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Category Vanadate mineral
(repeating unit)
Strunz classification 8.FA.10
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group A2/a
Unit cell a = 7.68 Å, b = 8 Å
c = 10.09 Å; β = 110.27°; Z = 2
Color Black; reddish brown to dark reddish brown in transmitted light; white in reflected light
Crystal habit Anhedral crystals and incrustations
Cleavage None
Luster Metallic
Streak Red brown
Diaphaneity Opaque
Specific gravity 3.86
Optical properties Biaxial
Refractive index 2.055
References [1][2][3]

Ziesite is a copper vanadate mineral with formula: β-Cu2V2O7. It was discovered in 1980 as monoclinic crystals occurring as volcanic sublimates around fumaroles in the crater of the Izalco Volcano, El Salvador. It is named after Emmanuel G. Zies (1883–1981), an American geochemist who studied Izalco in the 1930s.

Closely related is blossite, also a copper vanadate with formula of α-Cu2V2O7. It forms orthorhombic crystals. Blossite was also first described for specimens from the Izalco volcano.

Ziesite and blossite are polymorphs, different crystal structure for the same chemical composition and are quite similar in physical properties.

Associated minerals include stoiberite, shcherbinaite, bannermanite, fingerite, mcbirneyite, blossite, chalcocyanite and chalcanthite.[1]