An Article In Meteorite Times Magazine
by Jim Tobin

 

Carbonaceous Chondrites

Over the next few months there will be a lot of information and articles on the Moss Meteorite. But, I thought amongst other things this month that I would share a few thoughts and a couple photographs of Moss. On July 14 over Norway a meteoroid fragmented and as of now about 3 kgs have been recovered. It is being referred to as a CO type carbonaceous chondrite in most of the news articles. And I would agree from its appearance and description that carbonaceous is likely. At this point in my article I had to make some changes at the last moment. We have through the efforts of Mike Farmer who provided the material and Jeff Grossman who did the classification gotten preliminary results of what it is. It is a CO3. Probably a petrologic type 3.5. It is a shock stage S2 or S3. I will now take out a sentence or two about how it would not be prudent of me to make definite statements without some science being done. On with the article. The chondrules in my particular piece are small and they are well shaped several appear as smooth round domes protruding from the matrix on broken surfaces of the fragment. So I would be inclined to go with the grade 3 and avoid the CK type which are often grade 4-6 with less distinct chondrules. But, the color and texture of the matrix is sure reminiscent of Karoonda. I am (like everyone else) eagerly awaiting the results of the scientists about its classification. Actually I am not waiting so eagerly now.

    

Falls of carbonaceous chondrties are always exciting moments for meteorite lovers. They are among the most interesting and scientifically valuable meteorites. I remember when Allende fell in Mexico. I did not get a piece at that time, but should have. This time I got a couple pieces right away. Unlike Allende where there was a lot of material, this time I think there is never going to be much of Moss available to collectors. More may be found. I would be a little surprised if after months or years a few more pieces have not been added to the total known weight. But, it is going to remain I think a low total weight fall which to many collectors is just what they love.

Carbonaceous Chondrites have not been ones that I personally seek to collect. I am much more interested in irons and LL chondrites. But, of course I have a few carbonaceous meteorites. Many years ago I found a dealer selling some badly broken Allende individuals for a very good price and I got some. This was the material I learned to make thin sections with. It has a soft matrix it was easy to screw up by removing too much. That was good from a learning point of view. It taught me early on to be careful and go slowly at the end of the grinding.  And Allende gives a great reward in polarized light when the slide is made right. I basically ground away probably 30 grams of Allende learning to make thin sections, but it was worth it. I can now make slides of my NWA and other meteorites anytime with great results. I can make close estimations of what type meteorites they are; without sending them out. I still have the early experimental Allende slides. And some of them are really poorly made. But soon the quality got much better. All the ones made now have large surface areas and are flat and thin. I learned really fast that the surfaces need to be nearly polished if the slide is to work well. A rough ground slice is pretty useless and hard to get a good photo from.

One thing that can be said for carbonaceous chondrites of petrologic type 3 is that they will have great chondrules in them. Here are some picture I took from those early Allende thin sections I made. Some of these chondrule pictures have been published before but these are all new shots taken for this article. Seems like I see new things every time I view the thin sections. You can maybe understand from these pictures why I was immediately fascinated by meteorites in polarized light. Enjoy, till next month.