Fayalite
The iron-rich end of the olivine series, formed in meteorites mostly by water and heat acting on rock.
Key facts
- Formula
- Fe2SiO4
- Family
- Silicates
- Crystal system
- Orthorhombic
What it is
Fayalite is olivine in which iron has replaced the magnesium. It is denser than forsterite, about 4.4 times as dense as water, and melts at a much lower temperature, about 1,200 °C. It was named after Faial island in the Azores, where it was thought to occur, though that first sample was probably slag.
Where it is found
Almost pure fayalite grows in some CV and CO chondrites, where warm fluids moving through the parent asteroid reacted with metal and silicate. Because it holds manganese, the decay of short-lived manganese-53 in these grains can date that water activity, which happened within a few million years of the Solar System's birth. Iron-rich olivine is also found in some angrites and lunar basalts.
How it is recognised
It is yellow to brown or black, often in small veins and rims. Under the electron microprobe it appears with an iron content above about Fa90. Its presence alongside magnetite and hedenbergite is a sign of alteration by water.
Did you know?
Fayalite in meteorites gives some of the earliest dates for liquid water anywhere in the Solar System.
Fayalite is also common in iron-smelting slag, which is one reason slag is so often mistaken for a meteorite.