Meteorites – A Very Short Guide

Here is a short guide to the main types of meteorites that should help you to pick your way through the minefield of extraterrestrial jargon. I am still working on this section – it will be ready to go very shortly – thank you for your patience.

I have tried to keep technical jargon to a minimum in my book, A Meteorite Killed My Cow. Getting overly obsessive with the buzzwords puts many people off and makes the subject seem dense and unexciting, which it definitely is not. However, it is always useful to have a few definitions so we can all talk from the same page. And it is also important to understand some of the details of meteorite classification to get a better feel for the rich variety of materials that arrive on Earth from space. Meteorites are not all the same, far from it.

And so, with these noble aims in mind, we will first define a few key terms and then look at the main types of meteorites.

A FEW DEFINITIONS

First up, let’s start with the word meteorite itself. Basically, a meteorite is the rock you pick up from the ground, not the bit that flies through the air. To paraphrase one widely used definition of the term “meteorite”: it is a space rock that survives its passage through the atmosphere, and so can be picked up off the ground [1,2]. On the other hand, as it flies through the atmosphere doing its stuff, it is more technically referred to as a meteor [1,2]. Now this can be a bit puzzling, because when most meteorites arrive, the term “meteor” is not usually used to describe their atmospheric flight. Instead, the term most often used is fireball. But it turns out that this is not really as confusing as it might seem. A fireball is just a bright meteor [1]. You kind of know it when you see it. More strictly speaking, a fireball is a meteor that is brighter than any planet or star [1]. The brightness is given a magnitude number, but we won’t worry about that here [1]. But it doesn’t quite end there. Some fireballs are very big and as a result you get a bit more stuff going on, explosions, shock waves, etc. These larger fireballs are sometimes referred to as bolides [1,3]. A bolide is “a very bright meteor which fragments or explodes. Sounds of the explosion can be heard if the observer is close enough” [1]. But it can get even more exciting, because you can also have a superbolide [3]. These are rare, spectacular bolides and have a strict definition in terms of the amount of light given out, which again we don’t really need to worry about. The Tunguska and Chelyabinsk events were examples of superbolides. There probably should be a category called megabolides for the sort of dinosaur‑killing, giant‑sized, objects that have happily only turned up extremely rarely in Earth’s history. But as it’s very unlikely any of us would survive one of them to compare notes afterwards, that’s probably not a useful term.

Asteroids are mentioned a lot in A Meteorite Killed My Cow. These are basically chunks of space rock that hurtle around the Solar System. The term meteoroid has been introduced for mini‑asteroids, which range from the size of a grain of sand up to 1 m in diameter [2,3]. Asteroids, meteoroids, meteors, and meteorites – that’s starting to get a bit confusing. So, let’s think about a small chunk of rock hurtling through space. Away from the Earth, it’s an asteroid (or meteoroid if smaller than 1 m). As it descends through the atmosphere, it’s a meteor (or fireball if bright enough). Finally, when it gets picked up off the ground, it’s a meteorite.

A few other useful terms now. Many incoming space rocks fragment as they reach the lower, denser parts of the atmosphere. When those fragments fall to Earth, they define a strewn field [4]. This is an elliptical area elongated in the flight direction of the meteorite. When you look closely at the broken surface of many meteorites, you may be able to make out millimetre‑sized spherical structures, which are called chondrules [5]. These important objects formed very early in Solar System history [6]. They are widely believed to have started out as clumps of dust that were flash‑heated, and so melted to form hot, free‑floating spheres of liquid rock. They come in an amazing variety of textures and nothing like them has been formed since. Then there are calcium‑aluminium‑rich inclusions, or CAIs for short [6,7]. An awful name for astonishingly important objects. CAIs are rich in the elements calcium and aluminium, as you would expect, but more importantly, they are the oldest Solar System objects so far dated. In fact, CAIs are used to date the formation of our Solar System. The current most reliable date is 4,567 million years ago, which is pretty old [8].

DIFFERENT TYPES OF METEORITES

An important note before we get going. It is much easier to make sense of meteorites if you can see what they look like. A great way to do this is by visiting the Virtual Microscope and taking a look at the wonderful images it contains [9]. The site contains superb, high‑quality, rotatable images of a wide variety meteorite types. Here are the main areas to visit:

This is the Glatton meteorite discussed in Chapter 2 of A Meteorite Killed My Cow. You can see clearly the thin black fusion crust that covers the original outer surface of the specimen. The light coloured material is the interior of the specimen where the fusion crust has been removed. A fully rotatable image of Glatton can be viewed on the Virtual Microscope. Use the zoom function to really look in detail at the interior and exterior structure of the meteorite. It is hours of fun!

British and Irish meteorites This has not only thin‑section images, but also rotating object images.

EUROPLANET meteorites Includes thin‑section images of some important types of achondrites.

Lunar meteorites Just one sample at the moment, but a growing collection.

Martian meteorites Lots of thin‑section images. Check out the rotating Nakhla specimen.

Apollo Lunar Missions A collaboration with NASA. If you want to see Moon rocks, this is the place.

Naturally, any individual meteorite that lands on Earth and is then recovered for scientific study, represents a unique and important sample. But scientists have been studying space rocks for well over 200 years [10], and in that time, it has been recognised that differences and similarities exist between the various samples that we have in our worldwide collection. Finding connections between samples and trying to work out which samples are related to each other is a natural human process. We do it all the time. We try to bring some sort of order to what appears to be a chaotic world. If supermarket shelves were randomly filled with unrelated objects, we would pretty soon start complaining. We expect things to be ordered in some sort of logical way. It’s the same with space rocks. Set out below is a somewhat simplified scheme for dividing up the various types of meteorites. For those who would like a more in‑depth treatment, there are some very authoritative scientific papers [11] and websites [12] that can help.

Stones, stony-irons and irons

Meteorites can be divided up into stones (left), stony-irons (centre) and irons (right) – the stone on the left is the UK Crumlin ordinary chondrite fall (image: Virtual Microscope); the stony-iron is the pallasite Esquel (image: Wikipedia) and the iron on the right is the UK iron fall Rowton (image: Virtual Microscope).

A simple but effective way to divide up meteorites is on the basis of how much metal they contain compared to silicate (rocky) materials. This leads to three main divisions: (1) stones, which are predominantly composed of silicate‑rich minerals, and in some respects, resemble rocks found on the surface of the Earth; (2) stony‑irons, which are, more or less, 50:50 mixtures of silicate‑rich minerals and nickel‑bearing metal; and (3) irons, which are essentially composed of nickel‑bearing metal, but also contain various other minor minerals.

Stones – the importance of chondrules

In terms of number of samples, stones are by far the most important type of meteorite. But here is the catch! The fact that meteorites are made up predominantly of rocky material is not really very helpful when trying to understand the origin of these samples. It turns out chondrules are the key. As we discussed in Chapter 2 of A Meteorite Killed My Cow, these small silicate spheres, generally no more than a few millimetres in diameter, likely formed in the first few million years of Solar System history. They are usually easy to spot so that helps us to divide up the stony meteorite world into two major types. If a meteorite has chondrules, it is called a “chondrite”, and if it lacks chondrules, it is an “achondrite”. Phew! We have made a start. Let’s look at chondrites first.

Chondrites

Ok, so you might think that’s it. It’s got chondrules, so it is a chondrite. It turns out that’s not a bad way to look at things. But there are some complexities. Aren’t there always!

Most chondrites are stuffed full of chondrules. In fact, some types have almost nothing else but chondrules. However, there are other chondrites in which chondrules are much rarer and are separated from each other by fine‑grained material. We call that fine‑grained stuff, matrix. Some of these matrix‑rich chondrites are super dark and there’s a reason for that: they have a high content of carbon. And there are other differences. When you rotate the broken surface of a fresh chondrite, it will often glint. That is because it contains nickel‑bearing metal. The amount of metal in a chondrite also helps to define which particular group a chondritic meteorite falls into.

   So, you can see that chondrites have a range of features that can be used to subdivide them. This is not an exact science and there are sometimes disagreements about what features are the most important. But broadly, chondrites are divided into three major types (often called classes), two further minor types, and then a bunch of samples that don’t seem to fit in anywhere (they are called ungrouped). Major types (classes) of chondrites:

  1. Ordinary chondrites
  2. Carbonaceous chondrites
  3. Enstatite chondrites

Minor types: Rumututi‑type (R) chondrites and Kakangari‑type (K) chondrites and ungrouped chondrites.

We could look in great detail at the important features of each of these different types of chondrites. It is a fascinating subject, but there is a lot of information already out there [6,7]. The main features of each of the three main types of chondrites are set out below.

Ordinary Chondrites

First of all, this is not a very nice name. They are called “ordinary” not because they are boring, but because they are the most common type of meteorite that falls to Earth. At the time of writing (February 2024), the Meteoritical Bulletin Database [8] lists the total number of classified and officially approved meteorite falls as being 1,238. Of this number, 968 (78.2%) are ordinary chondrites. This compares with only 52 (4.2%) carbonaceous chondrite falls, and 17 (1.4%) enstatite chondrite falls. All other meteorite falls total 201 (16.2%). These numbers clearly show how dominant ordinary chondrites are in our meteorite collections. If finds are included, the dominance of ordinary chondrites increases slightly, such that they represent 84.5% of all meteorites that have ever been collected.

Characteristically ordinary chondrites are densely packed with chondrules. They contain very little fine‑grained matrix and almost no calcium‑aluminium‑ rich inclusions. They also have variable content of metal. Now we don’t really need to go into lots of detail on this, but the amount of iron they contain helps to subdivide the ordinary chondrites. Three groups are recognised: H group, which stands for high iron, L which stands for low iron, and LL which stands for low metal and low iron.

Carbonaceous Chondrites

This grouping covers a very diverse range of meteorites. You might think from the name they all contain a lot of carbon. Most do, but some don’t. Not much of a help then. They contain a lower fraction of chondrules than ordinary chondrites, and a lot more of that fine‑grained stuff we call matrix. Many of them contain abundant CAIs. There are some fairly coarse‑grained types with big CAIs, such as Allende (Chapter 11), and others which are highly altered, such as the CIs we discussed in Chapter 14. You are probably wondering how these meteorites are actually classified. Well, you have to use a range of characteristics, not just one. Their chemistry is important. They have a lot of calcium and aluminium compared to silicon. Their oxygen isotopes are also diagnostic. Eight main groups are recognized (CV, CR, CH, CI, CO, CB, CM, and CK), and there are also some smaller groupings that we don’t need to worry about.

Enstatite Chondrites

In a similar way to ordinary chondrites, enstatite chondrites often have lots of chondrules and can also have a lot of metal. The mineralogy of enstatite chondrites is important in classifying them; they are dominated by a type of mineral known as low‑calcium pyroxene, rather than olivine, as is found in ordinary chondrites. Once again, their oxygen isotope compositions are also a big help.

Achondrites

As their name implies, these meteorites do not contain chondrules. But apart from that, most of the different groups in this category have little in common with each other. Achondrites include planetary types derived from the Moon (Chapters 12 and 13), Mars (Chapters 12 and 15), and Vesta (Chapter 7), the second largest asteroid in the asteroid belt. There is an enigmatic group of very ancient achondrites that shares many characteristics with the planetary achondrites, except no one knows where they are from. These are the angrites. There is a group of achondrites that seem to be related to the enstatite chondrites. These are the aubrites. A number of achondrite groups share some chemical characteristics with the chondrites and are sometimes referred to as primitive achondrites. Again, these are a very diverse group and include brachinites, acapulcoite‑lodranites, winonaites, and ureilites. There are also an increasing number of so‑called ungrouped achondrites. These are an area of active research because they may sample long destroyed parent asteroids that formed very early in Solar System history.

stony‑irons

There are only two main types of stony‑irons: pallasites and mesosiderites. Mesosiderites are a mix of crustal rocks that resemble the HED meteorites with metal‑rich material (Chapter 8). Pallasites consist of a mix of coarse‑grained olivine and metal.

irons

Irons are classified based on the variation of a number of key elements, including nickel, gallium, iridium, and germanium (Chapter 5). Using this data, irons are divided into 12–14 principal groups. There are also a lot of ungrouped irons. It is estimated that iron meteorites are likely derived from about sixty parent asteroids.

NOTES