Meteorite Monday: Or not…

I’ll admit, I kinda let the ball drop on this week’s Meteorite Monday. Normally, I try to get this post done a few days ahead of time, but this weekend has been filled with work, chemistry, petrology readings and completing scholarship essays so I can get some research money.

However, I do have a neat post planned that talks about a recent paper that was published by some researchers at MIT. In the paper, “Chondrites as samples of differentiated planetesimals”, Elkins-Tanton et al., postulate that carbonaceous chondrites could have originated from asteroids that experienced differentiation. This challenges the current paradigm that such chondrites actually come from bodies that didn’t grow large enough to develop a core and mantle. Some have speculated that if true, this could change the way we view the formation of the planets. It’s a good read and if you have time, I’d recommend giving it a look over.

The Allende Meteorite- One of the meteorites used in the Elkins-Tanton et al., study.

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Meteorite Monday: Carbonaceous Chondrites

This post marks the first of, what I’m hoping to be many, a weekly series about various meteorites. I love space rocks as much as I love their terrestrial cousins because they recount the history of our solar system. In the simplest of terms meteorites are basically chunks of natural space debris (i.e., not from man made objects) that survived their descent through the earths atmosphere and the resulting collision. These objects come primarily from asteroids such as Vesta 4, but a few have been found of lunar and martian origin.

Allende, Mexico Carbonaceous Chondrite

Allende, Mexico Carbonaceous Chondrite

A carbonaceous chondrite is basically a big ball of space mud  that can contain up to 20% water (I’m not remembering where I read that, so that number may change when I find the source). They are fairly soft and don’t survive their sojourn on the earth to well. These chondrites also represent the most primitive matter drifting through the solar system and have undergone the least amount of  chemical and physical change when compared to ordinary chondrites. It’s estimated that they represent roughly 5% of the meteorites that are observed and collected upon entry to the atmosphere (1).

The Orgueil Meteorite

The Orgueil Meteorite from the Hoover study

As of last month a carbonaceous chondrite found itself in the center of a controversy when a NASA astrobiologist, Richard Hoover, declared that he found fossilized bacteria in a specimen discovered in the late 1800’s. I won’t go into his claims here because this isn’t  a biology blog and I don’t have the expertise to handle such a topic. If you’re really interested you can click here and read the abstract. Also a quick google search will bring up all the arguments for and against Hoover’s claims.

  1. Bischoff, A.; Geiger, T. (1995). “Meteorites for the Sahara: Find locations, shock classification, degree of weathering and pairing”. Meteoritics 30 (1): 113–122.

It’s done!

*There are some edits in this post. My values for enstatite, wollostanite and ferrosilite were calculated incorrectly. New ternary diagrams were generated to reflect the update along with the new values.*

With finals coming up in the next few days, I really shouldn’t have put so much time into processing my meteorite data and making the ternary diagrams. Was it a complete waste of time? Not really. But I definitely could have been doing something school related. Oh well.

As I noted in the last post, I got to do a meteorite analysis with an electron microprobe last summer. It was a fun project, but it left me with a huge Excel spreadsheet of atomic weights, oxide weights and a host of other numbers and abbreviations that I didn’t understand. However, all that changed with my mineralogy course.  After processing EMP data for the course lab,  I did the same for my space rock data and created these spiffy ternary diagrams with Mystat.

 

 

A ternary diagram basically shows the proportion of elements in a certain mineral. Each mineral has a basic structure, such as Si2O6 for pyroxene, but they also incorporate a certain amount of magnesium, calcium and iron into their crystal structure. A ternary diagram allows you to display the quantity of each element and determine which kind of species of mineral you’re working with.

The first diagram displays the proportion of magnesium to calcium to iron in a pyroxene crystal. These values are also known as enstatite, wollastinite and ferrosilite, respectively. In the case of my little space rock, the dot on the graph tells us that it’s primarily a magnesium heavy pyroxene, at around 90% ( 78%).

Enstate/Ferrosilite/Wollostanite value for the pyroxene in the meteorite sample

The second graph displays the magnesium and iron ratio in the olivine crystals. The magnesium value is known as forsterite (Fo) and the iron value is known as fayalite (Fa). This graph is a little wonky because technically I should have three minerals and not two. But I decided to run with the diagram anyways and label the z-axis as C. It still shows what my spread sheet calculated: that the olivines are pretty magnesium rich, but still contain a fair amount of iron as well.

Eventually I’ll get some pictures of the meteorite posted as well.

A meteorite by any other name…

I’m finally done with my meteorite analysis and write-up. What’s next? Blog it of course!

So, the whole point of my analysis was to determine the metamorphic grade of the meteorite. I was going to do this by using an electron microprobe to measure the calcium content of the pyroxene crystals. While the premise was simple, we had a rather complex problem. The meteorite was so shocked (probably between S4-S5) that the fracture of olivine crystals nearly mimicked the right angle cleavage of the pyroxenes. So, trying to tell the difference between the two was tricky. We had 43 points to scan and we hoped that about 15-20 would come back as pyroxenes. Here is where things get interesting.

Our meteorite should have had a composition that was roughly 60% olivine and 40% pyroxene. However, of the 43 points shot, only two came back as pyroxenes. And to top it off, the Ca content of the two was at just under one percent. This presents us with a rather vexing issue. What happened to all the pyroxene? Statistically we should have hit more than 2 crystals. Was it really bad luck on our part or are we dealing with a sample that has super-low pyroxene content? My two advisors that are assisting me with this project aren’t really sure. Either it’s an L6 chondrite or we have something really unique. And we won’t know until we get the thing on a Scanning Electron Microscope. So, until we can get a proper classification, the metamorphic analysis is on hold. I’m gonna keep my finger crossed that we have something more than an L6 on our hands.

Carbonaceous Chondrite

Carbonaceous Chondrite

Carbonaceous chondrites are the rarest of all the meteorite specimens. Like most meteorites, there are composed of a matrix and chondrules. The matrix is basically the body of the meteorite.  In the case of the carbonaceous chondrite, it’s composed of soft minerals very similar to serpentine or montmorillonite (John A. Wood, The Solar System, 1979). Due to it’s composition, very few of these meteorites survive the entry into the earth’s atmosphere. Those that do face further weathering damage at the surface of the earth.

The chondrules are the rounded minerals that are studded into the matrix. They are mostly composed of olivine and orthopyroxene and are generally rounded in shape (like the picture shows). However, not all CC’s have chondrules. Some have these irregular inclusions that are composed of uncommon minerals such as spinel and grossular. These minerals have been enriched in “calcium, magnesium, aluminium and titanium relative to silicon” (Wood, 1979).

Sources:

John A. Wood, The Solar System, 1979

Carbonaceous Chondrite Meteorites http://www.astro.washington.edu/courses/labs/clearinghouse/labs/Meteors/meteors.html

Space Rocks

This summer I get to do my very first independent research project. I’ll be helping one of my geology professors finish classifying a meteorite with the use of an electron microprobe. This is my first time doing such a project and as such I have a lot to learn. But that’s the exciting part of doing science: the continual learning process. So, as such I figured it was time for me to learn about meteorites. Or meteors. Or meteoroids. Each name means something different. And according to my instructor, the classification has been changing lately. So, in my efforts to learn the basics about these ancient pieces of space debris, I will be posting what I’ve learned in my blog. To start, those confusing names.

It turns out the terms meteor, meteorite and meteoroide are not interchangeable. They seem to refer to the phases of change space debris goes through as it enters the earth’s atmosphere. This rock can come from the moon, comets, asteroids or even other rocky planets (most notably Alan Hills 84001 from Mars- that deserves a post of it’s own). Some of it can be left over material from the formation of the solar system, almost 4.5 billions years old. While their origins differ they pretty much go through the same process upon encountering the earth’s atmosphere.

The difference between the three names used to be simple. The flash of light produced by the entering debris was a meteor. Any chunks of rock that broke off were the meteoroids. And any piece that didn’t disintegrate in the earth’s atmosphere and made it’s way to the surface was a meteorite.

However, a paper recently published in Meteoritics & Planetary Science by Alan E. Rubin and Jeffrey N. Grossman proposed a complete overhaul of the definitions. They suggest that a meteoroid becomes a “10 micrometer to 1 meter sized natural object traveling through interplanetary space”. A meteorite is a natural object that is larger than 10 micrometers whose parent was any rocky celestial body. The meteorite had to travel under it’s own natural means with enough velocity to escape the gravitational pull of its parent body. It then has to hit something that is larger than itself, natural or artificial, and survive the impact. What is most interesting is that the meteorite doesn’t have to hit a foreign object. If it hits the surface of it’s parent body, it’s still considered a meteorite*.

So, those are the differences between the terms. I think in the next post about meteorites, I’ll cover the classification system. And as a side note, this is my first time writing about anything of this nature. In the very unlikely chance that someone from academia (or anyone at all) reads this, please be kind with criticism. I’ll happily accept feedback if done in a professional manner.