Nakhlites

Augite-rich volcanic rocks from Mars, about 1.3 billion years old.

Classification

Stony meteorites › Achondrites › Planetary achondrites › Martian meteorites › Nakhlites

Share of witnessed falls

about 0.1%

In our catalogue

15 meteorites, 1 seen to fall

Heaviest

Yamato 000593, 13.71 kg

Earliest recorded fall

Nakhla, 1911

Type specimen

Nakhla, fell in Egypt in 1911

Nakhlites, named after Nakhla in Egypt (fell 1911), are made mainly of augite (a calcium-rich pyroxene) with olivine. They crystallised about 1.3 to 1.4 billion years ago, probably in a thick lava flow or shallow intrusion, and contain clays and carbonates formed by water on Mars. All seem to have been ejected by a single impact about 11 million years ago.

Mineralogy and chemistry

Nakhlites are clinopyroxenites: about 70% to 80% augite crystals, with 5% to 15% iron-rich olivine, set in a fine-grained mesostasis of feldspar, glass, magnetite, sulphides and phosphates. The crystals are aligned, as in a rock where crystals settled and were packed together. Unlike most Martian meteorites, nakhlites are only lightly shocked, so their feldspar is not converted to glass.

Veins of iddingsite, a mixture of clays, iron oxides and carbonates formed by water, cut through the olivine. Dating of these minerals suggests that liquid water flowed through the rocks on Mars about 600 to 700 million years ago, long after the planet had become cold and dry at the surface.

Origin and formation

All nakhlites crystallised about 1.3 to 1.4 billion years ago, and all have cosmic-ray exposure ages of about 11 million years, showing they were launched by a single impact from a single volcanic pile. Slight differences in cooling rate between members suggest they come from different depths in one thick lava flow or a series of flows. Chassignites share the same crystallisation and ejection ages and probably come from the same igneous complex. A young crater in the Elysium region has been proposed as the launch site.

Because nakhlites were only lightly shocked, they preserve original magnetic and chemical records of Mars better than most Martian meteorites, and they are compared closely with rocks examined by rovers on the Martian surface.

Abundance, type specimen and famous examples

Around two dozen nakhlites are known, several of them paired. Nakhla, the type specimen, fell near Alexandria, Egypt, on 28 June 1911 as about 40 stones totalling about 10 kg; a story that one stone killed a dog is often repeated but was never confirmed. Lafayette was found in a drawer at Purdue University in Indiana in 1931, and Governador Valadares in Brazil in 1958. Yamato 000593 from Antarctica, about 13.7 kg, is the largest.

How to recognise one

Nakhlites are greenish-black, medium-grained rocks of long, dark augite crystals with rusty orange-brown flecks where olivine is altered. They are weakly magnetic. Their unshocked, fresh-looking texture sets them apart from shergottites, but laboratory analysis is needed.

Famous examples

• Nakhla Martian (nakhlite), Egypt, 1911 · 10 kg
• Lafayette (stone) Martian (nakhlite), United States, 1931 · 800 g
• Governador Valadares Martian (nakhlite), Brazil, 1958 · 158 g
• Yamato 000593 Martian (nakhlite), Antarctica, 2000 · 13.71 kg

Related groups

• Chassignites Olivine-rich Martian rocks; the first known fell in France in 1815.
• Shergottites The most common Martian meteorites: young basaltic and olivine-rich rocks.
• Martian meteorites Rocks from Mars, recognised by gas from the Martian atmosphere trapped inside them.

Photos shared under Creative Commons licences (Wikimedia Commons).