Zirconium

Zirconium is a strong, shiny metal that most people encounter in two quite different forms: as cubic zirconia: the cheap, convincing imitation diamond, and as the cladding around the uranium fuel rods in nuclear power stations. In both cases it is exploiting different aspects of the same remarkable element.

  • Atomic Number4040 protons, 40 electrons
  • Atomic Mass91.22 uAbout 91× heavier than hydrogen
  • State at Room TempSolidshiny silver-grey metal
  • Density6.52 g/cm³About 6.5× denser than water
  • Melting / Boiling1854.8°C / 4408.9°CMelts at 1,855°C
  • Discovered1789Martin Klaproth, 1789

Zirconium is a key Period 5 transition metal. Compare its mass to neighbours.

Zirconium sits below titanium in Group 4 and shares many of its properties.

Atomic Mass Comparison
Yttrium88.9 u
Zirconium91.2 u
Niobium92.9 u
Molybdenum96.0 u
Iron55.8 u

Zirconium (91.2 u) sits just below titanium (47.9 u) in Group 4, and shares titanium's corrosion resistance and strength, but at roughly twice the atomic mass and somewhat higher density. It is transparent to neutrons, making it uniquely valuable in nuclear reactors.

What is zirconium?

Zirconium is a transition metal in Group 4 of the periodic table, sitting below titanium. It has 40 protons and shares titanium's impressive corrosion resistance, it immediately forms a stable oxide layer that protects the metal beneath. Zirconium has one extraordinary property that makes it irreplaceable in nuclear engineering: it is nearly transparent to neutrons. Most metals absorb neutrons and would reduce a reactor's efficiency, but zirconium lets them pass through almost undisturbed.

Zirconium gets its name from the Persian word zargun, meaning gold-coloured, referring to the golden colour of the mineral zircon. The mineral zircon (zirconium silicate, ZrSiO₄) has been a prized gemstone since ancient times, ancient zircon crystals from Australia dating back 4.4 billion years are the oldest known mineral samples on Earth. Martin Klaproth identified the new oxide in zircon in 1789, and Berzelius isolated the metal in 1824.

Fact The oldest known mineral on Earth is a tiny zircon crystal from the Jack Hills of Western Australia, dated to 4.404 billion years old, just 150 million years after the Earth itself formed. These microscopic crystals survived billions of years of geological upheaval intact because zircon is almost indestructible. They are the closest thing we have to samples from the very early Earth.

Where you find zirconium

In space

Zirconium is found in meteorites and across the solar system. The oldest mineral on Earth is a zircon crystal. Lunar rocks brought back by Apollo missions also contain zircon crystals that have helped scientists understand the early history of the Moon.

On Earth

Zirconium is the 18th most abundant element in the Earth's crust and is found in many minerals, most usefully in the heavy mineral sands of beaches and river deltas.

  • Zircon (ZrSiO₄). The main ore mineral, found in beach and river sands in Australia, South Africa and India. Australia is the world's largest producer of zircon.
  • Baddeleyite (ZrO₂). A rarer but important zirconium oxide mineral found in Brazil.

How we use zirconium

  • Nuclear reactor fuel cladding. The fuel rods in most nuclear reactors are tubes of zirconium alloy (zircaloy) filled with uranium oxide pellets. Zirconium's transparency to neutrons makes it ideal, almost no neutrons are lost to the cladding.
  • Cubic zirconia. Zirconium dioxide (cubic zirconia) closely resembles diamond in brilliance and is used as an affordable, ethical alternative to diamonds in jewellery.
  • Ceramics and refractories. Zirconium oxide ceramics are extremely hard and heat-resistant, used in knives, grinding media and high-temperature furnace linings.
  • Dental crowns. Zirconia ceramic dental crowns are tooth-coloured, strong and biocompatible, now the most widely used material for dental crowns.
Did you know? Zirconium and hafnium are chemically almost identical, they are the most similar pair of elements in the periodic table. They are so alike that when zirconium was first discovered, its hafnium content went unnoticed for over 130 years. Hafnium was not discovered until 1923, when Dirk Coster and Georg von Hevesy specifically searched for it by X-ray spectroscopy in zirconium ores.

How it was discovered

Zirconium was identified in 1789 by Martin Klaproth, who analysed zircon gemstones and found they contained an unrecognised oxide. He called the new earth substance zirconia. Jöns Jacob Berzelius isolated impure zirconium metal in 1824 by heating potassium hexafluorozirconate with potassium metal. Truly pure zirconium was not obtained until 1914, and the separation from hafnium (present in virtually all zirconium ores) was not achieved until the 1940s, specifically required for nuclear reactor applications.

Deeper dive: zirconium in nuclear reactors and zircon geochronology

The nuclear application of zirconium was developed during the Manhattan Project and post-war nuclear reactor programmes. The key requirement for nuclear fuel cladding is a material that is strong, corrosion-resistant at high temperatures, and does not absorb neutrons. Zirconium meets all three criteria perfectly. Naturally occurring zirconium, however, contains approx. 2% hafnium, which absorbs neutrons strongly and must be removed before zirconium is used in reactors. Separating zirconium from hafnium is chemically very difficult and adds significantly to the cost of nuclear-grade zirconium.

Zircon geochronology is one of the most powerful tools in geology for dating ancient rocks. Zircon crystals incorporate uranium (but not lead) when they crystallise. As uranium decays to lead over geological time, the lead accumulates inside the crystal. Because zircon is chemically robust and does not allow lead to escape, it preserves the uranium-lead ratio faithfully for billions of years. Measuring this ratio with extreme precision gives the age of the crystal, and thus the age of the rock it crystallised from. This technique has been used to date rocks over 4 billion years old and has transformed our understanding of early Earth history.

Zirconium is a versatile metal that spans nuclear engineering, gemstones and the oldest minerals on Earth. Moving to 41 protons brings us to niobium, a metal that gives steel extraordinary strength and is used in superconducting particle accelerators.