as the mineral is extensively twinned.