The largest single deposit of mercury ore anywhere on earth is at Almadén, Spain, where more than 250,000 tons of mercury have been produced over the past 2,000 years or so, totaling about one-third of all the mercury ever produced. It’s one of the longest operating mine complexes anywhere in the world. The name of the location, Almadén, comes from Arabic, meaning, “The Mine.”
The Almadén deposit is hosted in Silurian rocks. As with many mineral deposits, it’s often not entirely clear exactly when mineralization happened, and often enough it can take place over many millions of years. But we do know the age of the rocks that the mineral resources are in.
The ore at Almadén is mostly the mineral cinnabar, mercury sulfide. Cinnabar is a bright red mineral, and if you know where mercury is on the periodic table – atomic number 80, with a high molecular weight – you might not be surprised to learn that cinnabar is a very heavy mineral. Its specific gravity – that’s a way of looking at mineral density, the ratio between the density of the mineral compared to that of water – is 8.1, three times the specific gravity of quartz.
If you have a specimen that’s red and feels anomalously heavy as you hold it in your hand, there’s a good chance you have some cinnabar. Don’t eat it. Mercury is bad for you, although there are some medical compounds using mercury that are highly beneficial, and holding a piece of cinnabar in your hand won’t make it fall off. In fact, cinnabar, mercury sulfide, is a form that isn’t very bioavailable so if you did eat it, it would probably just pass through. But still, please don’t eat it.
The Romans ground the red mineral to use as pigment. Alchemists in the Middle Ages were fascinated by mercury – it is a liquid metal, after all – and by the 16th century, the discovery was made that mercury could combine with gold and silver, which were otherwise relatively inert, in a process called amalgamation. That made mercury important in extraction of gold from rock – and the Spanish were discovering tremendous deposits of gold and silver in the Americas in the 1500s. The mines at Almadén were owned by German bankers in the 1500s and 1600s, in return for loans they gave to the Spanish government. Beginning about 1566 convict labor was used to mine the ores. They were fed at least moderately well, but about a quarter of the convicts died, usually from mercury poisoning: pulverizing rock to powder, and breathing it over time, does make the mercury more bioavailable.
The cinnabar ore at Almadén is in cracks and pores in Silurian quartzites. Quartzite can be a metamorphic rock – basically, pure quartz – or it can be a sedimentary rock that’s really densely compacted. At Almadén, the rock was most likely originally a pretty clean quartz sand laid down in a delta, near a large river mouth. The rock is pretty brittle, easy to fracture, and the mercury ores at Almadén are in fractures within the quartzite.
There’s speculation that the mercury ultimately came from older black shales of Ordovician age. Something heated the water in these rocks, which percolated through the shale, picking up mercury, which was deposited in the fractures in the overlying quartzite. Alternatively, the mercury came from the earth’s mantle, much deeper than any of the near-surface sedimentary rocks.
Wherever the mercury ultimately came from, it seems that magmatic activity – moving molten rock – probably was the driving force that mobilized the mercury, or the hot waters that moved it around. The richest deposits are quite close to an explosive volcanic vent whose rocks are more or less basaltic in composition. The volcanic activity is almost certainly the most important factor in concentrating the mercury ore.
But why is there so much, right here in this one small district? As indicated above, close to a third of all the mercury known on earth was at Almadén, Spain. You can talk about modes of concentration, volcanoes and fractures, but to my mind the question of why so much is there is unanswered. What I tell tourists about the incredibly rich deposit of copper, silver, and many more minerals here at my home in Butte, Montana, is that I think it’s the luck of the draw – that there were inhomogeneities in the early earth, essentially blobs of the good stuff, that have been altered, changed, and moved around by geologic processes, but that must have been persistent concentrations from way back in geologic time. That’s my guess for the mercury at Almadén. But trust me, I definitely don’t know for sure.
The volcanism and hydrothermal activity that concentrated the mercury at Almadén were probably ultimately the result of the complex collisions between fragments of Gondwana (including parts of what are now the Iberian Peninsula, present-day Africa, and more) and western Europe in the Variscan Orogeny beginning about 380 million years ago, Late Devonian time (Palero-Fernández and others, 2015, Geological context and plumbotectonic evolution of the giant Almadén Mercury Deposit: Ore Geology Reviews Volume 64, p. 71-88).
Because mercury is so toxic, its use has declined precipitously in the past couple decades. The last time primary mercury was produced in the United States was in 1992, from a mine in Nevada. Mercury does have a lot of uses, however, and the biggest use in the U.S. is as a catalyst for the production of caustic soda. That’s essentially a closed system, so little new mercury is needed, and little old mercury is lost.
In the US about 43% of the mercury consumed still goes to dental amalgams, where like cinnabar it is (mostly) not bioavailable (but mercury vapor is evolved from dental fillings, so the question is controversial, and the several studies I’ve reviewed are inconsistent). Historically, snooze-alarm clocks used a lot of mercury, in a tilting switch that closed or opened an electric circuit using mercury, and because they were seldom recycled, they are sources of some mercury contamination in landfills. “Mercury” in some traditional thermometers is being replaced by “galinstan,” an alloy of gallium, indium, and tin. Electronics and fluorescent light bulbs also use mercury, but all uses are decreasing because of mercury’s toxicity and persistence in the environment.
Dr. Benjamin Rush’s Thunderclappers – purgative pills containing mercury chloride in a mixture called calomel – revealed the location of the latrine at Traveler’s Rest, near Lolo, Montana, where explorers Lewis and Clark camped September 9-11, 1805, and again June 30-July 3, 1806, before the captains began their separate return journeys across Montana. Mercury-laden medications were common in health care at a time before germs were understood, and despite mercury’s toxicity, the Corps of Discovery seems to have avoided ill effects from the pills. At ten cents a dozen – Clark bought 50 dozen for $5 in Philadelphia in 1803 – Rush’s pills were dispensed freely, although they cost more than five percent of the expedition’s total medical budget. Mercury’s toxic persistence in the environment pointed the way to the latrine southwest of today’s Missoula, Montana, and led to mercury’s disuse in modern applications.
The deliberate search for mercury in the soil at Traveler’s Rest paid off in 2002 for archaeologist Daniel Hall, who excavated a suspicious-looking rectangular trench at the site. Dark organic-rich soil half a meter thick was sandwiched between sand and gravel deposited by nearby Lolo Creek; the soil proved to have mercury concentrations far above background levels. “Finding the subsurface trench was serendipity in the first place,” says Hall, “because finding evidence of Lewis and Clark in a quickly-developing community like Lolo is like looking for a needle in three haystacks.” But mercury was the smoking gun that pinned down the campsite’s position – the only Lewis and Clark campsite with physical evidence of its location.
The European Union ended mercury production in 2003, and the mines at Almadén were closed. Almost 1,200 tons per year are still mined around the world – mostly in China, which produces 83% of the world’s mercury, with Tajikistan in second place with about 8% of the total.
The word cinnabar has ancient heritage. The earliest surviving use of Greek κιννάβαρι, kinnàbari, was by Theophrastus (c. 371 – c. 287 BC) in his treatise On Stones, but it was ultimately derived from Sanskrit or Chinese words, possibly by way of Persian zanjifrah.
Portions of this post were taken from my book What Things Are Made Of (2011).
Very interesting article. I've researched an uncle who was killed in a car crash outside Winnemucca, Nevada in 1940. He was working at a "quicksilver" mine somewhere in the area. I think I've traced the location, thanks to a U.S. Bureau of Mines publication, to the Bottle Creek District in the mountains about 65 miles northwest of town. I also learned from an oral history transcript from a miner's wife that the cinnabar was refined on site, trucked to the railroad and shipped in 76 pound flasks to California, where it went into munitions manufacture. This woman also reported that the mine had pretty much played out shortly after Pearl Harbor.
Good article Richard, I often drive by Sulpher Bank Mine in Clearlake Oaks, CA where they mined Cinnabar for mercury. Red soil, smells like rotten eggs, I always wondered about its history...looks like it too was one of the biggest sources of mercury in the world...also located just north of the biggest geothermal field in the world at The Geysers which I always thought would be cool to take a tour of if they allow that. Looks like geologists think the whole area is underlain by a magma body which makes sense. Here was wiki's article. Thanks again for another neat geology history lesson! The history of these mines boom and bust cycles are so fascinating to me as well. https://en.wikipedia.org/wiki/Sulphur_Bank_Mine