Local Residents

How do you know it’s a meteorite?

I’ve been looking at these things for so long, it is hard to remember the time I was asking the same question.

For now, let’s stick with Brenham Pallasite Meteorites.

OK, you located a place where iron or stony-iron meteorites have been found, you got a signal with the metal detector, you start digging and it’s not the usual piece of trash. But to most people it just looks like a rock with some dirt stuck to it:

Must have metal in it, right? You can test the metal and it contains Nickel, rare in Earth rock, common in meteorites.

You cut the specimen and see what it looks like inside (need to use a diamond or CBN saw because it is so hard):

OK, that’s different; the rock has shinny metal in it and some other material. A magnet sticks to it-this rules out lead, aluminum was never an issue because the material is very heavy, it rusts, all roads lead to iron. The metal is a mixture of mostly iron, some nickel and many trace elements (need a special laboratory to determine this, a good university can help).

A rock with iron, ever seen this before? If so, it was probably a meteorite. Many billions of years ago Earth rock had iron in it but after the action of oxygen and water it has been converted to iron oxide. Conversion of iron oxide (hematite, magnetite) back to iron is accomplished using a blast furnace at a foundry.

Anything unusual about the metal? If you etch the metal phase you get this:

It’s like developing a negative to get a photograph. The lines in the triangular region are called the Widmanstatten Pattern. It is a crystalline phase of iron and nickel that took a million years to cool only 3 degrees in temperature-you don’t find this in man made metal because we don’t have any foundries that have been in business for millions of years. In the upper left corner is a band of swathing (means surrounding) kamacite. The shiny veins are plessite, a granular form of the iron-nickel.

What’s the other stuff? Turns out to be olivine, a type of rock that is found on Earth. The more crystalline form is transparent and is know as peridot and is used to make jewelry. Some Brenham has green peridot crystals:

So, that’s how we know it is a meteorite. The knowledge gained from countless scientists over the last few hundred years allows us to look at this material and understand what it truly is, a meteorite from the asteroid belt.

I used to hang around Field Museum in Chicago, meteorite section, for hours, days, taking pictures of each specimen and learning as much as I could. One day a group came into the meteorite hall, looked at one of the larger iron specimens for a few minutes, and one member loudly proclaimed “it’s just a rock” and the group walked off. That’s when it hit me, without knowledge, it is just a rock, something to fashion into a tool or jewelry or throw at something. But our understanding allows the mind’s eye to see beyond the superficial to reveal the hidden beauty, a rare and wondrous object from outer space.