GLOSSARY – W-X-Y-Z
by: Charles O’Dale
A high-pressure phase of polymorphous Mg2SiO4. An orthorhombic mineral. It was first found in nature in the Peace River meteorite from Alberta, Canada.
Zircon, zirconium orthosilicate (ZrSiO4), is found in most igneous rocks and some metamorphic rocks as small crystals or grains. Zircon transforms into reidite when the latter is subjected to very high pressures.
Zircons have two convenient characteristics;
- They withstand metamorphosis-the brutal churning of rock over very long periods of time;
- Their atoms are naturally arranged in a neat cubic structure. This molecular box can trap atoms or uranium inside them, as few as ten parts per million.
A small proportion of radioactive uranium over time will decompose into lead. Zircons when they form, may include uranium but exclude lead. The radioactive uranium decays into lead over millions of years. The U/Pb ratio, therefore is a clock to determine the age of the zircon.
[see – SHOCK METAMORPHISM – reidite]