Chondrite meteorites are the most common accounting for 83.6% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), free Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such as Ca-Al-rich Inclusions (CAIs) found in many carbonaceous chondrites and a few rare ordinary chondrites). These components have remained largely unchanged since their formation within the accretion disk ~4.5 Ga at the beginning of our Solar System. CAIs and chondrules studied today experienced a complicated series of high-temperature events as isolated grains in the disk before developing into planetesimals that went on to experience additional thermal and aqueous alteration. They therefore provide important evidence of the constraints at multiple points in time and location on the evolving dynamics, physical and chemical processes during the formation of the accretion disk and early solar system1.
Except for the lightest elements (e.g., H, He), chondrites have the same elemental composition as the original solar nebula because they come from planetesimals that never melted or underwent differentiation. Chondrites are so named because they nearly all contain chondrules – small round droplets of olivine and pyroxene. Chondrules are one of the first solids to have condensed and crystallized in the solar nebula and all similar in bulk composition to the Sun’s photosphere. Variations in chemical composition among chondrites reflect formation of their parent bodies in different regions of the solar nebula. The main groups are carbonaceous chondrites (CC), enstatite chondrites (E), ordinary chondrites (OC) including the H, L, and LL groups, and rumurutiites (R). Other, rarer chondrite groups are F-chondrites, G-chondrites and kakangariites (K).
Each group is further subdivided into petrologic types 1 through 7. Types 1 and 2 show evidence of aqueous alteration to the extent that chondrules are either absent (Type 1) or rare (Type 2). Petrologic types 3 to 7 show evidence of varying degrees of thermal metamorphism, which is reflected by modification of the chondrules and chemical homogenization. Type 3 chondrites display unaltered and distinct chondrules, whereas the chondrules become increasingly indistinct due to recrystallization/equilibration in types 4 to 6. Type 7 chondrites, better termed metachondrites, in which chondrules are absent, are transitional between chondrites and primitive achondrites.
Aqueous alteration and thermal metamorphism used to be considered as separate processes leading to the linear petrologic type scale from Type 1 to Type 7. However, it is now clear that fluids of some kind were present during thermal processing of virtually all chondrite classes, and thermal alteration played a role in virtually all chondrite classes including many carbonaceous chondrite types.