Most meteorites seen to fall are chondrites that formed by the aggregation of stony globules, iron-nickel metal and iron sulfide some 4560 million years ago. The stony globules, "chondrules", are millimeter-size, sub-spherical objects that may constitute 75 percent of a chondrite meteorite. Their textures testify to their solidification from melts, in many cases during rapid cooling. The origin of chondrules has been the subject of debate and speculation; The English petrologist, Sorby, thought that they were a fiery rain from the Sun!
For the past thirty years, theories of the origin of chondrules have been dominated by nebular models. The Sun, planets and other Solar System objects were thought to have formed in an accretion disk of interstellar gas and dust. Chondrules were formed by fusion of dust-balls in the disk by collision, lightning or solar radiation. There are, however, counter arguments to these theories.
Many chondrules have compositions like that of the stony part of a hydrogen- and helium-free Sun but extremes exist and must be explained. Some chondrules are rich in refractory aluminum and moderately volatile sodium, but impoverished in refractory calcium. How could this compositon have been achieved in a dust-ball by nebular condensation?
Some iron meteorites that formed in solidifying cores on asteroids are as old as chondrules. In addition to chondrules, chondrites contain rare fragments of igneous rock. It is proposed that chondrules formed by interaction between already differentiated, partly molten planetary bodies. The debris from such interactions were compositionally variable from chondrule to chondrule but uniform on a centimeter-sized, chondrite scale. By some magic planetary break-up and recombination hardly fractionated stony material, but may have changed the metal to stone ratio.
