Poirierite
A rare form of olivine composition, halfway in structure between olivine and its high-pressure forms, found in shock veins of chondrites.
Key facts
- Formula
- (Mg,Fe)2SiO4
- Family
- High-pressure minerals
- Crystal system
- Orthorhombic
- First described
- the Suizhou, Tenham and Miami L6 chondrites, 2021
Poirierite was described in 2021 from shock veins in the Suizhou, Tenham and Miami L6 chondrites. It is named after Jean-Paul Poirier, a French mineral physicist who proposed in the early 1980s that olivine passes through an intermediate structure of this kind when it changes form. It is known only from meteorites and experiments.
What it is
Poirierite has the same chemistry as olivine, but its oxygen atoms are stacked in a pattern that sits between those of olivine and of its dense relatives wadsleyite and ringwoodite. Physicists had long suspected that olivine passes through such an intermediate arrangement when it is squeezed and changes form, and poirierite is the first natural example.
It seems to grow not while the pressure is highest but as the pressure falls, when olivine or ringwoodite is rearranged in the short moment of release after an impact.
Where it is found
It forms nanometre-thin layers inside grains of wadsleyite and ringwoodite in the dark shock veins of heavily shocked L chondrites, the meteorites thought to come from an asteroid broken apart about 470 million years ago.
How it is recognised
Its layers are far too thin for an ordinary microscope. It is identified by transmission electron microscopy, where its stacking shows up in electron diffraction patterns and high-resolution images.
Did you know?
Poirierite gives physicists a natural snapshot of how olivine changes shape under pressure, a process that also happens deep inside Earth's mantle.
Because it forms during decompression, it records the end of an impact rather than its peak.