Chondritic meteorites are fragments of ‘cosmic sandstone’ from space. They are chemically similar to the Sun and so have been regarded historically as samples from the first crop of planetesimals that accreted in the solar protoplanetary disk.

Their component ‘sand grains’ include rare objects known as CAIs (calcium aluminium-rich inclusions) and abundant ones called chondrules; both cooled down from magmatic temperatures. CAIs are the oldest dated solar system solids at 4567 to 4568 Myr old. They probably condensed in a hot vaporized region of the disk, and they incorporated radioactive 26Al. Most chondrules, in contrast, are frozen droplets of ultramafic magma. They also incorporated radioactive 26Al but often ~80% less, relative to 27Al, than in CAIs, so probably formed ~2 Myr after CAIs. Chondrules are widely regarded as having formed when clumps of dust in the disk became ‘flash-melted’. However, by 2 Myr after CAIs, to judge from 182W deficit dating of iron meteorites, much of the dust in the disk had already evolved into a host of substantially molten planetesimals intensely heated by the decay of 26Al. Planet-forming mergers between those bodies would have led to ‘splashing’ with ejecta plumes of molten droplets being recycled to the disk. Such droplets account plausibly for most chondrules, with chondrites being construction debris from planet building rather than primary raw material. Their solar chemistry probably reflects late remixing of processed planetary materials. They did not melt because they accreted after 26Al had largely decayed.