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.
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).
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 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.
- 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.