Cesium–A Critical Mineral

Cesium (“caesium,” outside the US) is not just a ringtone. Nor is it simply the atom used in ultra-precise clocks that make everything from the Internet to GPS possible, thanks to accurate time stamps.

Yes, cesium defines time itself.

Although it can be hazardous in some forms, cesium also is extremely useful in a variety of niche applications ranging from things like catalyzing chemical processes to cleaning sulfur out of crude oil to making fiber optic and other specialty glasses to building cellphone motion sensor devices.

In its manmade radioactive form, Cs137 is used for treating cancer and sterilizing equipment in modern medicine; and as cesium formate, it is rented out as a reusable brine to companies drilling oil and gas wells in high-temperature, high-pressure formations.

The United States depends on imports, mainly from a mine in Canada. And that mine closed in 2015, although its cesium stock is still sufficient to meet US and worldwide demand for the near future.

What is cesium?

It’s an alkali metal and must be specially stored so it doesn’t come into contact with either air or water. Why?

That’s why. It will ignite explosively in air, too.

Here is a slow-mo look at cesium as it is broken out of its air-tight container and falls into water.

As the chemist says, those vapor trails might have been from the metal reacting with oxygen, though in this particular case it was probably H2O.

Still, an element that leaves vapor trails–wow!

Where does cesium come from?

Cesium isn’t found as a reactive metal in nature. It generally occurs as pollucite, a mineral found in rare-metal pegmatites. These are intrusive geologic formations containing minerals–particularly lithium, cesium, and tantalum (sometimes abbreviated LCT)–concentrated by water and other fluids around an intrusion of granitic magma.

The oldest go back some three billion years. Peak deposits happen as supercontinents grow, but not as the continents break apart. Probably very little LCT pegmatite is forming today.

It certainly happened, back in Precambrian times, in what’s now Manitoba, Canada. A huge pollucite deposit formed there–about two-thirds of the world’s known reserves. There are also some high-grade lodes in Namibia and Zambia, as well as noneconomic occurrences elsewhere around the world, including pegmatites in Afghanistan, China, and Italy, hydrous opal in Tibet, and natural brine in China.

The Chinese have been mining salt from underground brines for centuries:

With modern methods, apparently they can also extract cesium from groundwater.

The US has some pollucite, too, but none has been mined yet.

Why is cesium critical to the US?

The critical minerals listing just says that cesium is “used in research and development.”

Despite the Canadian mine closing, there certainly is no looming shortage. This element has many uses, but none of them require large amounts of cesium, and in its major use–for well drilling–cesium formate is simply rented out over and over again!

Perhaps those making the critical minerals list have noticed cesium’s role in R&D history. It seems to be more of an enabler than a true pioneer.

After the element’s discovery in the late 19th century, nothing much happened with it until the 1920s, when it proved useful in evacuating vacuum tubes as well as coating cathodes. Then safer, more economic materials were used, and finally vacuum tubes went extinct.

In the 1960s, the very first ion propulsion engines used cesium as a propellant. Such engines today can power a spacecraft like the Dawn mission vehicle from zero to 60 mph–in four days. Still, such gentle power is excellent when maneuvering around a small target in the Solar System.

But cesium was replaced long ago by mateials like xenon, which are much are less corrossive to spacecraft components.

Today, cesium doping may help to move more efficient solar cells called perovskites out of the research lab and into full-scale industrial production:

Time will tell. In the meantime, it’s a wise move to keep an eye on this “Great Enabler” and its supply lines and global reserves.

Featured image: Alshaer666, via Wikimedia, CC BY-SA 3.0.


Bradley, D. C.; McCauley, A. D.; and Stillings, L. M. 2017. Mineral-deposit model for lithium-cesium-tantalum pegmatites (No. 2010-5070-O). US Geological Survey. (PDF)

Butterman, W. C.; Brooks, W. E.; and Reese Jr, R. G. 2005. Mineral commodity profiles: cesium (No. 2004-1432). US Geological Survey. (PDF)

Fortier, S. M.; Nassar, N. T.; Lederer, G. W.; Brainard, J.; and others. 2018. Draft critical mineral list–summary of methodology and background information–U. S. Geological Survey technical input document in response to Secretarial Order No. 3359. Open-File Report 2018-1021. (PDF).

Tuck, C. A. (2013). Cesium. United States Geological Survey Mineral Commodity Summaries (Ed.), 44-45. (PDF)


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