As has been the case the last couple of issues I have a short announcement before the actual article. A meteorite that Jason Phillips and myself purchased a few months ago has become official in recent days. We knew on sight that it was a eucrite but we are happy to have it all classified up and listed in the Meteoritical Bulletin. Its designation is NWA 14370. Total known weight of the meteorite is 3071 grams. There were 45 pieces. Some of the stones had patches of fresh fusion crust but most of the stones had signs of abrasion and little or no fusion crust remained. The interior as indicated by the low weathering grade in the classification is a fresh light-gray color with interesting clasts and mineral crystals. It is always fun to see a new listing for a meteorite you have submitted to a lab.
74 Years Old And Still Has Much To Say
This month’s article is a journey back in time. Not too far back, just to February 12, 1947. The Second World War is over and things are returning to normal. It’s 10:38 in the morning, northeast of Vladivostok, in Eastern Siberia. Suddenly a flash of light brighter than the sun is seen. A tremendous meteor is crossing the sky leaving a trail of smoke that will endure for several hours. The iron body is traveling at 14 kilometers per second (8.7 mps) on its inevitable course toward impact with the Earth. At approximately 5 kilometers altitude the main mass of the asteroid breaks up (“Hemmungspunkt”) creating thousands of pieces great and small, and dust. In seconds most of the incoming material is on the ground.
Over one hundred impact pits are in an area of about one-half square miles. Some of the large pits contain only small fragments as the impacting chunk hit with such force it exploded into thousands of pieces of shrapnel. Smaller pits do contain large meteorites. Remarkably the largest mass of 1,745 kgs will be found in 1950 in a rather small impact pit only 3.5 m across by 0.8m deep. The largest specimen landed quite remotely from the other large holes, actually 600 meters north-northwest. That is well outside the scatter ellipse for the majority of the specimens. The whole area of the event contains almost countless small iron meteorites. Based on the amount initially collected the estimate of the mass reaching the ground is 23 tons.
Coming back to our time. The pre-entry mass of the asteroid is estimated today at 200,000 to 220,000 pounds. That is the weight of two fully loaded and fueled Boeing 737-100s. The majority of that countless number of small meteorites will have to be found with metal detectors decades after the fall. They soon became invisible, buried on the forest floor.
That description is of course the story of the Sikhote-Alin meteorite fall. I realized as I was cleaning a big batch of Sikhote-Alins recently that I had never written an article about this fall. I also saw as I cleaned the meteorites features that have been a source of questions for decades. So that’s where this article is going.
The fall area of the Sikhote-Alin meteorites is forested, wet, still isolated today, and inhabited by a variety of pretty dangerous creatures. The climate of the Sikhote-Alin Mountains is not hospitable for iron meteorites. The meteorites I was cleaning had likely been buried for 60 years. But it was interesting how much information was still visible on the meteorites despite the rust and pitting created in the six decades.
I decided for this article not to discuss the shrapnel pieces that were torn apart by impact. I will devote this article to the flight-shaped meteorites.
All meteorites experience tremendous stress coming through the atmosphere. Stone meteoroid bodies often explode into small pieces and dust. Iron meteorites experience some different effects in addition to also often breaking up. Because the iron metal is ductile and not quite as rigid as stone, iron meteorites will sometimes show stretching and twisting from the pressures exerted on the metal. The photo shown below is of a Sikhote-Alin meteorite that had both ends torn off and was folded in half enough to create cracks at the bend.
I doubt that this particular specimen could have stood much more bending without failing in the middle as the ends did. Asteroids are cold bodies when they enter the atmosphere and despite the great temperatures that melt and shape the surface, just below that heated skin they often remain cold. Metals including iron in particular do not bend and fold well at low temperatures. They instead are brittle and break. I was sure if I searched there would be some data on the strength of the Sikhote-Alin iron alloy. I was able to find this information on its tensile strength. There is some commonality between ductility, plasticity, and tensile strength. Ductility is measured in some cases by the elongation of a sample at a specific percentage. Those happen to be the figures offered by Buckvald for Sikhote-Alin’s tensile strength and were taken from Book Three of “The Handbook of Iron Meteorites.” Plasticity might also be a better term than ductility since it concerns deformation and not drawing out of the material as ductility might describe. Small specimens of Sikhote-Alin range from 45kg/mm squared at 15% elongation to 49 kg/mm squared at 9% elongation. In larger specimens with inclusions and grain boundaries, the tensile strength is far less at just 5kg/mm squared. This is significant as we examine specimens. We often see them broken at grain boundaries or inclusion areas. Many I have seen are also broken with scars that resemble what happens in manmade iron and steel. From my observations, these last breaks occur when the meteorite metal has a small cross-section and fails because it is simply too thin to resist the forces applied. Some commonly seen breaks are shown in the images below. I have captioned them based on what I believe to be correct interpretations of the scientific descriptions of how the meteorites broke. I hope I got most of them right.
The early reporters wrote of breaks that showed linear marks where they separated at skeleton schreibersite inclusions. Secondly, they wrote of breaks on metal grain boundaries. These are breaks with relatively smooth separation surfaces that are angular and often form pockets or box-like cavities where adjacent metal crystals once intersected. Then there is the third type of break I have noticed that was not addressed in the early reports of Sikhote-Alin. These are seen less often and sometimes have a different set of external characteristics on the specimen. They resemble the breaks seen on sheared-off bolts and other failed industrial steel objects. I will attempt to describe this type of break. It is a surface with a fine granular texture and a ragged torn circumference. These breaks are not at locations where inclusions and mineral aspects of the meteoritic metal are involved. The breaks look to be purely mechanical failures of the metal under tremendous forces. Similar to twisting off a bolt with a wrench or bending a steel rod beyond its limits. Specimens showing these breaks may have smooth rounded surfaces with shallow thumbprints larger than usually appropriate to the small specimen size. I theorize that many of the mechanically broken-off specimens were part of larger meteorites. Also, that they broke off late in flight but while the mass was still traveling with high velocity. Slow speed would not exert the force needed to break the pieces off. After breaking off however these specimens slowed quickly. Meaning they no longer had the speed to further ablate and alter the broken surfaces.
I fell in love with this Sikhote-Alin slice soon as I saw it about 25 years ago. Light shines through the crack and it is held together in only a single area. It demonstrates quite dramatically the forces that were applied to the meteorite. The crack has the same angles seen so often in Sikhote-Alin meteorites.
Above are specimens with what I call “mechanical breaks.” They are very different from the other type of breaks which could perhaps be called separations instead of breaks. In the others, the resulting surface is not rough and the edges are not jagged. In the mechanical breaks, the material had no flaw to cause separation at a point of weakness. Instead, the metal was simply stressed until it failed.
In the complete one-sentence description of the Sikhote-Alin meteorite, Buckvald calls it an unannealed coarsest octahedrite. The metal just below the heat-altered skin shows a high hardness which even increases in some deformation situations. Buckvald suggested it had suffered cold working. Why is this strength and hardness information important? Sikhote-Alin specimens often show many characteristics in a single specimen. These features include twisting, bending, breaking, ablation with metal build-up, and orientation. I have on many occasions seen beautiful specimens with ablated surfaces and flowing metal, specimens with the domed cap of metal resembling wax running down a candle. They are offered for sale as oriented meteorites. But upon examination, some have a snapped-off broken surface. Are these flight-oriented specimens? This writer’s opinion is no. These are late flight break-off specimens from larger meteorites that dislodged at an inclusion, or grain boundary, or where support became too small to stand the pressures and they snapped off. But this happened too close to the end of the flight for continued ablation to alter the area where the break occurred. They are very nice and interesting meteorites worth the extra cost to purchase, but should they be called oriented?
It has been a while but there was a period when we used to see plier jaw marks on some exquisitely ablated pieces showing how they had been gripped and broken from their larger parent mass. Someone thought these selected portions were worth more sold alone than attached to what might have been an ugly larger meteorite. Or perhaps they were removed because the mass was so large that finding a buyer for the complete meteorite might prove impossible or a long-term endeavor.
These pieces were not oriented meteorites only ablated projections.
The term “oriented meteorite” is itself a bit ambiguous. Is it the shape of the meteorite that the name refers to? Or is it the process of forming the shape that should determine what the specimen is called? For me, it is the stable flight process and how that is seen in the finished meteorite that determines if it is an oriented meteorite. There is more involved than just that a surface faced one direction long enough to be ablated before breaking off. The whole specimen needs to show signs of stable ablative flight. That means at least a lip around the trailing edge or smoothing of the broken surfaces. With stone meteorites, it is more than just a smoothed curved side with some symmetry that is required to say that a fragment came from an “oriented” meteorite. It may be radiating thumbprints and flow lines on an incomplete stone that are enough to say it was a leading surface of a stable flight meteorite. Or it might be a bubbly accumulation of glassy froth with a perimeter lip on a fragment that is enough to confirm orientated flight on a partial stone. Complete stones are of course an easier matter.
Sikhote-Alin flight-oriented meteorites are fortunately quite common. Even these old and weathered specimens I have been cleaning are often truly oriented meteorites. Not all of them show the extreme dripping-wax look with fine-line detail, or splatter onto the back that exceptional specimens possess. But they have lip lines around the perimeter and sometimes worn built-up metal on the leading surface. Sometimes they even still have fading fusion crust on the grain break surfaces to show they continued in oriented flight after separation. Here are a few images of flight-oriented Sikhote-Alin meteorites.
The following meteorite is not too pretty today. Though it might have been beautiful 70 years ago.
During cleaning, I had to punch through some blisters of corrosion to get below and clear away the pocket of powdery rust. In doing so I had to work my way along the lip of melted metal at the nosecone tip. As I removed the corrosion some of the melted lip fell away. Just corrosion was holding it on. A tiny area was revealed that I found quite interesting and wish to share. I think that it demonstrates how the rolled-back structure is at least partially unwelded to the body of the meteorite. It just covers it. At least at the trailing edge of the metal flow. What was exposed when the flake of melted lip fell off were the lines of the regmaglypts underneath the melted lip. It was the stable flight in a single orientation that allowed the formation of the nosecone of flowing metal. But under the unwelded ablation metal are remaining traces of earlier flight markings.
There are several different forms of meteorites from the Sikhote-Alin fall. The thumb-printed individuals both large and small have always impressed people because they are just what a meteorite should look like. Maybe it is from SciFi movies or something else in popular culture but sculptured thumb printed, black stones are often what the general public thinks meteorites should be. The Sikhote-Alin fall produced thousand of individual meteorites just like that. The Sikhote-Alin event also produced thousands of meteorites that are not like that at all. Some have regmaglypts on only one side, many are torn and broken on one side, and have regmaglypts on the other. Many have no thumbprints at all. Occasionally, a meteorite will show up that has a large thumbprint but the meteorite is small. Regmaglypts are created by the action of the air as the metal is eroded during flight. Thumbprints are proportional to the size of the meteorite. Thus large Sikhote-Alin masses will have thumbprints that are several centimeters across and small individuals will have thumbprints that are a few millimeters across. When I see meteorites that are small but have one or two large flight marks it makes me question if the specimen is a piece that came off a large mass near end of flight. Below are a few images of nicely sculptured and thumb-printed Sikhote-Alin meteorites.
Shapeless iron blobs are common Sikhote-Alin meteorites too. Round, oval, elongated potato shapes and thin flat slab-shaped meteorites are fairly common. Some of these other shapes are seen in the following image.
Among my favorite shapes are the round-nosed cones. Sometimes these are decorated with flows of metal rolling back from the broad end. Many times they show no signs of ablation other than their basic form. Below is an image of a favorite where the meteorite metal spirals down to a sharp point.
The last images are of a beautiful Sikhote-Alin that I got years ago and originally thought was a flight-oriented meteorite. It is an amazing specimen but there are two breaks on fair size and one small on the bottom. By my current standard, this is not a flight-oriented meteorite. Before breaking off however it was a bridge across a hole in the larger stone. I am not disappointed that I can not call it oriented anymore.
The Sikhote-Alin meteorite event was one of the largest falls in recent times and was spectacular. It might not be the biggest recent event on record, others have been spectacular too. Chelyabinsk is more recent and a huge event with a fractional megaton explosion. But the Sikhote-Alin fall has given science and the meteorite collecting community plenty of beautiful information-packed meteorites to study and enjoy.