Chondrules

Chondrules are millimeter-scale bodies, generally spherical, which range in size from microscopic to a centimeter or more. The majority (80%) of meteorites that fall to Earth are composed primarily (70%) of chondrules. Chondrules show a range of internal structures. All appear to be drops of molten minerals that cooled over periods of minutes to hours, although some have a glassy composition suggesting flash cooling. Structures include fibrous or acicular crystals in a radial pattern, dendritic crystals, or plates which may alternate in composition. Some chondrules have a layered spherical pattern not seen in most Earthly minerals, but perhaps related to stalactitic forms. These have cores of minerals surrounded by layers of other minerals which sometimes alternate.

The formation of chondrules is problematic. Most show signs of rapid heating (in minutes) to a liquid at temperatures of 1500°C or higher, followed by slower cooling (over an hour or more). They may have undergone multiple heating/cooling cycles, but only the last is evidenced by the minerals present. The energy source of the rapid heating and the mechanism allowing relatively rapid cooling is unknown, since churning about in a cloud near the Sun would likely result in much slower rates of heating and cooling. A dense, turbulent cloud of chondrules where most particles are shadowed by other material has been proposed, as has the possibility that the young Sun may have shown extreme variations in brightness similar to an FU Orionis star. Such outbursts could rapidly blast large portions of the infalling solar nebula away in a high-energy burst. Also, note that slowly spinning grains could have rapidly heated on the sunlit side, then rapidly cooled on the dark side, contributing to a layered structure. Other mechanisms for forming chondrules have been proposed, including shock waves and even lightning. It is clear, however, that most asteroids formed as relatively cool chondrules aggregated together along with additional nebular material as a cement, with self-gravitation only becoming effective when the planetoids reached several kilometers in size.

Chondrules show a range of compositions, but the ones that survive falling to the Earth (the only ones we've studied) tend to be refractory minerals and metallic grains. Clearly, chondrules composed of ices would not survive transport to the Earth's surface, although I suspect that once we have visited and sampled pristine comets we'll find that their structure is also chondritic, with compositions of water ice, or ammonia ice, or methane ice, along with an assortment of refractory chondrules and metallic grains transported from the inner solar system. But perhaps the ice crystals will resemble snowflakes more than sleet or hail; only time will tell.

Type 1 chondrules appear to have formed under reducing conditions and are mostly fosterite (Mg₂SiO₄) and enstatite (MgSiO₃) with metallic iron or iron sulfides such as troilite (FeS).

Type 2 chondrules formed under oxidizing conditions and are composed primarily of olivine ((Mg,Fe)₂SiO₄) and hypersthene ((Mg,Fe)SiO₃), with iron oxides such as magnetite (Fe₃O₄).