Pigeonite
A low-calcium pyroxene that forms in quickly cooled lavas, typical of eucrites and lunar and Martian basalts.
Key facts
- Formula
- (Mg,Fe,Ca)(Mg,Fe)Si2O6
- Family
- Silicates
- Crystal system
- Monoclinic
What it is
Pigeonite is a pyroxene with a little calcium, about 5 to 15% of its metal sites. It is stable only at high temperature. In lavas that cool slowly it breaks up into thin alternating layers of low- and high-calcium pyroxene, called exsolution lamellae. Their width records how fast the rock cooled. It is named after Pigeon Point in Minnesota.
Where it is found
It is the main pyroxene of the basaltic eucrites from Vesta, and is common in shergottites from Mars and in mare basalts brought back from the Moon. Its layered textures show which eucrites cooled at the surface and which were buried and reheated.
How it is recognised
It is brown to black in hand specimens. In thin section it is colourless to pale brown with moderate interference colours, and often shows fine exsolution lamellae of augite. A microprobe confirms its calcium content.
Did you know?
Exsolution lamellae in pigeonite can be thinner than a thousandth of a millimetre.
Studying them shows that many eucrites were heated again long after they first erupted on Vesta.