TETRAHEDRITE-TENNANTITE: Which is Which?

Unassuming, handsome, and confusing minerals

- Unassuming, handsome, and confusing minerals

The problem with the two common copper sulfosalt minerals, tetrahedrite, and tennantite, is their chemistry. Chemically, they are both copper iron sulfides. They are the more common black copper species, and it’s important we treat them separately from the other dark copper species.

Tetrahedrite and tennantite use the same elements: copper, iron, zinc, and sulfur to develop molecules on an internal structure. But they differ by incorporating either arsenic or antimony in their chemistry. Each of these metal elements can fit into the atomic structure of either or both these minerals as part of the chemistry of these two sulfides.

Theoretically, pure tetrahedrite contains only antimony, while pure tennantite contains only arsenic. To make matters confusing, arsenic and antimony substitute for each other in varying amounts because their electron structure and atom size closely match. In fact, both elements occur in the two species, with one dominating, deciding if the specimen is tennantite or tetrahedrite. To further cause confusion, iron and copper can also substitute for each other, so either can be the dominant metal cation determining the species. As if that is not confusing enough, other metal elements also get into the act including silver, zinc, and mercury in trace amounts — no wonder these two copper species are so daunting.