Chondrules are the oldest objects in the Solar System. They are small, generally are 0.1 to 1 mm in diameter. (Chondrules outside of this range have been observed, but are rare.) They are spherical rock-based granules that make up around 60-80% of a typical meteorite. They are usually formed from olivine and/or pyroxene, sometimes with smaller amounts of glass, iron and nickel.
Because of their spherical nature, it's clear that they were at some stage molten - but explanations as to their formation mechanism(s) are not yet agreed.
“[…] two general categories of models have been proposed: those that form chondrules in energetic events occurring on or near planetesimals or planetary embryos and those that predict that chondrules were formed by some process that occurred within the solar nebula itself and operated on chondrule precursors that were suspended within the gas. Both planetary processes and nebular processes have their merits, but they both also have deﬁciencies that need to be addressed.”
See: Chondrule-forming Processes–An Overview NASA, Chondrites and the Protoplanetary Disk ASP Conference Series, Vol. 341, 2005
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