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Differentiation: chondrites & achondrites

Posted by Dustin Dickens on


When I first started learning about meteorites, I seemed to always be getting the terms "equilibrated" and "differentiated" mixed-up. Both terms can seem to describe a similar process at first glance, even while they are really quite different. If you've ever found it difficult to keep these terms straight, or just want to learn a little something about planetary science, this is pretty good place to start.

A good understanding of what differentiation actually is, will likely resolve the confusion. I'll get into the term 'equilibrated' in another post that tackles 'chondrites'. For this post though, we'll stick to the process of differentiation.

Keeping it simple, differentiation, also known as 'planetary differentiation' is the process of an asteroid accreting (growing) to the point that most of the metal distributed throughout its mass is pulled by gravity to its center to form a 'core'. This leaves the lighter rocky material to float above the iron-nickel core forming a 'mantle'. This little bit of information is a keyhole that allows us to glimpse just how intertwined planetary science and meteorites truly are.

So, what is a meteorite really? A meteorite is just a piece of an asteroid or planet that strikes another asteroid or planet, often ejecting bits of what it hit out into space. In a seemingly never ending cycle of cause-and-effect, one meteorite begets another meteorite and so on and so forth. This means a meteorite will either be ejected from a larger differentiated parent body or a smaller undifferentiated parent body. 

Leaving out massive collisions that utterly destroy one or both of the bodies involved in a cosmic smashup, a meteorite will generally originate from the surface of an asteroid or planet it's ejected from. If the metal has been gravitationally pulled to the center to form a core, there will logically not be much metal in a meteorite originating from its surface. 

Of all the types of meteorites known to us here on Earth, the most commonly found are known as 'chondrites'. There are many different kinds of chondrites, but the one thing nearly all of them share in common is something known as a 'chondrule'. All chondrites are essentially made of chondrules stuck together during the process of accretion and are considered to be undifferentiated. This is because their parent bodies have not undergone the process where metal separates (or differentiates) from silicates to form a core and mantle, a process that also destroys (or encrypts) the chondrules. 


Any meteorite that is not a chondrite is technically an 'achondrite'. The prefix "a" will typically indicate "without" in scientific terms. So a meteorite with the designation of 'achondrite' signifies a meteorite that has no chondrules. This is important to note when trying to get a handle on differentiation because it is this very process of differentiation that creates the achondrites. And conversely if an asteroid never gets big enough to undergo the process of differentiation, it remains chondritic. 

If we accept the idea that the entire Solar System was created or 'accreted' from the same giant molecular cloud of gas and dust particles, then we can make the leap to say that until any given accreted body in our Solar System grows large enough to undergo differentiation, it remains a potential chondritic meteorite parent body. When one of these same chondritic bodies in our Solar System grows large enough to undergo the process of differentiation, it can no longer produce chondritic meteorites, transforming instead into a potential achondrite parent body.

This is an extremely simplistic way of looking at planetary differentiation that does not take volatiles and gasses into account. It's also a very meteorite-centric way of looking at the Solar System and it's otherwise very diverse group of asteroids and planets. However, it is essentially a valid and useful distinction. 

When ejecta is created by an impact event, some of which is ultimately destined to become a meteorite when it hits the Earth, if it is ejected from the surface of a smaller asteroid that hasn't undergone differentiation, it will be a chondrite. This means that as a rule all chondrites are undifferentiated because their parent bodies have not undergone the process of differentiation and their chondrules remain intact.

When ejecta from the surface of a larger asteroid or planet that has undergone differentiation hits the Earth, it will have had most if its metal removed and chondrules eliminated, making it an achondrite.

This, in a very simplified nutshell, is the process of differentiation and how it relates to the distinction between chondritic and achondritic meteorites.




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  • After the initial differentiation processes have fully encrypted the chondritic material and a core, mantle, and sometimes crust have been differentiated from the relatively homogeneous chondritic precursor materials, it can be said that the differentiation process is complete.

    We would then begin to discuss further processing of the already differentiated asteroidal or planetary material as undergoing active or past geologic processing via magmatic / tectonic mechanisms within the mantle and crust of the respective parent body. It might help to think of it as a butterfly. The early chondritic precursors accrete to form the ‘pupa’ asteroids which grow and grow as chondrite pupa until they reach a certain stage and then they transform into achondrite butterflies. There are some one-time processes that occur to transform the pupa into the butterfly, but after the butterfly (achondrite parent body) formation is complete, it does’s just remain static. It changes and goes through all of its various butterfly processes, but it will never revert to a chondrite pupa again. Unless of course it is cataclysmically destroyed and its particles are cast by vast shock waves into the mix of another molecular cloud waiting for gravity and chance to bring together a new star system, and maybe a home for future beings.

    Dustin Dickens on
  • Wonderful facts that open a discussion of a further process of differentiation from achondrite. Mr Dickens does this cycle repeat within meteorites?

    Benjamin Roy Shideler on
  • Thanks so much for the “Meteorites 101” explanation of these concepts. It has cleared up several questions I have including the ‘High’ or ‘Low’ metal content of chondrites and almost no metal content of HED-class and achondrites in general. I really enjoy these simple lessons as a beginning meteorite student!

    Martin Lollar on

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