Properties and Types of Meteorites

I.              Stony

IA.            Chondrites.  Composed of chondrules, CAIs (refractory inclusions), metal, and matrix.  Chondrules are spherical, ~1mm diameter ‘droplets’ composed of Fe,Mg silicates [olivine (Fe,Mg)₂SiO₄ and pyroxene FeMgSi₂O₆] and glass.  They are believed to have formed in a high-temperature ‘flash’ heating event(s) in the solar nebula, before the formation of planets.  CAIs are generally not spherical, several mm’s in size, and composed of Ca, Ti, Al oxides and silicates.  The minerals in CAIs are identical to what is predicted to crystallize from a slowly cooling gas of solar composition, and probably formed in this manner.  Metal is Fe,Ni-rich and Fe-sulfides also exist.  Matrix is fine-grained (<1µm) and has low-temperature components, including organic compounds, and is found between the other components.  These four components are believed to have come together in a cold environment, most likely in the solar nebula, before planets were formed.  Chondrites are considered the ‘building blocks’ of planets.  Chondrites also show evidence of shock, suggesting that when they did accrete into a planetesimal, the planetesimal was bombared by incoming meteorites.  They probably come from smaller asteroids in the asteroid belt (<50-100 km diameter)

IA1.  Ordinary chondrites.  The most common type of meteorite by far (see the table on meteorite falls and finds, especially the Antarctica finds).  CAIs are a rare component in ordinary chondrites.  Their abundance suggests that the asteroid belt may be populated by ordinary chondrite ‘parent-bodies’ (where these meteorites came from).

IA2.  Carbonaceous chondrites.  Much rarer than ordinary chondrites, but very important because they have abundant CAIs, as well as carbonaceous matter.  Spectra of Comet Shoemaker-Levy 9 (the comet that hit Jupiter a few years ago) showed evidence of carbonaceous chondrite material, suggesting that they may be an important part of comets.

IA3.  Enstatite chondrites.  Enstatite chondrites are more reduced than ordinary or carbonaceous chondrites (oxidized and reduced is a function of how much Fe is in silicates vs. metal), meaning that all the Fe is in metal in these meteorites, not in the olivine or pyroxene.  They are important because the bulk composition of the Earth is more reduced than ordinary or carbonaceous chondrites, and is best modelled as a mixture of ordinary and enstatite chondrites.

IB.  Achondrites. 

            There are several types of achondrites, and they are all ‘igneous’ meteorties.  They are very similar in appearance to basaltic rocks on Earth (rocks from mid-ocean ridges or Hawaii), but differ in composition.  Some achondrites look exactly like what you would expect if you heated up and melted an ordinary chondrite.  They likely come from large asteroids (>50-100 km diameter); asteroids large enough to have undergone melting.  There is a good connection between HED achondrites and the asteroid Vesta, and many believe this is where they come from.  Other achondrites are SNC’s which come from Mars, as well as lunar meteorites.

II.            Stony-iron

The stony-iron meteorites are composed of olivine in a matrix of Fe-metal.  It is believed that they come from the core-mantle boundary of obliterated asteroids.

III.         Iron

            Iron meteorites are composed of large crystals of Fe and Fe,Ni metal with Fe-sulfides (and phosphides and carbides) that cooled much more slowly than can be duplicated in the laboratory.  They are believed to be from the cores of obliterated asteroids.