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Jul 09, 2023

Professor Hanington's Speaking of Science: The rare

Samarium is the next element on our list to investigate, just after the radioactive promethium that we did two weeks ago. Discovered in 1879 by French chemist Paul-Émile Lecoq de Boisbaudran, samarium

Samarium is the next element on our list to investigate, just after the radioactive promethium that we did two weeks ago. Discovered in 1879 by French chemist Paul-Émile Lecoq de Boisbaudran, samarium was named after the mineral samarskite, radioactive rare earth mineral series whose chemical composition has the chemical formula (YFe3+Fe2+U,Th,Ca)2(Nb,Ta)2O8.

The mineral itself was named after a Russian mine official, Colonel Vassili Samarsky-Bykhovets, who thus became the first person to have a chemical element named after him, although you can say it was done indirectly, the next one being gadolinium, named after Finnish mineralogist Johan Gadolin in 1886.

Samarium is a moderately hard silvery metal that slowly oxidizes in air much like all the rare-earths we have covered. Being a typical member of the lanthanide series, samarium usually has the oxidation state of +3. Samarium is the 40th most abundant element in Earth’s crust and more common than metals such as tin. You can find it in concentrations up to 2.8% in several minerals including the principal ore of cerium, cerite. Very few minerals have samarium being the most dominant element with possibly the Brazilian mineral florencite being the exception. In addition to that, samarium is usually found in monazite and bastnäsite, along with other rare-earth elements, the most common commercial sources of the element. Both minerals are found primarily in China, although the United States does produce some. One large active mine at Mountain Pass, can be seen from Interstate 15 just a little past Primm after you cross the California border.

The main use of samarium is as an alloy with cobalt to make magnets that are almost as strong as those made from neodymium, but can withstand significantly higher temperatures, above 700°C without losing their permanent magnetic properties. Samarium has a hardness and density similar to zinc and a low boiling point, only 1,794 °C, which helps separation from its ores. It is also paramagnetic, meaning it is weakly attracted to the poles of a magnet although not as strongly as iron which is actually ferromagnetic.

In air, samarium slowly oxidizes at room temperature and will spontaneously start burning at 150°C when the oxidation process gets out of control. The metal will tarnish yellow even when stored under mineral oil. About the only metallic appearance of a sample can be preserved by sealing it in a glass ampule under an inert gas such as argon.

There are seven naturally occurring isotopes of samarium with Sm-152 being the most abundant at 26.8%. Two of the isotopes are slightly radioactive with enormous half- lives, Sm-147 is 1.06E11 years and Sm-148 even longer at 7E15 years.

The half-lives of samarium can be put to good use in a process called samarium-neodymium dating, a technique used for determining the ages of rocks and meteorites. Based on the alpha decay of the long-lived samarium isotope Sm-147 to the stable neodymium isotope Nd-143, the ratio of the elements present in a sample evolves in a way that depends on the new ratio of samarium to neodymium in crustal material as opposed to that in mantle material. Scientists say that samarium-neodymium dating allows one to determine when the crustal material was formed.

Samarium has other uses as well. The radioisotope Sm-153 (made in a reactor) is the active component of the drug samarium lexidronam, sometimes called Quadramet and is used to kill cancer cells as a treatment in lung cancer, prostate cancer, breast cancer and osteosarcoma because it gives off beta particles. Instead of giving off emissions, the isotope Sm-149 can absorb them as well and is used as an effective neutron absorber in control rods used in nuclear reactors because it is less “poisoned” than boron or cadmium rods can be.

As mentioned earlier, about the most important use of samarium is to manufacture magnets, which are nominally SmCo5 or Sm2Co17. This technology was developed in the early 1960s based on work done by Karl Strnat at Wright-Patterson Air Force Base. Since they are almost as strong as neodymium magnets they are usually found in small motors, headphones, and high-end magnetic pickups for guitars and other musical instruments because they resist de-magnetism better when exposed to mechanical shock. When you see an electric guitar pickup called Samarium Cobalt Noiseless Pickup you can tell your friends what you now know about the element.

The James Webb Space Telescope has spotted a rare and tumultuous sight 15,000 light-years away from Earth.

Gary Hanington is Professor Emeritus of physical science at Great Basin College and Chief Scientist at AHV. He can be reached at [email protected] and [email protected].

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