An Article In Meteorite Times Magazine
by Jim Tobin, Editor
Introduction to Meteorite Classification
Coming in the next issue will be a feature structured along the lines of Meteorites 101. Sort of like a beginnerís course in meteoritics. In the same spirit my article this month will be; I hope a primer for that.
Several years back we obtained a meteorite at the Tucson show to cut and sell. It was an unclassified chondrite with nice fusion crust and was pretty large at 6 kilograms. We had to find someone to characterize it. I thought Maybe if we made it as convenient as possible a busy researcher might be more receptive to doing the work. So I sent a sample off to have some thin sections for a microprobe made. We knew it was a pretty interesting stone after seeing the inside when I cut it open. So we had some thin sections made to sell as well. As it turned out months were required to make the process work and I had plenty of time to examine the thin sections and look at the stone. It became a time of learning for me as I put some of the factual book data to practical use in my own homespun analysis.
For ordinary chondrites there are three basic types, H, L and LL. I wanted to know what we had and was getting impatient as the months went by. In the past it was the free metal that was used to make the determination of class. Today, with modern analytical equipment it is a measure of things like the percentage of iron in Fayalite mineral crystals that determines the class. But, I was limited to an old style free metal analysis.
Here is the way it breaks down for the three groups of chondrites. H chondrites, have the highest free metal and that is what the H stands for. The range of free metallic iron for H types is 15-19%. The low metal L type chondrites have a range of free iron from 4-10%. The LL type which stands for low metallic and low mineral iron have a range of free metal between only 1-3%. I did my analysis by weighing out the separated metallic iron from a sample I had crushed, and doing the arithmetic to arrive at the percentage. Since it came out 4.8 percent I would have had an easy answer if this had been a hundred years ago. It would clearly have been an L type.
As I was doing this I spent a great amount of time looking at the slices in my collection and there was the rub. I had Lís that had a lot of metal showing and I had Hís that looked like they had about the same as some Lís. What I finally determined was this and it is not very scientific. The Lís had metal sometimes quite a bit. But it often was in larger and fewer grains. The Hís on the other hand had smaller and more numerous grains. Some of the Lís were responsive to a magnet on one spot more than another, and were overall much less responsive then Hís. Hís on the other hand were generally very responsive to a magnetís effects
The characterization of a chondrite involves more than just the amount of metal the meteorite contains. It also requires the determination of shock stage and the petrologic type. The former of these I was less than confident of my ability to determine, but with a whole bunch of slices at my disposal for months I was more confident about determining the petrologic type. It was a type 6 as it turned out, a very common type. Type six means that the chondrules are in bad shape and are hard to make out. In the case of our meteorite the chondrules were little more than loose collections of mineral crystal grains clumped in the same general area. The area though was still more or less a circular space. But, occasionally there would be one lone better defined chondrule. This can be seen in these photographs of Sahara 99676.
As can be seen from the following pictures of
chondrules found in slides of Cole Creek which is only one type less
metamorphosed as a type 5. The chondrules are in much better shape and the matrix
rock is much less recrystallized and still distinct from the chondrule itself.
Ordinary chondrites are almost always classified today with a shock level designation also. This was the hardest for me. I knew some areas of the meteorite were very shocked. There was a thin zone showing compression and long parallel black shock veins. In fact the meteorite had been reassembled from several pieces and it had cleaved along one of these black lines producing an almost perfectly flat surface. But, the vast majority of the mass was more or less lacking in naked eye shock indicators. It would be a microscopic examination of the same slides to make a guess.
This is a picture of one large chondrule made of nearly a single grain of mineral. It displays cracks which are indicators of shock.
Sahara 99676 was characterized officially as an L6-S2-W2. I had a lot of fun experimenting with this meteorite. And I learned a great deal as well.
Once you move from ordinary chondrites to carbonaceous and enstatite chondrites and R chondrites, it gets more complicated. However, some of the principles remain constant. Recognition of the type is something that can be learned over time. It is possible to make a close guess in many cases from a good slide. With even more unclassified meteorites available at the present time having them in our collections is a very attractive idea. Especially since they are generally reasonably priced. With time and practice and the cost of slides it is possible to know pretty well what some of these nameless, locationless meteorites really are if you have a microscope.