Clan · Stony meteorites › Chondrites

Ordinary chondrites

The everyday meteorite: four in every five falls. Three groups, H, L and LL, from at least three asteroids.

Classification: Stony meteorites › Chondrites › Ordinary chondrites
Share of witnessed falls: about 80%
In our catalogue: 40,278 meteorites, 856 seen to fall
Heaviest: Jilin, 4 t
Earliest recorded fall: Nogata, 860
Most recent fall: Chelyabinsk, 2013

Ordinary chondrites are the most common meteorites: about 80% of all falls and the great majority of finds. They are made mainly of olivine and pyroxene with flecks of iron-nickel metal and troilite, so almost all of them attract a magnet.

They divide into three groups by their iron content: H (high total iron), L (low total iron) and LL (low iron, low metal). As the metal content falls, the silicates contain more iron, which can be measured in their olivine. Most are petrologic types 4 to 6.

The three groups come from at least three parent asteroids. Dust brought back from asteroid Itokawa by the Japanese Hayabusa mission in 2010 matched LL chondrites, the first direct link between a meteorite group and a known asteroid. Common S-type asteroids in the inner asteroid belt are the likely source of all three.

Mineralogy and chemistry

Olivine and low-calcium pyroxene make up most of every ordinary chondrite, with smaller amounts of plagioclase (or glass in the least heated ones), iron-nickel metal (kamacite and taenite), troilite, chromite and phosphates. Chondrules are abundant, typically 60% to 80% of the rock, and matrix is scarce, only about 10% to 15%.

The three groups form a sequence. From H to L to LL, total iron and metal fall while the iron content of olivine and pyroxene rises, a pattern first described by Harvey Prior in 1916 and now called Prior's rules. The olivine composition, measured as mol% fayalite, separates them cleanly: about 16–20 for H, 22–26 for L and 26–32 for LL. Chondrules also get larger on average, from about 0.3 mm in H to about 0.9 mm in LL. Oxygen isotopes rise slightly through the sequence too, all just above the terrestrial line.

Parent bodies and formation

Each group comes from its own asteroid, and probably from several fragments of each. They formed around 2 million years after the first solids and were heated from inside by aluminium-26. In the simplest model, an 'onion-shell' asteroid became hottest in the centre, where type 6 rock formed, and stayed cool near the surface, where type 3 rock survived. Many have since been broken up and reassembled by impacts, so breccias of mixed types are common.

Their source asteroids are the stony S-type and Q-type asteroids of the inner main belt. Dust returned from Itokawa in 2010 was LL chondrite, and studies of young asteroid families point to likely sources of H and L chondrites in the inner belt.

Abundance and famous examples

About 80% of witnessed falls are ordinary chondrites, and they dominate collections from Antarctica and the hot deserts. Famous ones include Ensisheim (LL6, France, 1491), the oldest fall with surviving material; L'Aigle (L6, France, 1803), whose shower convinced scientists that stones fall from the sky; Barwell (L5, 1965), the largest fall recorded in Britain; Sylacauga (H4, 1954), which struck a woman in Alabama; Peekskill (H6, 1992), which hit a car in New York State; and Chelyabinsk (LL5, 2013).

How to recognise one

Ordinary chondrites usually have a black, matt fusion crust over a pale to medium grey interior with visible chondrules and scattered bright metal. They are noticeably heavy for their size and pull on a magnet. Over years on the ground, metal rusts and stains the stone brown, so finds are graded for weathering. Telling H from L from LL needs laboratory measurement of olivine composition, though metal content gives a hint.

Groups

Famous examples

Chelyabinsk LL5, Russia, 2013 · 100 kg
Peekskill H6, United States, 1992 · 12.57 kg
Jilin H5, China, 1976 · 4 t
Barwell L5, United Kingdom, 1965 · 44 kg
Sylacauga H4, United States, 1954 · 5.56 kg
Ensisheim LL6, France, 1491 · 127 kg
L'Aigle L6, France, 1803 · 37 kg

Related groups

H chondrites High-iron ordinary chondrites: about a third of all meteorites seen to fall.
L chondrites Low-iron ordinary chondrites: the single most common group of meteorites.
LL chondrites Low iron, low metal: the ordinary chondrites matched to asteroid Itokawa.
R chondrites (rumurutiites) Highly oxidised chondrites with almost no metal.
Acapulcoites Fine-grained rocks that only just began to melt.
IIE irons Irons full of silicate fragments, possibly from asteroid Hebe.

Photos shared under Creative Commons licences (Wikimedia Commons).