Green chlorothionite associated to melanothallite from the Vesuvius volcano in Italy. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY 4.0
Melanothallite
Melanothallite is an IMA-recognized mineral species in the halide class, with the chemical formula Cu₂OCl₂.
It crystallizes in the orthorhombic crystal system (space group Fddd).
Key properties: specific gravity 4.08; Vitreous luster.
IMA symbol Mlth, grandfathered status — known before the IMA’s founding, named in 1870.
Sublimed on crater walls formed in 1868 and 1906 (Vesuvius, Italy); in volcanic fissures (Tolbachik volcano, Russia).
It is found associated with Eriochalcite, chalcocyanite, euchlorine, dolerophanite (Vesuvius, Italy); euchlorine, chalcocyanite, dolerophanite, tenorite (Tolbachik volcano, Russia).
Its name comes from the Greek for black and a young shoot, for the mineral’s property of turning green on exposure.