Reading Rocks: Zircons are Forever

Zircon crystals survive the processes of erosion, transport and metamorphism, preserving a record of the geological past. They truly last forever!

Oldest minerals in the solar system

The chemical formula for zircon is ZrSiO₄. Zirconium (Zr) atoms are occasionally replaced by uranium (U) as the mineral forms. Uranium decays to lead (Pb) over time. By examining the ratio of uranium to lead, scientists can determine how old the zircon crystal is.

Although both uranium and lead are present in lava, only uranium fits into the zircon structure. Over time, some of the uranium decays to lead. (KelvinW 344 2010, public domain)

 The oldest zircons on Earth are found in the Jack Hills of Western Australia. Many of these zircons are 4.0 billion years old, but the oldest of them all is dated at 4.4 billion years. Zircons are found on meteorites, too. A zircon from a meteorite discovered in Morocco was dated at 4.47 billion years. Dates from Earth and meteorites help us to determine the age of Earth.

Evidence for early events

The oxygen atoms in zircon can also provide evidence of the past. The ratio of Oxygen 18 to Oxygen 16 provides clues about the temperature of early Earth. Zircons from the Archaean suggest that early Earth was cool enough for liquid water to exist.

Clues to the history of life can also be contained within zircons. Tiny grains of graphite in a 4.1 billion-year-old zircon shows a carbon isotope signature suggesting that life had arisen at this very early time (it should be noted, however, that this was a single sample).

X-ray image of zircon crystal RSES 61-18.8 with arrows pointing to graphite inclusions that indicate biological carbon. (Bell EA, Boehnke P, Harrison TM, Mao WL. Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon. PNAS Nov 24, 2015 112(47): 14518-1452, https://doi.org/10.1073/pnas.1517557112)

The inclusion of relatively high concentrations of aluminium in zircons is used as a marker for the start of plate tectonics 3.6 billion years ago. Aluminium is included in zircons when rocks melt deep beneath the Earth’s surface at subduction zones. Thus, zircons high in aluminium indicate the presence of modern tectonics.

Durable gems and valuable metals

Zircon is usually reddish brown but can range in colour from clear to purplish red to blue. Zircon has been popular as a gemstone for centuries due to its variety of colours and high refraction. Its sparkle is similar to diamond, but zircon is softer and has double refraction. Heat treatment is often used to create blue and green colours. Zircons are mentioned in the Bible and a Hindu legend tells of presenting green zircon leaves as a gift to a god.

Zircon (ZrSiO₄) should not be confused with Cubic Zirconia (ZrO₂), a colourless synthetic mineral used as a diamond substitute.

Zircons come in a wide variety of colours, with blues sometimes produced by heat treatment. (Rob Lavinsky, iRocks.com – CC-BY-SA-3.0)

Zircons provide clues about the chemistry of underground rocks, helping researchers find valuable deposits of copper and gold. Zirconium metal extracted from zircon resists corrosion. Zirconium is used in a huge variety of products such as specialised pipes, nuclear reactor cladding, computer disc drives, wear-resistant knives and ballpoint pens.

Zircon is truly an amazing mineral that not only has a pleasing appearance but tells us much about the history of Earth.

Explore

• Geoscience Australia has all the facts about zircon.
• Knowable Magazine talks about keeping time with zircons.
• Learn about the structure of silicate minerals with this modelling activity.
• Read about how oxygen isotopes are used to determine past temperatures and try an activity to model this.