Ziesite is a mineral, copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 vanadate: formula β-Cu₂V₂O₇. It was discovered in 1980 as monoclinic crystals around fumaroles in the crater of the Izalco Volcano

Izalco (volcano)

Izalco is a stratovolcano on the side of the Santa Ana Volcano, which is located in western El Salvador. It is situated on the southern flank of the Santa Ana volcano. Izalco erupted almost continuously from 1770 to 1958 earning it the nickname of "Lighthouse of the Pacific", and experienced a...

, El Salvador

El Salvador

El Salvador or simply Salvador is the smallest and the most densely populated country in Central America. The country's capital city and largest city is San Salvador; Santa Ana and San Miguel are also important cultural and commercial centers in the country and in all of Central America...

. It is named after Emmanuel G. Zies (1884-1981), an American mineralogist.

Closely related is blossite, also copper vanadate; its formula is given as α-Cu₂V₂O₇. It forms orthorhombic crystals. Blossite was also first described for specimens from the Izalco volcano. is named after mineralogist F. Donald Bloss (1920– ).

Ziesite and blossite are polymorph

Polymorphism (materials science)

Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...

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