Ziesite is a mineral, copper vanadate: formula ?Cu2V2O7. It was discovered in 1980 as monoclinic crystals around fumaroles in the crater of the Izalco Volcano, El Salvador. It is named after Emmanuel G. Zies, a mineralogist.
Closely related is Blossite, also copper vanadate; its formula is given as ?Cu2V2O7. It forms orthorhombic crystals. Blossite is named after mineralogist F. Donald Bloss.
The available literature does not explain the significance of the ?/? forms other than the different crystal forms.