Before there were asteroid and planets, there were CAIs and chondrules. These early solids are the building blocks of planets, moon, and asteroids, and, luckily for researchers, are preserved in the meteorite record.

CAIs (calcium-aluminium-rich inclusions) are the oldest solids in the Solar System, dating back to 4.568 billion years ago, and are found in carbonaceous chondrites. Calcium and aluminium are some of the first solids to condense in the protoplanetary disk, but minerals like anorthite, perovskite, spinel, calcium-rich pyroxene, and magnesium-rich olivine are also included under the CAI umbrella.

Chondrules are spherical blobs of olivine and/or pyroxene, usually with some feldspar too. There are lots of different types of chondrule which are distinguished by composition and appearance, and these go on to make chondrite meteorites if enough of them stick together. Even though chondrite meteorites are very common, the actual formation mechanism of chondrules is still a mystery. So far, scientists have proposed that chondrules form by impacts of planetesimals, by shockwaves in the protoplanetary disk, magnetic flares, outbursts of energy from the Sun, and even by nebular lightning or radiation from supernovae!

Over time, chondrules and CAIs were able to stick together to form chondrite meteorites. From there, partial melting was able to occur in large enough bodies to create achondrites which went on to turn into planets. Chondrites are the most abundant class in the global meteorite collection (over 85% of all classified meteorites!) and are incredibly diverse. They might seem simple at first, but there's still lots to learn from these early rocks.

Chondrites that formed in the inner Solar System are the most common type of meteorite. Made from chondrules and a dusty matrix, inner Solar System chondrites are broadly divided in ordinary, enstatite, and R chondrites (with a few newer groups too).

As the name suggests, ordinary chondrites are the most abundant type of chondrite and make up over 75% of the meteorite collection. Ordinary chondrites are further broken down by mineral and chemical composition into H (highest iron), L (lower iron), and LL (lowest iron).

Enstatite chondrites are much less common (only 1.5% of meteorites) and have chondrules made from the high-Mg pyroxene, enstatite. These meteorites formed in exceptionally oxygen-poor conditions and as a result they contain a number of unusual sulfides such as oldhamite (CaS), niningerite (MgS), and rare alkali sulfides. Like ordinary chondrites, enstatite chondrites are split into two subgroups. In this case, they're grouped into EH (high iron) and EL (low iron).

The rarest inner Solar System chondrites are the K and R chondrites. Named after the Kakangari meteorite, the K chondrite group is identified by high amounts of dusty matrix between chondrules, mineral compositions similar to enstatite chondrites, and refractory lithophile element contents in keeping with ordinary chondrites. R chondrites, named after the Rumuruti chondrite, contain fewer chondrules than enstatite chondrites and are significantly more oxidised than ordinary chondrites. Importantly, R chondrites appear to represent the regolith (unconsolidated material on the surface of an asteroid of planet) of their parent body.

Although carbonaceous chondrites only account for less than 4% of the total global meteorite collection, these fascinating rocks preserve CAIs - the oldest solids in the Solar System - and have helped researchers understand a great deal about early development of solid material and the evolution of planets.

Carbonaceous chondrites are dark in appearance due to their high carbon content which includes graphite, carbonates, and organic compounds. They also contain water and many of the minerals in these rocks show evidence of aqueous alteration - a rare thing in the meteorite world!

There are a nine of different subgroups of carbonaceous chondrites in total but the four most common/unusual are discussed here. Meteorites from the CI chondrite group contain up 22% water and are chemically similar to the Sun's photosphere which makes them the most primitive type of meteorite we know of. It's also estimated that these rocks didn't exceed temperatures of 50°C, suggesting that they formed in a very cool part of the solar nebula.

CV chondrites include the meteorite Allende, from which the oldest CAI has been identified and dated to 4.568 billion years ago, and CM chondrites like the 2021 Winchcombe meteorite contain complex organic compounds as well as pre-solar grains which originated from outside of our Solar System!

In contrast to these stony examples, the CB chondrites are made from up to 75% metal. This Fe-Ni metal exists as large chondrules alongside Mg-rich silicate mineral chondrites, and could be the result of a huge impact in the early Solar System. Although similar to mesosiderites, CB chondrites are kept separate due to their carbonaceous components.

Similar to the ungrouped achondrites, there are also some ungrouped carbonaceous chondrites that don't fit into the established classifications. With new meteorites arriving on Earth all the time, there's always the chance to find something completely new!

As well as all the different groups and subgroups of chondrites, there are also petrological distinctions within each subset that refer to how much metamorphism and alteration a particular meteorite has experienced on it's parent body. These petrologic types are denoted by a number after the classification abbreviation. For example, CM2 means a CM carbonaceous chondrite that is petrologic type 2.

Carbonaceous chondrites are typically Type 1 or Type 2. Petrologic Type 1 chondrites only have a few chondrules and have high concentrations of water and carbon, with evidence of extensive aqueous alteration between 50-150°C (well below the temperatures needed to melt minerals). Similarly, Type 2 chondrites also record aqueous alteration but not all of the olivine and pyroxene will have been affected. This style of alteration occurred at around 20°C, so not even warm enough for a bath!

Type 3-6 chondrites underwent thermal metamorphism rather than aqueous alteration. Type 3 chondrites (further subdivided into 3.0-3.9) are the least metamorphosed and some meteorites are effectively pristine examples of formation conditions throughout the Solar System. Type 4-6 chondrites have undergone increasing levels of heating and thermal metamorphism. Type 4 chondrites show recrystallisation in the matrix between chondrules, Type 5 records the metamorphism of chondrules which start to become harder to identify, and chondrules in Type 6 are largely integrated with the recrystallised matrix.

Chondrites that have experienced the highest degrees of heating are considered to be Type 7. These rocks represent the tipping point between chondrite and primitive achondrite.