The Chondrites

Ordinary | Carbonaceous | Enstatite (E) | Rumuruti (R) | Kakangari (K)

A very rare unequilibrated LL 3.00 ordinary chondrite | NWA 8576

Meteorites come in all shapes, and sizes. Sometimes we discover them after they have been on Earth for hundreds, thousands, or even millions of years. Sometimes in moments of great serendipity, we look up and see them fall out of the sky like a celestial apple fresh from a cosmic tree. 

Recent lidar based studies predict as much as 60 tons or more of extraterrestrial material accretes on to the Earth everyday. Most of it is in the form of tiny dust grains and micrometeorites. However some larger pieces do make it through, and so we have the good fortune of studying these meteorites.

According to Washington University's Department of Earth & Planetary Sciences, "Most stony meteorites (94.1%) are chondrites, and most chondrites (94.3%) are ordinary chondrites. Put another way, 89% of stony meteorites are ordinary chondrites."

This means that the non-chondrites, known as 'achondrites', such as the martian, lunar, aubrite, angrite, eucrite, howardite, diogenite, primitive achondrites, and all the other meteorites I failed to mention, make up only about 5.9% of all known meteorites. Irons, pallasites and mesosiderites add only another 2.2%.

So when we discuss the chondrites, we are discussing the bulk of all the meteorites that have landed on this planet (or at least those we have recovered). However it must be noted, that from a perspective of diversity, the ordinary chondrites are only one small part of a large and diverse landscape of meteorite types, each with different mineralogies, textures, and parent bodies.

CV3 Carbonaceous Chondrite with CAIs | NWA 3118

 

The Chondrites are a diverse group of undifferentiated meteorites that still mostly retain their namesake spherical chondrules. The most common, the ordinary chondrites are comprised of three groups: LL, L, and H. These letter designations indicate fairly straightforward classification concepts. LL indicates the meteorite has a very low relative metal content. L indicates the meteorite has a low relative metal content. H indicates the meteorite has a high relative metal content. These designations are relative to the ordinary chondrites which typically fall into one of these three groups naturally. 

Ordinary chondrite metal content designation system:

LL = Very low metal content
L = Low metal content
H = High metal content

You will also see numbers ranging from 3 to 7 following the letters, examples of this would be; L4 or LL3.15 or H6, etc. These letter/number combinations indicate both the chondrite's metal content as well as its degree of equilibration.

The less equilibrated a chondrite is, the closer it will be to having a designation of 3.00. A completely pristine unequilibrated chondrite with a designation of 3.00, such as the one pictured above, is very rare and only a few are known to exist.

A more common type with many thousands of specimens known to exist, would be something with a 4 through 6 designation, such as L5 or H4. The higher the number gets, the more equilibrated the chondrite is. The scale maxes out at 7. Beyond this point the processing and chondrule encryption is so complete, it would be difficult to distinguish from a primitive achondrite. 

I have been coy about delving into a definition of 'equilibration'. This is because it involves chemistry and I am kind. Fortunately if you have any real background in chemistry, you will immediately grasp the concept of equilibration as it is applied to chondrites.

If you don't have a knack for chemistry, and perhaps even now are starting to drift off, never fear! It is really not too difficult to grasp and will require no actual mathematical computations in order to understand it. However, we must tackle it, even if only briefly and with contained vigor. So please bare with me as we at least sketch out the basics of chemical equilibration.

Equilibration

This is a tough topic to tackle. I'll do my best. In the beginning of the Solar System there was a massive molecular cloud spinning around our newly ignited Sun. This was the very molecular cloud from which the Sun itself was made. This molecular cloud offered up the the ingredients that came together to form the meteoroids, asteroids, comets, lesser planets and planets. These chemically heterogeneous constituents; chondrules, dust, etc... are thought to chemically interact only after coming together physically through the process of accretion.

The constituent chondrules, which make up a large part of the composition of an ordinary chondrite, are thought to have been accreted in such a way that chemically heterogeneous chondrules will randomly seat next to each other. Only after physical contact will the process of equilibration start to seek chemical balance between the previously disperate chondrules.

But wait you say, didn't this accretion process happen billions of years ago? Well, yes it did...very observant of you. So here is the deal, even after physical contact between the constituents has been achieved, equilibration can be greatly accelerated or slowed depending how much heat/pressure is put into the system as well as how reactive any given group of co-located constituent components might be. So two chunks of the same asteroid composed of ordinary chondritic material can have a completely different states of equilibration depending on how close to the sun it may have gotten, if it suffered multiple impacts, did it have a bit more radiogenic nickel, etc...

Author Hypothesis: my best guess is that the 3.00 and lower sub types of ordinary chondrites must have had the benefit of having accreted relatively late with respect to other non-type 3 ordinary chondrites. With less material around to accrete, perhaps these bodies stayed smaller thus avoiding heat from pressure and allowing for exterior temperatures to slow down interior chemical processes. A smaller size and less time in existence may have also helped them to avoid collisions that would have added heat to the system and increased the speed of equilibration. This is just my simple hypothesis, please take it with a heavy dose of skepticism and scrutiny, as it has not in anyway been tested. I sincerely invite those with greater wisdom than I to evaluate and critique this little bit of self-indulgent hubris.  

Type 3

Type 3 is a widely used term in the parlance of meteoritics and indicates an ordinary chondrite with an equilibration in the range of 3.00 to 3.9. In recent years though, the Meteoritical Society's Nomenclature Committee has adopted a policy of classifying anything between 3.3 and 3.9 as simply "3". This means a meteorite that may have previously been classified as say an L3.4 or H3.7 will now simply be classified as L3 and H3. Only if the chondrite has a significantly low state of equilibration, between 3.00 and 3.2 will an ordinary chondrite be awarded the coveted decimal point and numerals to the right.   

This begs the question, is Type 3 now better defined as 3.00 - 3.2, or do we in fact need a new way to refer to and indicate this new distinction in the way classifications are denoted? Perhaps Type 3 can still refer to anything with a 3 in it, and classifications with a decimal and numbers to the right of the 3 can be referred to as,  "Sub Type 3" or maybe "Low Type 3. I leave this up to others.

I don't think the term Type 3 is used often in research papers, but does seem to get used by researchers in conversation, though perhaps not as much as in collector circles. I also here researchers using the term "sub-type" to indicate the numbers to the right of the decimal point, and it seems to now be functionally interchangeable with the term "Type 3" in technical conversation.

Note on Type 3's & Equilibration: the term "unequilibrated" is often used for type 3 chondrites, but a meteorite cannot be considered truly unequilibrated unless it is classified as 3.00. We can take this further, and argue that there would likely be some areas of minor equilibration to be found if a broader sampling were taken of the few ordinary chondrites to ever be classified as 3.00. The point is, that a truly unequilibrated chondrite with a 100% pristinely heterogenous chemical make-up at each chondrule boundary with absolutely no indications of variation from its initial state of accretion, is probably a unicorn.

There is some bit of debate as to the appropriateness of the current system of classification of Ordinary Chondrites when it comes to those less equilibrated Type 3 stones. However, that is another story all together, and I will have to leave you with a...to be continued.

Stay tuned for more to come on: carbonaceous, enstatite, and the elusive R and K chondrites!