Chassignites
Olivine-rich Martian rocks; the first known fell in France in 1815.
- Classification
- Stony meteorites › Achondrites › Planetary achondrites › Martian meteorites › Chassignites
- Share of witnessed falls
- about 0.1%
- In our catalogue
- 2 meteorites, 1 seen to fall
- Heaviest
- Chassigny, 4 kg
- Earliest recorded fall
- Chassigny, 1815
- Type specimen
- Chassigny, fell in France in 1815
Chassignites are dunites, rocks made almost entirely of olivine, that formed as crystals settled in a Martian magma chamber about 1.3 billion years ago. Chassigny, which fell in France on 3 October 1815, was the first Martian meteorite known, though nobody realised it came from Mars until the 1980s. Only a couple more have been found since.
Mineralogy and chemistry
Chassignites are dunites, made of about 90% iron-bearing olivine (about 30–32 mol% fayalite), with minor pyroxene, chromite and plagioclase. Within the olivine are tiny pockets of trapped melt that crystallised to minerals including kaersutite, a water-bearing amphibole, evidence that the parent magma held some water. Their oxygen isotopes match all other Martian meteorites.
Origin and formation
Chassignites formed as olivine crystals sank and piled up at the bottom of a magma chamber or thick lava flow on Mars. They crystallised about 1.3 to 1.4 billion years ago and were ejected about 11 million years ago, the same as the nakhlites, so the two groups probably come from one igneous body. Chassigny contains xenon whose isotopes differ from the Martian atmosphere and are thought to reflect the planet's interior, making it a key sample for how Mars obtained its volatile elements.
Abundance, type specimen and famous examples
Chassigny, the type specimen, fell near the village of Chassigny in the Haute-Marne, France, on 3 October 1815; about 4 kg was recovered. It was the first Martian meteorite to fall, although its origin was recognised only in the 1980s. Northwest Africa 2737, found in Morocco in 2000 and sometimes called Diderot, is the second: its olivine was turned black by intense shock. Only a few chassignites are known.
How to recognise one
Chassigny is a pale yellow-green, sugary, crumbly rock of small olivine grains, rather like terrestrial dunite. Northwest Africa 2737 is dark, almost black. Both are weakly magnetic. Only laboratory study can separate them from Earth rocks or from olivine-rich achondrites such as brachinites.
Why they matter
With just a handful of specimens, chassignites are among the rarest meteorites. Their trapped melt and noble gases give a rare view of the Martian mantle and of the water dissolved in its magmas, which helps scientists estimate how much water the interior of Mars contains.
Orbiting spacecraft have mapped olivine-rich rocks across parts of the Martian surface, and comparing them with chassignites helps scientists interpret those maps and the volcanic systems that made them.
Famous examples
- Chassigny Martian (chassignite), France, 1815 · 4 kg
- Northwest Africa 2737 Martian (chassignite), 2000 · 611 g
Related groups
- Nakhlites Augite-rich volcanic rocks from Mars, about 1.3 billion years old.
- Shergottites The most common Martian meteorites: young basaltic and olivine-rich rocks.
- Brachinites Olivine-rich rocks from an asteroid whose melt drained away.
- Martian meteorites Rocks from Mars, recognised by gas from the Martian atmosphere trapped inside them.
Photos shared under Creative Commons licences (Wikimedia Commons).