Collecting the periodic table’s rarest elements: osmium, rhodium and iridium #IYPT2019

by Richard Herrington, Head of Earth Sciences

At the Natural History Museum our work is based on scientific collecting to support research on the natural world. In our collections we hold naturally occurring chemical elements in many different forms, compounds and combinations, so with many others, we are marking the 150 years since the periodic table was published in 1869 by the Russian chemist Dmitri Mendeleev by celebrating the International Year of the Periodic Table.  We are aiming to cover all of the elements in our collections in a blog, starting with osmium, rhodium and iridium.

Why should we celebrate the periodic table? To millions of people, it’s instantly recognisable, a rational and intriguing way of presenting the elements, an icon for chemistry and wider science. In scientific terms, the periodic table is a visual and logical presentation of the chemical elements on the basis of their atomic weights and common properties.  Mendeleev organised the known elements in a way that allowed him to predict the occurrence of undiscovered elements. Although the number of naturally occurring stable elements is limited to 91 (Uranium is the heaviest natural element found in significant amounts on the planet), scientists have since created further artificial transient radioactive elements using nuclear fusion and we now have a periodic table containing more than 100 elements.

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The modern periodic table (CC0 Wikimedia Commons)

The Earth is not a simple homogenous globe. Its central core is surrounded by a mantle which in turn is enclosed by the crust. Within each of these different layers, elements are unevenly distributed in different materials and rocks. In particular, elements are found in different proportions and combinations within distinctive chemical compounds of unique composition, compounds that we term minerals. The outer part of the Earth that we live on­—the crust—is made up of minerals dominated by common elements such as silicon, aluminium, iron, calcium, potassium, sodium and magnesium in compounds with oxygen and hydrogen. Other elements are much rarer in the crust and again are only found in very specific minerals in particular locations.

The rarest elements in the Earth’s crust are the platinum-group metals. These are concentrated in the Earth’s deep mantle (up to 2,890km below the surface) but are also common in metallic meteorites: fragments of primitive planetary material. These platinum-group metals are so rare and so chemically inert that they were not isolated from the minerals that contained them until the early 19th century, with important work being undertaken by the British scientists William Hyde Wollaston and Smithson Tennant.

The collections at the Natural History Museum contains concentrates of palladium (Pd), platinum (Pt), osmium (Os) and iridium (Ir) – which are probably linked to some of the earliest work including the discovery of these elements by Wollaston and Tennant in the early 1800s. These are currently being investigated by our curators to determine how these samples came to the collection and how historically significant they really are.

Osmium BM 36720-002-20022019
Osmium – tiny scales mixed with platinum. Sverdlovskaya Oblast; Nizhniy Tagil, Russia.  Museum specimen BM.36720

Osmium, rhodium and iridium are probably the rarest metals found in the Earth’s crust with average concentrations of 0.0001, 0.0002 and 0.0003 parts per million by weight respectively. These very rare metals are now very important industrially and command very high prices with rhodium the highest priced metal at more than twice the value of gold.

Since the initial discovery of these ultra-rare platinum-group metals, research into the NHM’s collections using state-of-the-art analysis still occasionally yields new minerals containing such rare elements. The elusive elements rhodium and iridium were found in a mineral hiding in the collection in 1983 by the NHM researcher, Alan Criddle. Alan named this mineral bowieite after Stanley Bowie, former assistant director of what is now the British Geological Survey, and the specimen in the NHM collection in which it was discovered now becomes the ‘type specimen’ (the definitive scientific reference specimen) for all other research that follows, whether in the Museum, universities, research institutes or industry.