TETRAHEDRITE-TENNANTITE: Which is Which?
Rock&Gem Magazine|July 2021
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.

Both species develop crystals in the isometric system, so that’s no guide to distinguishing one from another, making identification difficult. Tetrahedrite has one feature that sometimes helps. It is often very lustrous, while tennantite usually borders on dull, but that is not much help. Additionally, both species can occur in the same ore deposit. Both minerals can occur as important ores of copper but are usually only a small fraction of final production, as other copper minerals tend to be more abundant. In some cases, their silver content is significant. The tetrahedrite from the Yaogangxian mine in China is reported to be silver-rich.

You may already realize the two minerals form a series. Tetrahedrite, the copper iron antimony sulfide, is theoretically at one pure end with tennantite, the copper iron arsenic sulfide in pure form at the other end of the series. But all-natural specimens are somewhere between the pure forms.

Over the last few decades, geological and mineralogical research has answered many of the mysteries how ore deposits develop. The discovery of countless super hot hydrothermal springs called black smokers along the plate boundaries of the earth’s crust under the oceans and seas explains why many rich sulfide ores are found in chimney-like and thick blanket-like deposits. These unusual hot springs form as waters penetrate the rocks of the crust, gaining more and more heat and dissolving more metals as they travel. As continental plates move inexorably against each other, this mineral-rich water that is under pressure gets hotter and hotter, dissolving more and more metals. The waters follow cracks, joints, and faults, surging toward the earth’s surface. As the waters encounter the cooler crustal rocks, they deposit their minerals in veins and joints. The remaining waters, still mineral-rich, burst forth into the cool ocean waters and abruptly drop their remaining mineral load. This function accounts for many major ore deposits composed of sulfide minerals formed millions of years ago.

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