Professor Hanington's Speaking of Science: The rare-earth mineral gadolinium
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2023-12-21
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The next element in our series of rare-earth elements is gadolinium.Rare-earth elements are those at the bottom of the periodic chart, those funny two rows that don’t seem to be connected to anything.Actually, if you really wanted to be correct, these two rows should stick vertically out of the page right from the lanthanum and actinium positions. Nobody does this, of course, so you are stuck with the two bottom rows wondering why they are placed there.Gadolinium is a relatively rare element that is only now being noticed for its help in MRI procedures and to make exotic alloys with iron and chromium.Just in the last few years, metallurgists have found adding as little as 1% gadolinium to steel improves both the workability and resistance of the steel to high temperatures and oxidation. Also recently discovered is that gadolinium can serve as an electrolyte in solid oxide fuel cells – something that puts hydrogen and oxygen together yielding plain old water and an amount of electrical energy that could propel an automobile. Using gadolinium as a dopant for materials like cerium oxide, furnishes an electrolyte having high ionic conductivity while working at low operational temperatures.Gadolinium, having the symbol Gd, is a silvery-white metal when not oxidized that is malleable like silver and ductile like copper. Chemically, gadolinium reacts with atmospheric oxygen to slowly form a black coating, something like magnetite (Fe3O4), but in this case it is Gd2O3 showing the +3 oxidation state common to all rare-earths.Cold gadolinium is highly ferromagnetic like iron and will attract a horseshoe magnet at temperatures below its Curie point of 20 degrees Celsius. Above this temperature it suddenly switches to become paramagnetic meaning it will only be slightly magnetic but for the record, gadolinium is the most paramagnetic element of all.As we have found with all the rare-earths covered in this column, gadolinium is found in many minerals such as monazite sands and bastnäsite, where the metal is considered too reactive to exist free in nature.It was named after the mineral gadolinite, from which it was first discovered spectroscopically in 1880 by Swiss chemist Jean Charles Galissard de Marignac. Unexpectedly, the mineral specimen he was using actually contains very little gadolinium but it must have been enough to show new lines in a spectroscope so he knew he had a new element at hand and named it for the mineral he was studying, which in turn was named after the Finnish chemist and geologist Johan Gadolin. Thus, Johan got his name not only on a mineral but an element as well — certainly a rare privilege one would say.Later chemists found gadolinium in the mineral cerite, and by 1886 the French chemist Paul-Émile Lecoq de Boisbaudran, famous already for his discovery of gallium, carried out the separation of gadolinium metal in 1886.For the record, the abundance in the Earth’s crust of gadolinium is about 6.2 mg/kg making it about 10 times more common than iodine. The main mining areas are in China, the U.S., Brazil, Sri Lanka, India and Australia, with reserves expected to exceed 1 million tons. Because world production of pure gadolinium metal is about 400 tons per year, there should be enough to go around for a while — unless a very significant use is found and people start scrambling for it. The only known mineral with a high percentage of gadolinium is the very rare ore lepersonnite, found mainly in the uranium mines of the Congo.So what is gadolinium good for? Well, if you are going to have an MRI done it’s a good bet they will use a gadolinium-based contrast agent to help the doctor see abnormal tissues in the scan with greater detail. During the scan the technicians inject you with a solution of an organic gadolinium complex (typically Magnevist) to enhance the image created in magnetic resonance procedure.Paramagnetic ions, (remember gadolinium is the most), increase nuclear spin relaxation rates, making the black and whites of the picture sharper. This is important when looking for anomalies that are relatively small within the scan.Normally, gadolinium compounds are toxic to mammals, but gadolinium in these procedures go through a process called chelation, making it safe for use by surrounding the poisonous atom with a molecular cage of organics that can be expelled through urine before it can cause any toxic reaction.Research is being conducted on magnetic refrigeration that uses gadolinium-based materials such as Gd5SiGe that have a large magnetocaloric effect. A block of this material becomes cold when the magnetic field it is placed in suddenly switches off. A refrigerator with no moving parts (and no freon) seems like it has some advantages over what exists today.In addition, it must be said that of all solids, gadolinium has the highest neutron cross-section of all elements, meaning it absorbs incoming neutrons with high efficiency. You will find it used in shielding of nuclear reactors that are within structures where humans exist, such as on submarines, where it is also employed in emergency shut-down mechanisms.
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