Metavoltine
Metavoltine is an IMA-recognized mineral species in the sulfate class, with the chemical formula K₂Na₆Fe²⁺Fe³⁺₆O₂(SO₄)₁₂·18H₂O.
It crystallizes in the hexagonal crystal system (space group P3). It is a member of the Metavoltine structural group.
Key properties: Mohs hardness 2.5; specific gravity 2.51; Resinous luster; optically uniaxial.
IMA symbol Mvt, grandfathered status — known before the IMA’s founding, named in 1883.
Typically an alteration product of pyrite in arid climates; formed as a fumarolic sublimate and solfataric precipitate; may be a post-mining product.
It is found associated with Voltaite, botryogen, ferrinatrite, coquimbite, copiapite, sideronatrite, metasideronatrite, jarosite, alunogen, pickeringite, sulfur, gypsum.
Its name comes from the Greek for with and voltaite, as originally associated with that species.