TRAHEDRITE-TENNANTITE
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 allnatural 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 chimneylike 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.
In my collecting trips into Mexico, I’ve noted that many of the mines I’ve visited - Fresnillo, San Martin, Niaca, and others - line up in a more or less straight line north to south in that silver-rich country. I can’t help but suspect those deposits were formed along old crustal faults long since uplifted to form the present land. All these deposits are rich in the sulfides pyrite, chalcopyrite, galena (including tenantite, tetrahedrite, and silver minerals) and present as blanket-like layers with chimney off-shoots rich in metal
ores. Gangue minerals like calcite, dolomite, siderite, barite, quartz, and fluorite are also found with the sulfides.
The surging hydrothermal solutions contain various metal elements; this accounts for the presence of silver, mercury, and other trace elements in developing crystals of tetrahedrite and tennantite. It’s no wonder these two species contain both arsenic and antimony.
In some cases, there are one or two minor but important differences between tetrahedrite and tennantite, which helps distinguish between them, luster being one. Tetrahedrite crystals tend to be very smooth faced and lustrous while tennantite is less so. Another odd distinguishing feature is that brassy chalcopyrite seems to have an affinity for tetrahedrite, so it often coats tetrahedrite crystals with a brassy look.
As you have probably guessed, tetrahedrite gets its name from its crystal form; the tetrahedron is a shape that looks like a pyramid with one corner of the pyramid pulled out to distort or stretch the triangular faces. Tennantite is named for British scientist Smithson Tenant. Initially, tennantite was well known in the early silver mines of Germany and elsewhere but not recognized by that name. In fact, every black to gray sulfide mineral with tetrahedron crystals was called tennantite. There was no tetrahedrite as such! It wasn’t until 1819 that tennantite was formally identified as a valid mineral; and, in 1845, a scientist did more studies on the minerals and distinguished between them — making tetrahedrite a separate mineral.
Switching gears a bit, there is a large group of unusual sulfides called sulfosalts. The sulfosalt group, in general terms, have three main chemical or elemental components in the group’s minerals and are given the letters A, B, and S. A is any one of a dozen metals that serve as cations; B is any one of a half dozen semi-metals in the composition; and S is the sulfide anion member. Members of the group form at fairly low temperature and are often rich in silver. Sulfosalts include such noteworthy species as proustite and pyrargyrite, two exceedingly rich silver species that are colorful and far more eagerly sought than our two black copper iron sulfides. Other available and very handsome sulfosalts, which collectors are eager to own, include bournonite, enargite, and rare gratonite.
Enargite was a major ore at Butte, Montana, but not common in most copper deposits. Gratonite is quite rare and seldom found. Bournonite is found in quantities of collectible specimens in China, Cornwall, England and Bolivia. Tetrahedrite and tennantite are among the more common of the sulfosalts, so they are available to collectors but not as costly as proustite, pyrargyrite, and bournonite.
Our two copper iron sulfides have been found in a large number of ore deposits and readily available for decades. As mentioned, tetrahedrite and tennantite crystals form in low- to medium-temperature ore deposits, but one major exception to this is the copper mine of Cornwall, England. This deposit contains a host of minerals like cassiterite, formed in the high-temperature range, well above the normal environment where we usually find tennantite and tetrahedrite. Yet, both are found in Cornwall.
As mentioned, tennantite was the first of these two to be identified. It was encountered during silver mining in several deposits in Germany and was given several names depending on the metals it yielded during smelting. The chemistry applied to the mineral in Cornwall showed it was an argentum, arsenic, cupro, ferro mineral, so the elements it gave up when smelted were silver, arsenic, copper, and iron. Thank goodness, two Phillips brothers in England identified it in 1817 from the mines of Cornwall and named it tennantite for scientist Smithson Tennant.
In the U.S., the nicest tetrahedrite came from the big copper mine at Bingham, Salt Lake County, Utah. Crystals averaged about a half-inch on an edge and had a fairly bright luster. In Mexico, Mina Bonanza of Zacatecas produced fine black lustrous tetrahedrite crystals approaching an inch. These often had a grayish coating that might be tennantite. They occurred with other sulfosalts, including fine bournonite. Even though they lack high luster, they proved to be tetrahedrite!
The mines of Germany produced the very best tetrahedrite crystals to an inch or more. The finest specimens have sharp tetrahedron crystals coated with bright brassy chalcopyrite that rival the Cornish specimens of chalcopyrite coated tetrahedrite from the Heroodsfood mine, Liskeard, better known for its amazing bournonite. The chalcopyrite-coated tetrahedrite from Cornwall and Germany are considered classics and are eagerly sought.
Several of the metallic ore mines of Peru, high in the Andes Mountains, have also yielded wonderful tetrahedrite and tennantite. Mines in the Ancash District produce crystals to an inch on an edge. The Casapalca mine, Lima Department, is the source of the largest tetrahedrite crystals, with a few reported to approach five inches.
Though some of the Peruvian crystals are of excellent size, they do not hold the record for crystal size for tetrahedrite. That honor goes to a couple of older small French metal mines few collectors have heard of, as they did not produce specimens in quantity. When I was at the Pierre and Marie Curie Museum in Sorbonne, France, curator Pierre Bariand pulled out a tetrahedrite crystal that was a perfect tetrahedron with slightly modified faces. It was textbook perfect and was nearly six inches across. The faces were not bright and had lightly colored blotches of green and yellow chalcopyrite on the surfaces. This was an example of just a few of the large crystals that had been found during a brief mining period at Irazein, Ariege Department, France.
For pseudomorph collectors, a source of tennantite has on occasion been Tsumeb, Namibia. Here it has been found in fine crystals and also as a replacement for azurite. These specimens are dull and rough-surfaced but very interesting.
With many metal ore deposits long since depleted, the supply of these two copper sulfosalts in the mineral market is limited. Specimens from old collections do appear from time to time and should be considered for your collection. The potential for a good tetrahedrite is more possible now, since there is a strong flow of minerals from China. The Yaogangxian mine, better known for fluorite, scheelite, ferberite, and bournonite has also produced fine tetrahedrite rich in silver.
As you well know, this writer recognizes that black to gray minerals, particularly the copper sulfides, are not as popular or as frequently displayed at shows as they deserve. After all, they have curious chemistry, interesting crystal forms, are sulfosalts that are well worth collecting as a group, and add another attractive dimension to your collection.
Good examples of tetrahedrite and tennantite are very attractive and are found in well-crystallized clusters that exhibit nicely. Collecting them encourages you to look into their chemistry. Any wide-ranging collection should have more than a showy facade of minerals. It should include these two fascinating black to gray copper species with an interesting chemistry and crystal form.