|
Crystallization started already 4.6 billion years ago in the solar system, in which cosmic dusts formed from the nebula gas followed by chondrule from the melt droplets. On the earth, most of mineralization is the result of crystallization near equilibrium and thus the process is slow. However in space the situation is quite different because the crystallization takes place in a free space. In another word, crystallization starts without any substrates, walls or seeds, which behave like heterogeneous nucleation centers. We know from microgravity experiments that nucleation rate in such an environment is four order of magnitude smaller, which results in the nucleation at very large supercooling as large as ~1000K for chondrule formation. This large supercooling leads to a typical barred olivine chondrule or a radial pyroxene chondrules within seconds. Similar result was obtained for the cosmic dust formation from a gas phase, though again extremely high supersaturation was needed for the nucleation. In summary, crystallization of minerals in space is not controlled by the growth rate but by nucleation rate and therefore the nucleation kinetics has experimentally be investigated.
|
