Zircon
A tough zirconium silicate that holds uranium, making it one of the best natural clocks.
Key facts
- Formula
- ZrSiO4
- Family
- Silicates
- Crystal system
- Tetragonal
What it is
Zircon takes in uranium when it crystallises but rejects lead, so all the lead it contains has been made by radioactive decay since it formed. Measuring uranium and lead in a single grain gives its age. It is extremely hard-wearing and survives erosion and metamorphism.
Where it is found
In meteorites zircon is rare but valuable. It occurs in eucrites from Vesta, in mesosiderites, in lunar rocks and lunar meteorites, and in some Martian meteorites. Zircons in the Martian breccia NWA 7034 and its pairs are about 4.4 billion years old, among the oldest known pieces of Mars's crust.
How it is recognised
It forms tiny, clear to brown crystals, rarely larger than a fraction of a millimetre in meteorites. It is found with electron microscopes by its brightness in backscattered electron images, because zirconium is heavy.
Did you know?
Zircons from Western Australia are the oldest known pieces of Earth, about 4.4 billion years old.
Shock can turn zircon into its high-pressure form, reidite, or break it into tiny grains of baddeleyite.