Tuesday, May 29, 2012

Meteorwrong? No it's a Meteorite for once!

One of the nicest services we (the Mineral Sciences staff) provide is trying to identify random mystery rocks that people bring to the museum.  Nine times out of ten the people are hoping they have found a meteorite.  They usually haven't. In the 34 years Tony has worked here he estimates only about 4 real meteorites have walked through our doors.  This is not counting a few cases where meteorites were purchased and the buyer asked us for verification.  Usually people bring in nodules of magnetite or hematite, they "pass all the tests", they attract a magnet, they are black, they are dense...but they are boring old earth rocks.  We also get pieces of slag (mining industry byproducts) that can be very weird looking and metallic but none the less are also from this planet.  So many meteorwrongs!

If you think you might have found a meteorite here is a great checklist for self-evaluating your mystery rock:  http://meteorites.wustl.edu/realities.htm
Sorry, the truth hurts.

I wish they were really meteorites, believe me it is much more fun and exciting to get a real one!

Each new meteorite found is another piece of the puzzle of trying to figure out how our solar system, our planet (and eventually us) got from a cloud of gas and dust to what we are today.

So anyway, a couple days ago a man came in with some ugly rocks he found in storage.  They weren't labelled but someone had carefully wrapped them and packed them away.  They were covered with rust and sorta look like maybe they could be meteorites...or not.  Normally we can tell right away just by looking at a rock that it cannot be a meteorite.  We will not do further testing since it is time consuming and expensive.  In this case we decided to do more tests.

We broke a small piece off one of the rocks.  The entire surface was rusty - oxidized...a mix of magnetite and hematite (iron oxides).  Those minerals are not found in meteorites originally but as meteorites sit on earth exposed to air and water, the iron-nickel metal they contain can turn into to iron oxides.   Using our small file I was unable to get through the coating to expose a fresh surface.  Tony spotted some well-formed colorless crystals and was able to pluck them out. They looked like quartz and if so this rock can NOT be a meteorite, there is no quartz in iron-rich meteorites.   X-ray diffraction analysis revealed they were feldspar group minerals...inconclusive.  Finally, he used the EDS (energy-dispersive spectrometer) attachment on the museum's scanning electron microscope to check the iron oxides for nickel.  Metal in meteorites always contains some nickel.  As the iron in the metal oxidizes, the nickel does not, and it can't go anywhere, so you end up with rust that contains a little bit of nickel.  You don't find nickel in rust from earth metal (almost all earth's nickel is in our planet's core), so nickel would be excellent evidence that the rock is not from earth.

A tiny fragment of rock is placed in the EDS and hit with a high energy electron beam.  The electron beam interacts with the atoms in the minerals.  If you remember some chemistry atoms contain a nucleus of protons and neutrons orbited by a cloud of electrons in discrete "shells".  The beam ejects electrons from inner shells and electrons from outer shells  fall in to fill the hole.  The excess energy from these falls is emitted as photons.  The photons are collected by a detector in the EDS and their energy is measured.  This energy is different for different atoms so the result is an elemental composition of the material analyzed.  Neat, huh?

Here are the results, the fragment of mineral in the EDS contains oxygen (O), iron (Fe), just a tiny bit of sulfur (S) and nickel (Ni), yay!

Now our work is finished.  EDS can tell us this is a meteorite but it is not a tool that can be used for further study.  Most likely this is a well-known meteorite, it has already been discovered, other pieces are already in museums and for sale on the internet.  I can't be certain what type of meteorite it is as I can't even make a polished slice, but it looks like an ordinary chondrite, which almost 90% of meteorites are.  It is up to the owner to figure out the meteorite's name or try to find someone to study it or to buy it (or both).  There is still work to be done to figure out how rare and valuable this particular meteorite will turn out to be.

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