Arsenic: A Critical Mineral

This element is colorless, tasteless, inexpensive, and deadly–oh, and as a compound it can turn electricity into light (as in LEDs, fiber optics, telecommunication, and laser scanners).

That last “Star Trek”-style application is just one of the reasons why this “Murder She Wrote” element is on the 2018 US critical minerals list.

What is arsenic?

You might be thinking, wait, arsenic is a mineral? It’s not a nefarious white powder?

That powder exists, but it’s artificial. Arsenic itself is a metal–one of the weird ones that have a few nonmetal properties. As the chemical element As, it belongs in the same group of metalloids as antimony (Sb), another critical mineral.

Metalloid table image
The metalloids are green. Reading vertically, from top to bottom, arsenic is in group “15”–the nitrogen group–on the periodic table of elements. This matters for arsenic’s use as a semiconductor in electronics and telecommunications. (Links are live at image source.)

There’s a bit of gray arsenic metal out there naturally, but arsenic usually combines with sulfur to form a mineral–unlike silicon (which has a thing for oxygen), and like antimony, silver, gold, and some other metals.

These sulfide ores build up wherever volcanic activity heats circulating water, say, a hot spring or hydrothermal vent. (Here is a more technical description of how this process works with arsenic and gold.)

Yes, there is a little bit of natural arsenic coming out of Yellowstone’s fumaroles! (In general, though, mining and industrial uses release about ten times as much arsenic into the environment as active volcanoes do, and there is also some in the ground from weathered rocks. Many regions have arsenic in their ground water, too. These environmental issues, while serious–especially in Asia–are beyond the scope of this post.)

Metal sulfide ores also build up in anoxic water–“the dark depths” as this geologist calls it.

He doesn’t mention arsenic by name until the very end, but that’s appropriate for us, since the white arsenic powder that murder mysteries describe is a byproduct of smelting arsenopyrite and several other ores, including but not limited to lead, zinc, gold, silver, and copper.

Whether the ore it’s in comes from hydrothermal deposits or an oxygen-starved ancient sea bed, arsenic vaporizes out of the crushed rock when it is heated, as unfortunate metal workers down through the centuries have found out the hard way.

Arsenic goes straight from a solid form to a gas because it’s one of the few elements with a boiling point (at standard atmospheric pressure) that’s lower than its melting point. And you definitely do not want to breathe in that garlicky-smelling stuff or otherwise ingest this poison.

After it vaporizes during the smelting process, arsenic then reacts with oxygen in the air and collects on the furnace chimney and flues as a grayish-white dust (arsenic trioxide). This is the material fictional villains and real-life murders slip into food and drink, and it also has many legitimate purposes.

In addition to this manmade “white” arsenic, there are two natural forms of “red” arsenic: bright yellow-orange orpiment and ruby-colored realgar (sometimes of gem quality).

arsenic threefer
See? Arsenic *is* a metal, one that’s as beautiful as it is deadly. Top left, realgar (James St. John, CC BY 2.0); top right, orpiment (James St. John, CC BY 2.0); bottom, arsenopyrite, most common source of white arsenic (James St. John, CC BY 2.0, via Wikimedia)

Down through the centuries humanity has employed all three forms of arsenic for better things than murder (although that has been quite a popular use, too, at least until the Marsh test arrived in the 1830s).

Here are just a few:

  • Ancient Egypt: Possibly both for embalming (it has a drying effect) and to harden copper, although this is still under debate.
  • Metal work: Arsenic and antimony are both strengthening alloys, and they both make lead shot round (arsenic has been used for this purpose since the 17th century). Both are also used to make frictionless ball bearings and other Babbitt metals.
  • Leatherworking: Removing fur from hides. Needless to say, arsenic is no longer used this way.
  • Pigments: Realgar and orpiment make bright colors that were widely used until safer alternatives like cadmium yellow came along. Mixed with copper, arsenic gave Van Gogh, Monet, and other 19th century artists wonderful shades of green, but it also killed many Victorians when used as a food coloring (candy and pudding in some parts of Scotland) and in wallpaper and clothing dyes.

    Monet Irises
    A hundred years ago, Claude Monet had to use arsenic-based paint to get these green colors. (Source)

  • Pest control: Killing rodents, insects, snakes, and other pests has been a major historic use of arsenic, though the old practice of distilling it into a liquor and drinking it to repel evil probably killed some people, too.
  • Medicines: Arsenic trioxide was part of traditional Chinese medicine more than five thousand years ago, although its toxicity was eventually recognized and “arsenic” became a synonym of “poison.” Hindu medicines also used it. In the West, various forms of arsenic were used medicinally and occasionally as a tonic from at least the 18th century on. In the early 1800s, arsenic became the first effective chemotherapy against syphilis, parasites, and leukemia and remained the go-to medication for these purposes until the early 20th century, when better treatments took over. However, arsenic still has some veterinary applications. It also may be therapeutic for human hematology and oncology treatment.
  • Miscellaneous: Fireworks (red and white), explosives, printing, correction fluid on yellow pape (in ancient China)r.

What are arsenic uses today?

Arsenic is present all over the world, but let’s focus on its uses in the United States.

The trioxide (white arsenic) was produced in 1901 first by smelting gold and silver ores in Washington State, and then in nonferrous metal smelting elsewhere in the US. (Yes, some of these places are now Superfund environmental cleanup sites.)

There was a domestic market for the stuff.

Early in the 20th century, arsenic was still considered a medicine. Between 1910 and 1920, as an insecticide, it helped Southern cotton farmers fight the boll weevil. Other agricultural applications were herbicides and as a cotton desiccant, while organic arsenic was fed to some farm animals to promote weight gain and treat parasites.

As the environmental movement got going, these uses declined (although arsenic remained in poultry feed until 2015).

In the mid-1970s, arsenic became the “A” in CCA pressure-treated lumber, serving as an insecticide and antibacterial. Chromated copper (the “CC”) also protected the wood and gave it a greenish hue. By 1990 the wood preservation industry accounted for 90% of US arsenic consumption.

rotting 2 x 4
Once rot sets in, it’s hard to stop, and replacement isn’t always practical (though it is always expensive). That’s why an effective preservative, even if it contains arsenic, is so important. (Pete Brown, CC BY 2.0)

During pressure treatment, arsenic and other toxins are locked up in the wood by chemical stabilization reactions that make them insoluble in water. This takes a few hours at high temperatures (150° F/66° C) or weeks to months at low temperatures (60° F/16° C).

Because of public health concerns around the turn of the 21st century, the Environmental Protection Agency withdrew its approval of CCA-treated lumber for residential purposes, while stating that existing structures did not need to be removed. Today other compounds are used to protect lumber destined for homes, schools, and similar places, while CCA-treated wood is used for industrial purposes (mainly posts, poles, and in highway construction).

Arsenic is still used to preserve lumber, and it’s also still in some insecticides. The biggest change from its historical use is a result of the semiconductor revolution that began in the 1950s.

Arsenic and other group 15 elements make good silicon dopants. The physical chemistry is complicated. Suffice to say that arsenic and some other members of the nitrogen group (group 15, formerly known by the Roman numeral V) can be combined with elements in the boron group (group 13, once the III group) to make the III/V compound semiconductors.

As far as I’m concerned, these are magic, but people who know more about them (see source list) say that compound semiconductors are faster than those made out of silicon and they provide natural insulation between the circuit and the device that protects against signal loss, especially at high operating temperature.

Gallium-arsenide and other compound semiconductors are radiation resistant, which is very handy when building spacecraft, and their physical structure can be fine-tuned down to the atomic level.

The ability to make your own electronics material has led to amazing results. For instance:

  • Gallium-arsenide (GaAs) semiconductors convert electricity directly into coherent light. They are used in everything from LEDs to the light sources that carry lots of data on short- and long-distance optical networks.
  • GaAs lasers, operating at near infrared wavelengths, work in devices like market scanners and disc players.
  • Gallium arsenide was also used in the first microwave integrated circuits that enabled mobile communications. Silicon is being used more often now, not because it’s faster but because it’s a less expensive material.
  • GaAs thin-film solar panels hold the modern world record for efficiency. In the late 20th century, the USSR’s lunar rovers, as well as its Venus lander, were powered by gallium arsenide photovoltaics, as is the Mars rover Opportunity and most Earth-orbiting satellites launched since the 1990s.

At the best of times, Opportunity needs all the efficiency it can get from its GaAs solar panels as it rolls over the loose soil of Mars. This image was taken in 2014. At the time of writing, Oppy has powered down to sit out a huge dust storm that is completely blotting out the distant Sun. Elsewhere on the Red Planet, Curiosity is seeing some dust, too, but this rover runs on plutonium. (NASA/JPL-Caltech/Cornell University/Arizona State University)

Compound semiconductors are very expensive to make, which is why a certain region in California is still called Silicon Valley, not III/V Valley, but their future looks promising.

Why is arsenic critical to the US?

According to end-use statistics compiled in 2005 by Matos and Brooks, there were peaks in arsenic consumption in 1991, 1994, 1998, and 2001. I’m just speculating here, but those peaks seem to correlate with some US military actions.

If that’s valid, the peaks could be explained by increased use of arsenic during wartime to produce ammunition, metal alloys, and electronics.

More credibly, the US Geological Survey reports that arsenic is important to the sectors of national defense (semiconductors), energy (solar cells), and telecommunications/electronics (cellular phones, integrated circuits, and optoelectronic devices). (Fortier and others)

And the US relies totally on imports. We haven’t manufactured arsenic for decades. China supplies most of our arsenic metal, while the arsenic trioxide comes in almost equal parts from Morocco and China (with about 6% imported from Belgium). There are no government stockpiles.

So, what happens next? The US Commerce Department is working on it.

According to the USGS,

. . . the Commerce Department is responsible for organizing the interagency responses into a final report which is due Aug. 16, 2018, to the President. The report will include:

  • a strategy to reduce the nation’s reliance on critical minerals
  • the status of recycling technologies
  • alternatives to critical minerals
  • options for accessing critical minerals through trade with allies and partners
  • a plan for improvements to mapping the United States and its mineral resources
  • recommendations to streamline lease permitting and review processes,
  • ways to increase discovery, production, and domestic refining of critical minerals

This report will, as appropriate, include analyses and strategies to strengthen and sustain the supply chains for all minerals, and analyses and strategies targeted to minerals deemed critical based on this 2018 analysis . . .

We’ll only be up to gallium in this Mineral Monday blog series, so it’s a good idea to check out the whole critical mineral list (at the USGS link above) if you want to have input into the process through your congressional representatives.

For now, though, we’re through with arsenic. It has accompanied us in our journeys for millennia, first as we traveled all across the Earth, and then on spacecraft that we sent to the Moon, Venus, and Mars.

Arsenic is a little like human nature, too. It can be deadly, but when well understood and used carefully, it holds great promise for our welfare in the present and our hopes for the future.

Featured image: A syphilis medication bottle. Wellcome Collection, via Wikimedia, CC BY 4.0 DE.


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