By Bob Jones
Today proustite, commonly called “Blood of the Bull” mineral, is hard to find. Native gold is rarely found as a compound, preferring instead to remain aloof from joining other elements. Silver, on the other hand, forms a variety of wonderful compounds that collectors treasure: waxy looking chlorargyrite, dark gray to black acanthite, stephanite, polybasite, and two fine red sulfosalts pyrargyrite and proustite.
Seldom-Seen Silver Compounds
The red color of these last two silver minerals makes them the most appealing and eagerly sought silver species. Collectors are overjoyed when they obtain a specimen of either of these silver compounds since today’s silver mines rarely encounter such specimens. Many found in the early centuries of silver mining have long since ended up in museums or in private collections.
Plus, there is the problem of proustite and pyrargyrite darkening with age and gradually losing their vibrant red colors. Of the two, proustite exhibits the richest red color and when found in quantity in Chanarcillo, Chile, was given the name Sangre de Torro, or Blood of the Bull. Pyrargyrite is a somewhat darker red color and is still occasionally found today in silver mines of Mexico.
Some 30 years ago, I was traveling in Mexico visiting silver mines with my son, Evan, and Bill Panczner and his son, Chris. This included a visit to a silver mine at Fresnillo, Chihuahua, Mexico.
Prior to going underground, we were being escorted through the mill by an engineer. As we stood by the conveyor belt watching silver ore come from the depths, I asked the engineer if proustite or pyrargyrite crystals ever showed up on the conveyor belt.
He said they did and they would pick them up. I immediately had visions of being able to buy some fine specimens from him.
So I asked what he did with what he collected, expecting a positive answer. I almost fell on the belt when he said they save the red silver crystals so when the silver production numbers dropped below expectations, they tossed the collected red silver specimens on the belt to “sweeten” the production numbers. I never got over that comment!
Pyrargyrite is a silver antimony sulfide, and proustite is a silver arsenic sulfide. Both exhibit a lovely red color with proustite being the brighter red, hence the general name ruby silver. Both species are rare, with pyrargyrite a little more common and a slightly darker shade of red.
Unfortunately, both these silver minerals are heat- and light-sensitive, tending to darken when exposed to strong light.
These external influences only hasten an inevitable internal change that gradually happens regardless of external influences. Research has shown that darkening of these species can even happen regardless of careful protection from long exposure to light because of a slow, natural darkening process. Even when carefully protected, specimens of these silver sulfosalts darken anyway. This is because another mechanism is at work.
Briefly, silver atoms in these species are not locked in place. They tend to move, and that causes them to begin to form a second mineral internally within the red crystals. This movement is due to a thermal effect, and it happens regardless of the external protection given the red silvers.
The thermal effect actually causes silver to develop another mineral, forming dark gray silver sulfide, acanthite. As this dark mineral becomes an integral part of the red crystal’s internal structure, it affects a slow color change, unlike so many minerals that change color only due to external influences like radioactivity, heat, or light. Since the color change in proustite and pyrargyrite is an internal function causing the darkening of color, the internal change is permanent and cannot be reversed.
This simple summary of information can be found in much greater detail in a very interesting article available on the internet. For collectors who are serious about learning more about the process of color-change in proustite and pyrargyrite that I briefly explained here, I invite you to read the article entitled “Chanarcillo, Chile.” Under that heading, you’ll find the article titled “Wandering in the Mountains” by Terry Wallace. Terry, a longtime member of the Tucson Gem and Mineral Society Show Committee, has visited Chanarcillo, Chile, the source of the world’s finest proustite specimens.
The article, “Wandering in the Mountains,” does an excellent job of giving the reader a lot of information on the famous Chilean locality, and the technical information therein will help you understand what happens inside ruby silver, causing a darkening of the specimen.
The Wallace article is about his journey to Chanarcillo and is accompanied by a number of choice proustite specimen photos. It contains information on the history of the deposits, their geology, discovery, and development of the mines. I found the article fascinating, full of information, and Wallace does an excellent job of clearing up misunderstandings of why red silver specimens darken in spite of all our efforts to prevent it from happening. I consider the Wallace article a must-read for anyone who owns a proustite.
I’ve been lucky enough to handle and own quite a few specimens of these two silver species. Of the two, proustite is my favorite for perhaps the oddest of reasons. Yes, it has the richest red color. Yes, it forms extremely attractive, tapering hexagonal crystals. True, the finer tapering crystals resemble red dog tooth calcites. But proustite is my favorite because of a single proustite specimen that had no crystals at all.
The specimen is about 5” across, 2” thick, and maybe 3” front to back. It has a grayish acanthite exterior “skin” and was composed internally of a solid mass of red proustite. No crystals – just a solid chunk of red proustite. It is half of what probably was a proustite nodule mined well over 100 years ago in Colorado.
Have you ever seen a solid mass of silver arsenic sulfide proustite? In my dozens of years of collecting, visiting private collections and museums around the world, this solid mass of proustite is the only such mass I’ve ever handled. I first saw it about 1948 when I joined the New Haven, Ct., Mineral Club and met collector Russ Jones.
In those days, the club met at Yale’s Peabody Museum and member Russ Jones worked in Yale’s powerhouse. He had been collecting since the late 1920s and invited me to see his collection.
While visiting him, he brought out a piece of black velvet cloth wrapped around a rock. The rock turned out to be that solid mass of red proustite described above.
Russ and I kept in touch after that visit, even after I moved to Arizona. As a school teacher, I had summers off and when school would close I would load my family in the car and drive to Connecticut to escape the heat and work on my dad’s animal farm. That gave me plenty of opportunities to spend time with Russ Jones, visiting mineral locations and Eastern museums.
Fast-forward to the 1990s when Russ’s health was so bad, he was close to death. I was in Arizona when I got a call from Russ’s lawyer, who informed me that Russ was in a nursing home and wanted me to put his collection in storage. He wanted me to do it because he had willed his collection to me! That’s how Evan and I ended up owning the massive, unique chunk of proustite.
The specimen was not from one of the world famous silver mines like Kongsberg, Norway, Chanarcillo, Chile, or Saxony, Germany. It had been mined in the late 1800s in Colorado in a Fall River District mine, Clear Creek County west of Denver. Even when it was found, it was considered so unusual that it was actually put on display in a local library for a time. At some point, a Denver mineral dealer, Mitch Gunnell, came into possession of it, and when he advertised it for sale Russ Jones, who loved Colorado minerals, bought the piece for $200, and it eventually ended up in the Evan and Bob Jones collection.
I relate this brief history of a proustite for two reasons. The piece is
what actually introduced me to want to own more proustite. It also is a really good example of the importance of the provenance of minerals, the journey of a mineral from mine to owner, an always fascinating sidebar and adding value to a specimen.
While some Colorado silver mines did produce proustite of note, the major sources of proustite are far better known in the collector world and include Germany’s Saxony and Harz Mountains, Joachimthal, Bohemia, and most important of all, Chanarcillo, Chile. Each of these localities yielded clusters of superb proustite crystals groups as well as fine single and small sprays of crystal clusters.
History in the Making
Keep in mind that these mines, except for Chanarcillo, were first discovered starting over 1,000 years ago. Specimens from these mines that you see today are old, been out of the ground for many hundreds of years in some cases, so lack the vibrant color they originally had.
These older mines certainly yielded plenty of proustite as evidenced by what we see in museums and collections today. But the world’s greatest source of proustite is Chanarcillo, Chile. One of the more important producers was the Dolores mine, Chanarcillo, Chile. Lucky is the collector who has a colorful Dolores Terera mine proustite. Of course, hundreds of other mines developed here, but this mine is best known for producing proustite in quantity and exceptional quality.
The silver deposits of Chile were discovered by Juan Godoy in 1832. The discovery was accidental, an interesting tale of discovery recounted by Wallace in his article.
The silver species Godoy actually found was chlorargyrite, a silver chloride that is easily scratched with your fingernail and usually a dull, waxy material. The chlorargyrite led him to the Chanarcillo silver veins and silver mining boom in the area from the 1830s to about 1890.
During the more productive years in Chanarcillo, miners had breached the supergene environment where proustite most often forms in quantity. The high heat and pressures of the supergene zone facilitate proustite formation.
Specimens of proustite from Chile can be seen today in most museums as well as in private collections. I can recall visiting the Peabody Museum of Harvard in the 1980s and enjoying the lovely suite of proustite specimens well-protected in a wooden lidded box. You had to lift the lid to view the proustites. Today we now know such protection only delays but does not stop the slow darkening of proustite.
Though proustite specimens are rarely available today, collectors keep searching. To know better what you are looking for, I suggest you check the Terry Wallace internet article “Wandering in the Mountains” under “Chanarcillo, Chile.”
Author: Bob Jones
Holds the Carnegie Mineralogical Award, is a member of the Rockhound Hall of Fame, and has been writing for Rock & Gem since its inception.
He lectures about minerals, and has written several books and video scripts.