It has been a while since I wrote an article focused on my favorite place; Meteor Crater. Over the last fifty years, I have added to my collection many specimens of meteorites, rocks, and ephemera from the crater. But, one of the items that I had just a single representative specimen of was the graphite nodules. Recently I was given as a gift a large nodule in as found condition and a somewhat smaller nice specimen cut in half. Suddenly I had something new to play with, rework, and photograph.
The large nodule specimen weighs 192.3 grams and has a nice orange exterior. If it was not a perfect half-sphere I would think from the color that it was an as found meteorite fragment.
The smaller cut nodule is 110 grams forming identical halves. Here is an image of how the halves looked when I received them. Later is an image of the faces after I relapped and polished them.
Unlike some other common iron meteorites which are almost inclusion-free, Canyon Diablo meteorites can have considerable amounts of inclusions along with the solid iron metal. Graphite nodules are often seen in slices where they have been exposed by the cutting. Some of the graphite nodules found around the crater are ones that came out during the flight of the asteroid through the air. Others likely weathered out of the masses of iron over the nearly 50,000 years since the asteroid crashed. Some of the deep round hollows seen on larger masses of Canyon Diablo meteorites may once have been locations filled by graphite nodules. Certainly, not all the concave features seen on Canyon Diablos had graphite nodules. It appears that the creation of concaved depressions is part of the weathering process as the layers of rusted iron exfoliate off the larger masses. Even many smaller masses are sculptured beautifully into spiny and spikey sharp-edged fragments where concave curves intersect.
When I began researching this article I believed there was a consense on the formation mechanism for the metal veins in these large graphite nodules but it would appear from what I read that no consense exists. I still personally like the theory that the metal was injected as a liquid into the graphite during collision events in space ages before it struck Arizona. But there are other ideas about the creation of the spiderweb veining. Vapor deposition at a much lower temperature than in the injected metal theory being one of the newer candidate theories. I find myself looking at the spiderwebbing and seeing a similarity to the spiderweb shock veins seen in some chondrites. Maybe that is not a valid comparison but it is where my mind goes. Below are images of two chondrites with spiderwebs of shock and melt. I could have used Chelyabinsk or several other more recent falls that have melt veins and melt pockets but these will let the reader see meteorites not show as often.
Graphite is carbon the same mineral as that used for making pencils. It is soft and would not be expected to survive for thousands of years of exposure to the widely varying weather at Meteor Crater. However, it was found by analyzing a very large one found southwest of the crater that the graphite nodule was covered with a layer of nickel-rich taenite which provided a corrosion-resistant protective surface.
I suppose there are loose graphite nodules found at other meteorite locations however, we seldom hear of them or see them for sale. I know that Nantan graphite nodules exist and graphite is found as small inclusions in many iron meteorites besides Canyon Diablo. But there is something uniquely cool about a large Canyon Diablo graphite nodule with its spiderweb veins of metal.
One might expect that graphite nodules would cut like butter with a diamond saw and most of the time that surely is the case. However, it is near the graphite that the aggregates of diamonds are sometimes found in Canyon Diablos. And as already discussed the large and the small nodules seem to have different histories. Only meteorite fragments found really near the slope of the crater seem to have diamonds. They are the fragments that suffered the most effects of the heat and pressure of the Meteor Crater impact. The large masses found out on the surrounding plain for many miles often fell as individuals escaping serious damage from the underground explosion of the main asteroid. So my first thought would be no diamonds in graphite nodules recovered at a much greater distance. Still with that said, I got a phone call from a meteorite dealer friend, who might get a kick out of this if he reads it. I answered the phone and said hello. He said, “How would you like to wreck every piece of lapidary equipment in your lab in the first few seconds that you work on a stone.” I replied, “That would not be high on my list of things to do.” As we chatted longer he told me about a graphite nodule that had already wiped out a lot of lapidary equipment. It destroyed a brand new expensive diamond and metal band saw blade. The slices were finally cut if I remember correctly using a batch of silicon carbide discs resulting in rough surfaces. After being cut with such great difficulty one of the faces was put on a diamond lapping disc and it tore the diamond bort right off in about two seconds. That was it for that disc. A couple more discs were destroyed following the first. The owners of the material wanted to get the slices smooth and polished. I was pretty sure after hearing the story that there was no way that I could do anything better with my mechanical equipment. I suggested to my friend that it might be possible to grind and polish down the nodule material by using loose diamond powder with water on a marble or granite slab or even a thick piece of glass. The diamond powder would not be gone in an instant as dust thrown into the air as with the lapping discs. It might take a while to work the surfaces down. I told him I was not even sure about the availability of coarse diamond abrasive powder. I only use the fine diamond paste as a polishing compound. I did a brief search of the internet after the phone call and found that 100 grit diamond abrasive is available to purchase so my idea might work. But it is still diamond grinding diamond and that is tough or impossible work. I have ground and polished slices of meteorite that way with aluminum oxide grits. I only have to do it when the slices are too large to go on my lapping equipment. It is messy and takes a long time to do. It is just about the same as the telescope mirror grinding I did as a kid. I am pretty blessed in one respect that as hard as my childhood was I did have many opportunities to do neat things and learn skills that formed a foundation for much of what I still do now.
So I was just a little cautious as I approached the regrinding and polishing of the cut nodule I had received. But it had already been cut in half so that was a good indicator there was nothing in this nodule to cause me to fear for my equipment.
I do have a tiny Canyon Diablo meteorite of just 3.4 grams in my collection with what might be a small carbonado diamond. It has a flat surface I windowed onto it. I sanded and polished it repeatedly when I was young trying desperately to get a nice etch on it. I used both nitol and ferric chloride to etch it but it has really no pattern other than a thin border of partial crystals on two sides, a few thin shiny lines, and a granular field across the rest. The tiny black crystals might be cohenite or something else but I have never had any problem smoothing cohenite down to the level of the metal. The one small black crystal is about one-half millimeter square and always wants to stand up above the surrounding metal enough you can feel it if you rake the surface with your fingernail. I have never been able to grind it completely flat. I know that it was found on the northwest slope of the crater in the early 1960s. That would be a classic area for shocked and heat-altered small fragments. But it will require more than what I think to be sure it is a carbonado. I like the idea that it is though. Unlike the metal veins in the graphite nodules which have revealed a very ancient (>4b years) creation time. The diamonds are believed to have formed in mere seconds during the impact into the Arizona Plateau.
I have a beautiful slice of Canyon Diablo with graphite nodules scattered through it. None of the nodules in the slice have shiny metal-rich veins. The graphite inclusions in my iron meteorite slice are featureless flat gray with a bit of bronze sheen from the associated troilite. But no bright iron spiderwebbing. It seems again from my reading that there is a real difference in nature and history between the two types of graphite nodules found at Meteor Crater. The images below show that there are portions of the graphite nodules that appear to not have the troilite. They are just as gray as the graphite in a pencil and there is a distinct separation from the zone with the bronze color. Tiny blebs of metal can be seen in the small graphite nodules of my slice at higher magnification.
I acquired another iron slice years ago that was in terrible shape. I have always suspected that it was a shellacked Ninninger slice. I have little doubt about it being a Canyon Diablo. It too has a large graphite inclusion about 2.5 cm across. Again there is a bit of sheen from troilite in part of it. Like the graphite nodules of the other slice, there are rings of Schreibersite surrounding the nodules. In this slice there appear to be regularly arranged but scattered crystals of what I think is cohenite and the etch is just a bit off. I have always thought it was a bit shocked. It took some work to clean and restore the slice. I have always wondered if I should have left it alone just in case someone could identify it as a Ninninger slice. I cleaned, lapped, polished, and re-etched it anyway since it had no number painted on it. It really was awful originally.
In the lab, the halves of the cut nodule were an interesting bit of work. Especially since I had never worked on a graphite nodule before. I have a Bondoc nodule that gives me nothing but trouble. And I have sliced a great many pieces of Bondoc and they have nodules of something in them. This was different. I found I had to treat the graphite nodule halves a little like fresh carbonaceous chondrite slices. To see what is going on after smoothing to 600 grit the nodules needed a cleaning with repeated applications of 99% isopropyl alcohol and a rub with a clean soft rag until no more gray stain came off on the rag. Then you could see the results of the lapping. A water and graphite film was covering the surface obscuring the metal veins. Graphite is simply too soft to polish on a polishing machine. However, the metal veins needed that polishing. After the veins were polished a little brighter the graphite was rubbed up to high polish on an old tee shirt rag. As to the presence of diamonds I have to say as I ground the halves down on my 600 grit lapping disc there were some hard particles trying to get out from under and through the space between the lapping disc and the nodule surface. The particles were small but you can easily feel them come loose and get under the stone as you hold the rock to the lap. I could feel the granules between my fingers when I wiped off the mud on the face of the nodule. I don’t know what they were, just that they were not graphite, not metal, and that they plucked out and stayed unground up. They could be something like fine sand, or some mineral grains, or maybe that something else more interesting.
Science calls all living things on Earth carbon-based life forms. Carbon comes in many forms and more uses and forms are being discovered all the time. Carbon nanotubes, carbon fiber composites, and hexahedral crystals of carbon harder than diamond are some of those. Barringer Meteorite Crater was formed by an asteroid that contained a lot of carbon in several different forms. I have enjoyed working and playing this month with the graphite varieties.