Primitive achondrites
Rocks that began to melt but were never fully processed: halfway between chondrites and igneous rocks.
- Classification
- Stony meteorites › Achondrites › Primitive achondrites
- Share of witnessed falls
- about 0.3%
- In our catalogue
- 494 meteorites, 9 seen to fall
- Heaviest
- Winona, 24 kg
- Earliest recorded fall
- Lodran, 1868
- Most recent fall
- Almahata Sitta, 2008
Primitive achondrites have igneous textures, but their bulk chemistry is still close to that of chondrites, and some even contain relic chondrules. They come from asteroids that were heated enough to begin melting, partially losing their metal and basaltic melt, but not enough to separate fully into crust, mantle and core.
The groups are the acapulcoites and lodranites (from the same parent body), the winonaites (linked to IAB irons), the brachinites and the ureilites. Ureilites are sometimes placed separately because they melted extensively but kept unusual carbon-rich chemistry.
Between chondrite and igneous rock
Imagine heating a chondrite slowly. The first liquid to form is a mixture of molten iron-nickel metal and iron sulphide, at around 950 °C. A little hotter, and the first silicate melt appears, rich in plagioclase and pyroxene. Primitive achondrites are rocks caught at these stages. Their grains have recrystallised into interlocking igneous textures, and some melt has drained away, but their bulk chemistry is still close to the chondrites they came from. A few still contain recognisable relic chondrules.
Different groups record different amounts of melting. Acapulcoites lost little more than some metal-sulphide melt; lodranites lost basaltic melt as well. Winonaites are similar rocks from a separate body. Brachinites are olivine-rich residues from yet another, and ureilites are mantle rocks from a body that melted more extensively.
Oxygen isotopes and parent bodies
Primitive achondrites keep some of the isotopic variety of their chondritic starting material. Acapulcoites and lodranites share oxygen isotopes, as do winonaites and the silicate inclusions in IAB iron meteorites, so these pairs come from common parent bodies. Ureilites are the extreme case: their oxygen isotopes vary from stone to stone along a line with a slope of about 1, the same line traced by carbonaceous chondrite inclusions, showing that their parent body never mixed thoroughly even while it melted.
Their ages, mostly around 4,555 to 4,565 million years, show that partial melting happened early, while aluminium-26 was still active.
Abundance and recognition
Together the primitive achondrites make up well under 1% of falls, with ureilites by far the most common among them. Famous members include Acapulco, which fell in Mexico in 1976; Pontlyfni, which fell in Wales in 1931; and Almahata Sitta, the ureilite shower from the tracked asteroid 2008 TC3. To the eye they resemble fine- to medium-grained, recrystallised chondrites with few or no chondrules. Many contain metal and so attract a magnet. Only laboratory study can identify them.
The primitive achondrites matter because they show the first step of planet building that turned dust into layered worlds. Comparing how much melt each group lost tells scientists how quickly metal and silicate began to separate, and therefore how fast cores formed. The winonaites are paired with the IAB irons, linking the stony and metallic records of the same body.
Groups
Acapulcoites
Fine-grained rocks that only just began to melt.
56 in the catalogueLodranites
Coarse-grained leftovers after melt drained away.
50 in the catalogueWinonaites
Primitive achondrites related to the silicate inclusions in IAB irons.
28 in the catalogue
Brachinites
Olivine-rich rocks from an asteroid whose melt drained away.
33 in the catalogueUreilites
Carbon-rich rocks with diamonds, from an asteroid shattered by a giant impact. One was tracked in space before it hit Earth.
327 in the catalogueRelated groups
- Acapulcoites Fine-grained rocks that only just began to melt.
- Lodranites Coarse-grained leftovers after melt drained away.
- Winonaites Primitive achondrites related to the silicate inclusions in IAB irons.
- Brachinites Olivine-rich rocks from an asteroid whose melt drained away.
- Ureilites Carbon-rich rocks with diamonds, from an asteroid shattered by a giant impact. One was tracked in space before it hit Earth.
- IAB irons Irons with stony inclusions, from a partly melted asteroid. Canyon Diablo and Campo del Cielo.
Photos shared under Creative Commons licences (Wikimedia Commons).