Over the last twenty-five years, I have cut a great many Allende stones. In the last month, I have cut a 600+ gram stone and a nearly 300 gram stone of Allende into 60 or so slices. Allende is easily in my top ten favorite meteorites for visual interest as far as chondrules. The slices come off the saw not very attractive because they are clogged and covered with black dust. But a good cleaning will remedy that. The densely packed surface of chondrules and CAIs will pop like crazy when a slice is clean and then dry after a bath in silica gel.
At some point in the 1980s, I began to save the dust. It was difficult in those days for me to collect it. I was not trying to control the coolant water and much of the dust was simply lost. With the saw I have now I get all the dust. Screens catch any particles of meteorite big enough that they might damage or clog the pump or water jets while the fine powder goes to the bottom of the water tank. So for the last fifteen years since I made this saw I have collected a pretty good supply of Allende dust. It takes a few days for the dust to fully settle but then removing the water is not hard and in a few more days the powder is dry and ready to bottle. Of course, I do the same collecting with lunar and Martian dust from stones I cut for myself. Any stones I cut for the business or for the handful of friends I do cutting work for have their dust saved for them. They generally remind me nowadays with a note that says “Save my dust!” I save and use the dust from the ordinary chondrites I cut too. That is used in my ceramics. The bits of iron and meteorite chips make an interesting and unusual texture in the clay and glaze when fired. And the chondrite powder turns white body clay a nice terra cotta color. I do not know what might be interesting in the lunar and martian dust other than where it comes from which is interesting enough. The minerals in lunar and martian meteorites are mostly ordinary. But there is something very interesting in the dust of primitive chondrites like Allende.
There was a science program on TV many years ago where a liquid-filled glass ampule was shown which according to the narrator contained trillions of nanodiamonds recovered from Allende. I have never forgotten that scene of the program. Especially, when the container was shaken and the liquid took on a strange sheen as the diamonds floated around in suspension. I put it on the “want-to-do list” in the back of my mind to try and extract some nanodiamonds from the cutting powder I was starting to save. Of course at that time thirty years ago I knew nothing about the process involved in isolating nanodiamonds.
Decades have passed since I watched that TV show. I cut a large Allende in September and as always my mind moved to the nanodiamonds that were in the dust. I was not going to get to keep any of the dust this time. It would go to the owner of the meteorite but there was my personal supply of saved dust. By now I have a rough idea about how the meteorite is treated to extract the tiny diamonds. The process is the reason I do not try here at my little lab. There are chemicals I have used and there are chemicals I have always said I would never use. High on the never to use list is HF, hydrofluoric acid. The HCL does not bother me I have used it. Sulfuric and nitric acids I have used off and on all my life for various things, but there is something especially scary to me about HF. Even in extremely low concentrations such as 5mg/L, it is a seriously dangerous irritant to the mucosa and respiratory tract. So I won’t be extracting any nanodiamonds from my saved powder unless a better way is found than using hydrofluoric acid along with hydrochloric acid to dissolve the meteorite and create the diamond containing residue. That process will have to stay in mineralogy laboratories other than my little lab.
Along the way, in my recent research about the nanodiamonds, I discovered how exceptionally tiny and abundant they are in primitive chondrites. One image I saw online from a museum was captioned that there were 60 quintillion nanodiamonds in the tiny glass ampule pictured and that only 5-6 grams of Allende meteorite had been used to obtain that vast number of diamonds. My reaction to that image which showed a very easy-to-see pile of white material at the bottom of the glass ampule was “Wow! That is a lot of diamonds.”
I use diamond pastes as polish now almost exclusively in all my lapidary work. Only the tumbler still uses other abrasives to grind and cerium oxide to polish. The lapping disks use diamond particles from 100 grit to 1500 grit. And I use diamond polishing paste on hard felt discs at meshes of 3000 to 100,000. I thought the 100k mesh diamond paste was made from very small particles; and it is. Many manufacturers of diamond powder polishes hesitate to say what the exact size of the particles is for the 50k and 100k mesh pastes. The separation process is seemingly so difficult for those grades that they can not guarantee sometimes that all the particles are uniformly small. They often give a range of particle sizes. But, for general discussion purposes such as this article 100k mesh is around .25 microns (¼ micron). In other words ¼ millionth of a meter. How small are the nanodiamonds in meteorites like Orgueil, Murchison, or Allende? They are called nanodiamonds because they are measured in nanometers. One micron is 1000 nanometers. So the finest size diamond powder I use for polishing precious stones is .25 microns or 250 nanometers. The size range for nanodiamonds in primitive chondrites is 1-10 nanometers with an average of about 2.5-3 nanometers. The nanodiamonds in meteorites are one hundred times smaller than the finest diamond polishing particles I use for the final polish on hard stones such as ruby.
Things such as human hair are enormously thick by comparison to nanodiamonds. A hair at 70 microns is in fact over twice the thickness of meteorite thin sections we study. This may give those thinking that thin section preparation is doable by anyone something to consider. Thin section preparation is a bit of art mixed with great care and patience. Human red blood cells are fully 8 microns and also enormous compared to nanodiamonds. I know I am not going to resolve any nanodiamonds with microscopes optical or digital at my disposal. These cosmic diamonds need something special in the way of equipment to be studied.
Now there are other diamonds in meteorites besides these tiny nanodiamonds. There are the diamond particles that were created in some Canyon Diablo meteorites, likely during the impact which formed Meteor Crater. The aggregates of diamond particles in CD irons are not measured in nanometers but millimeters and microns. The diamonds in ureilites that destroy our diamond saw blades are again not always nanometer-size they are often 100 microns. And in a recent report researchers concluded that ureilite diamonds are also the product of collisions meteorite graphite experienced but out in space. The strange and tremendously abundant diamond particles in Allende are truly small to the extreme. They are thought to often contain only1000 carbon atoms each.
Scientists think that these nanodiamonds are particles that have come here from beyond our solar system. Presolar grains that are older than anything else we have handled. They have survived the tremendous temperatures of our solar system’s beginnings and are still in some meteorites that have remained primitive. Many of the metals and other elements in meteorites are measured as traces with a few or even just a fraction of parts per million (ppm). The average amount of nanodiamonds across the group of meteorites that contain them is 400 ppm with a range that can be as high as 1500 ppm in some stones. The nanodiamonds make up a significant and measurable portion of the carbon in these meteorites. But there are great difficulties in studying objects so small. Yet the scientists do study them in great detail. During my research, I found that there were sometimes additional things done to the diamond residue after it was extracted from the meteorite. The scientists sometimes treated the residue with phosphoric and sulfuric acids to remove spinel and corundum which contaminated the diamonds. I found it amazing that the scientists were later able to determine that the surface of the tiny nanodiamonds was etched by these later acid baths.
The goals in the decades since the initial discovery of the nanodiamonds have moved from studying the diamonds as material by itself to studying the diamonds as they are found in the meteorites. This is amazingly fine work. I can not imagine working with such small particles. I have made dozens of thin sections in my lab over the years and I can deal with microns. I can take a thin section that is just a little too thick and I can grind and polish off another fraction of a micron without difficulty. But to examine a thin section for particles several thousand times smaller than the 30 microns of a normal thin section. That is hard to imagine. There is fantastic equipment available today for scientists. What an exciting time it must be for those already in the field and new students entering meteorite investigation.
These are images from Allende thin sections under polarized light.
Meteorites with diamonds! The phrase itself is sort of a joke for some of us in the meteorite community. It seems like about every two or three weeks I get another email from someone who has found a meteorite containing diamonds, gold, and platinum. They want to sell the thing for some ridiculous amount of money. They formerly got upset when told they had a piece of worthless rock. I guess they would still be getting upset but I no longer answer any of that mail. The nanodiamonds in meteorites are not going to ever be a concern to the diamond brokers here on Earth. But they are of tremendous scientific value. They have carried to Earth information from farther away than anything else and have survived unchanged for longer than anything else.
I was already collecting meteorites when Allende fell in 1969. For just a few dollars I bought a stone within weeks of the event. It is black and very fresh. It is not perfect. It has broken spots on the surface. It might not be the stone I would buy today, but as one of the first meteorites I bought it is special. It has not seen the light of day much for decades. It has been in a gray plastic 35 mm film container. Only us older people would even remember them. But I have brought it out for a photoshoot on this occasion.
I would love to own as meteorite ephemera a glass ampule of nanodiamonds. These minuscule gems of carbon could be among the most abundant particles out in space. They shine at a couple of narrow bands in the infrared. There may be a lot of them out there. And that could be saying something if less than 6 grams of Allende can yield 60,000,000,000,000,000 nanodiamonds.
