Acapulcoites
Fine-grained rocks that only just began to melt.
- Classification
- Stony meteorites › Achondrites › Primitive achondrites › Acapulcoites
- Share of witnessed falls
- about 0.1%
- In our catalogue
- 56 meteorites, 1 seen to fall
- Heaviest
- Northwest Africa 2656, 7.5 kg
- Earliest recorded fall
- Acapulco, 1976
- Type specimen
- Acapulco, fell in Mexico in 1976
Named after Acapulco in Mexico, which fell in 1976, acapulcoites are fine-grained rocks of olivine, pyroxene, plagioclase, metal and troilite, roughly chondritic in chemistry. They were heated just past the point where melting starts, and some still contain relic chondrules. They come from the same parent body as the lodranites.
Mineralogy and chemistry
Acapulcoites are fine-grained, even-textured rocks with grains about 0.1 to 0.3 mm across, made of olivine, orthopyroxene, calcium-rich pyroxene, sodium-rich plagioclase, iron-nickel metal, troilite, chromite and phosphates. Their olivine and pyroxene are relatively poor in iron, intermediate between enstatite chondrites and H chondrites. Bulk compositions are close to chondritic, though some acapulcoites have lost part of their metal and sulphide.
Some contain relic chondrules, proof that they began as chondrites. Their oxygen isotopes lie below the terrestrial line and match those of the lodranites, with a Δ17O of roughly −1 per mil, distinct from any known chondrite group. Their precursor material was therefore a kind of chondrite not otherwise sampled.
Parent body and formation
Mineral thermometers suggest acapulcoites reached about 1,000 to 1,170 °C, just enough for metal and sulphide to melt and for the first silicate melt to form, but not enough for much of it to escape. Their radiometric ages show heating around 4,557 to 4,562 million years ago and fairly rapid cooling, consistent with a small body perhaps tens of kilometres across that was later disrupted. Lodranites are coarser rocks from deeper or hotter parts of the same body, and some stones are transitional between the two.
Acapulcoites are valuable because they record the very start of melting. Melt pockets and metal veins in them show how the first liquids moved through hot rock along grain boundaries, a process that, carried further, would eventually have separated a core. Their cosmic-ray exposure ages, similar to those of the lodranites, support the view that both were launched from one parent body in the same collision.
Abundance, type specimen and famous examples
Acapulco, the type specimen, fell on 11 August 1976 near Acapulco in Mexico, a single stone of about 1.9 kg. Several dozen acapulcoites are now known, mostly finds from Antarctica and the deserts of Africa and Oman. Monument Draw, found in Texas in 1985, is a well-studied example. Like the other primitive achondrites, they are rare among falls.
How to recognise one
An acapulcoite looks like a fine-grained, sugary grey rock with small bright metal grains and no clear chondrules. It is magnetic and somewhat heavy because of its metal. To a hunter it can easily pass for a strongly heated ordinary chondrite; only analysis of mineral compositions and oxygen isotopes can tell them apart.
Famous examples
- Acapulco Acapulcoite, Mexico, 1976 · 1.91 kg
- Monument Draw Acapulcoite, United States, 1985 · 525 g
Related groups
- Lodranites Coarse-grained leftovers after melt drained away.
- Winonaites Primitive achondrites related to the silicate inclusions in IAB irons.
- Primitive achondrites Rocks that began to melt but were never fully processed: halfway between chondrites and igneous rocks.
Photos shared under Creative Commons licences (Wikimedia Commons).