Meteorite Times Magazine
Contents Paul Harris
Featured Articles
Accretion Desk by Martin Horejsi Jim’s Fragments by Jim Tobin Meteorite Market Trends by Michael Blood Bob’s Findings by Robert Verish Micro Visions by John Kashuba Norm’s Tektite Teasers by Norm Lehrman Mr. Monning’s Collection by Anne Black IMCA Insights by The IMCA Team Meteorite of the Month by Editor Tektite of the Month by Editor
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The Felix Carbonaceous Chondrite: A 3.3 from 1900. Martin Horejsi
Thirty minutes before noon on May 15th 1900, a single stone flew down from space through a clear blue sky. It attracted the attention of those in that region of Alabama having generated the sound of thunder during its supersonic passage through the thin atmosphere of earth. Using the metaphors of the day, one witness described the meteor’s noise as “a big piece of red-hot iron being struck with a hammer, causing many sparks to fly in all directions.”
The classic dark matrix of Felix contains many colorful and well-formed chondrules. As a CO carbonaceous chondrite, its namesake is Ornans, but was often compared to Lance’ in the literature. And in the case of Merrill’s work, the Warrenton.
In 1901, George Merrell published the authoritative account of the fall of Felix, as well as a chemical analysis and a few pictures. From what I’ve read, not only is Felix the only CO3.3 witnessed to fall, but it seems Felix is the only CO3.3 period.
The British Natural History Museum’s catalog initially listed the classification of Felix as a CO3.2 which would have provided it some overseas relatives including Kainsaz and Rainbow, but the Meteoritical Society suggest Felix be known as a CO3.3. On the far end of the family, there is a transitional hot desert find known as NWA 062 that straddles the line between 3.3/3.4 Crust is always welcome on a historic fall, and as 115 years old, Felix is historic.
Below is the full text of Merrill’s tome on this Felix, Alabama fall. I especially appreciate the descriptions of the fall provided by the witnesses.
Until next time…. Meteorite Times Magazine
Meteorite Imaging James Tobin
One of the wonderful things that the Internet has brought to us is the vast amount of images available on every topic. For meteorites many different issues can create challenges for getting images done well. Even as I write this I realize that I will not be able to show the images in this article at their hi resolution. We are forced by bandwidth and speed to compress our images and reduce their size. Still it is possible to capture and offer nice meteorite images.
This is an image of a fascinating impact rocks from Australia. This is Jeerinah Layer spherule rich rock that is very old. The spherules that are so noticeable in this image are not even recognizable with the naked eye. The image was taken with a 50mm Macro lens set at f22 with a lifesize extension ring which makes the set up about f32. When this set up was used with 35mm film cameras the image exposed on the film was the same size as the real subject being shot. Thus the name “lifesize” extension ring. Sensor size and other factors change that some when the ring is used with a digital camera.
I thought for this month’s article I would do it on one of the areas of imaging that we use frequently with meteorites. Close up, macro and ultra close up, are terms often seen. They are different in slight ways but they are an area that has many similarities and can be discussed as a single topic. For the sake of simplicity lets just call it macro photography. No microscope will be involved.
I went digital years ago and had a point and shoot digital camera that did acceptable macro as one of its dedicated functions. By today’s standards it did not give me much control of exposure or lighting. And when the camera was so close to the object at .7 inches it was difficult to light the subject. With most point and shoot cameras there was no way to take the picture except by pushing the actual shutter button. I could put it on timed delay and that helped with vibration some if I used a tripod. But the camera was not very macro friendly.
I got a Canon DSLR a while back to be my general photography camera. I use it for astrohotography and much of my meteorite imaging. I have normal and telephoto lenses for outdoor pictures to use with it. I was considering what to do for my macro and ultra close up work. Did I want to get a dedicated macro lens, extension tubes, bellows and all the rest? I should mention that I have a very nice 5 megapixel digital microscope for when I need to get in really close, but I enjoy using a regular camera more. How deeply did I want to get invested in new macro equipment?
I had kept all my lenses from film days and had a full line of equipment for macro and closer work to fit my Minolta SRT 201. A little exploring on the net and I found that conversion rings were available for my old Minolta lenses to let them fit on my Canon T4i. If I wanted to use my Minolta normal and telephoto lenses I needed a conversion mounting ring that had a corrector lens in it. Otherwise it would not be able to reach infinity. You don’t have to reach infinity with macro photography. The lens-less converting mount was perfect. So suddenly I had all my old equipment back to use with the Canon DSLR.
My biggest problem with that Nikon point and shoot digital which did macro was it had a small diameter lens and only went to f8. That is not going to ever be very good for advanced macro work. You need a little larger lens that you can really stop down to at least f22 or f32. And it will go to even slower settings when you start putting on extension tubes or bellows. So I needed the wide aperture control of a DSLR with a manual mode. My Minolta stuff mounted on the Canon gave me that. I was back in business for only a few dollars investment in the conversion mounting ring.
In close up photography there are two topics that are just more important than anything else. Focus and depth of field. And they work hand in hand. In outdoor photography there might be times when the background is of no interest or even something you do not like. Well you can crank open the lens and focus on the foreground and like magic the background is blurry and not a problem. Other times you may want the background and the foreground to both be in great focus. So you squeeze that aperture down and the camera will take a longer exposure or you run up the ISO and there you are; the whole shot front to back is in focus.
In close up photography you have the issues of the last paragraph to deal with. But the distances from foreground to background are compressed to usually a fraction of an inch or at most one or two inches. If your close up subject is a 3D object that you want well focused in all parts you have a real challenge. Even if you learn to control the depth of field. Briefly this is how it works. When the lens is wide open you have rays of light coming into the lens at all different angles which are bent to strike the film or sensor. The rays hitting the center of the picture will be focused at one setting but those hitting the edges of the film or sensor will not be focused at that same setting. When the aperture is stopped down so that only the very center of the lens is used the rays of light enter the camera nearly parallel without having to be bent by the system of lens elements very much. The light is much decreased in strength but it focuses across the whole piece of film or the digital sensor very close to the same focal point. This is an image taken at f8 and 1/160th of a second exposure. The camera was focused on the front part of the bright metal mounting ring. Note the words “Minolta” and “Japan” are also quite sharp. The internal parts down in the extension tube are so blurred that you can barely make them out. This is the same framing and the lens is set for f36 and the exposure has moved to 1/8th of a second. It is quite easy to see the difference. The whole item is now focused and the internal working parts are actually quite sharp. This was taken a few inches from the subject making the possible depth of field rather broad. This will not be the case when the subject is only 1-2 inches from the lens.
With macro photography you are never going to get very huge amounts of focal depth range. You are dealing with factions of lifesize or lifesize, or even larger than lifesize images being put on the film or sensor. Features such as the rolling away edges of meteorites will quickly move from focused to blurred. This is a nice image of the fusion crust of a Chelyabinsk meteorite. The top surface is well focused and the detail sharp. Even with everything on the shot set for best depth of field the meteorite quickly becomes blurred on the sides going away from the foreground in the shot.
You need to stop down the lens and get the best focus on the most interesting part and basically accept the amount of blur on the other parts. While it is true that there are programs that do “focus stacking” which will stack slices of an object at different focus points and make a very nice fully focused image of a deep three dimensional subject. For day to day work we are shooting a single aspect or in fact a flat slice or polished face of meteorite. You still want to use the tight small lens aperture (high f number) so that the edges of the image are as sharp as the center of the image. This is an image of a Chelyabinsk meteorite where the flat top surface is the only aspect that is of interest. The problem here is to just make the whole image focused from edge to edge of the frame. Here is an image of loose Saratov chondrules. Without some depth of field they would not have their nice sharp shape and of course would not show many of the details that they do. This image was shot with the lifesize ring at an effective f32 and an exposure of 1/20th of a second.
I don’t think there is a feature that I love for both astrophotography and macrophotography as much as the magnified live view of focus on the LCD screen of the camera. I use it all the time. Focus is such an important thing for making it a great image or having it be a poor image. And being able to view the focus and the framing as you adjust the camera in and out from the subject is just wonderful. Many DSLR cameras also have a button to let you do an aperture preview. You can see the effect of the f-stop settings on the depth of field. The image will darken on some models when you push the button but look to see the focus sharpen. Unfortunately this feature is not available to me when using Minolta equipment on my Canon camera. But it is available to me when I use Canon lenses of course. The first time you see the way that the background and deeper parts snap into focus when you push the aperture preview you will be sold on using a high f-stop number. Now we have to figure out the lighting to make it work. For much of my imaging I am using lights like this picture shows. They are color corrected for daylight and are adjustable in brightness. They take AA batteries and will also work with rechargeable batteries. Two lights one on each side of a specimen is a great way to go, but a lot of work on meteorite slices and windows can be done with just one light. This image was done with a 30mm lens setting at 16 inches from the subject and focused on the LED plastic lens in the center of the lamp. It was shot at f22 with a 2.5 second exposure. The long exposure was to demonstrate how tripod and cable release switch eliminated any vibration in the shot.
The biggest real problem for most people with macro or even other types of imaging is illuminating the subject. To some extent it is an artistic thing that is true, but if you can not see areas of the subject because of surrounding shadows or the whole image is dark then it stops being so artistic and becomes something to explore. Once you start to really stop down the lens to get the depth of field you are cutting off the amount of light in a mathematical relationship. Each f-stop move is half or twice the light entering the camera depending on if you are opening one stop or closing one stop. So if your outdoor snap shot of the kids is fine at f8. When you change it to f22 to get the distant snow covered mountains also focused you have moved the aperture through f11, f16 to f22. You had half the light at f11 a quarter at f16 and so on down the line. Sometimes you can find yourself at f32 or even f64 in macrophotography. How do you make up for that loss of light. Well the simple answer is longer exposure time. There are subtle caveats to this that make it not exactly right but for the sake of this article twice the time or half the time is equal to one f-stop of aperture. Again it depends of if you are opening or closing the lens aperture. This image of Murchison is of course very dark naturally and the challenge is to illuminate the shot so that the interesting inclusions will stand out. This shot was taken at f22 at 1/2 second and some contrast processing was done in Photoshop. But the focus is fine and the long exposure gave me all the inclusions nice and defined. Here is a small unclassified NWA meteorite that is really nice. It has well defined chondrules and might be a type 3 stone. It deserved a sharp image. Here is where some artistic treatment is possible. It could get a nice background and some framing. I did none of that for this shot which was taken with a 50mm Macro lens at f22 and a 1/20 second exposure.
Outdoors for that snapshot of the kids it is not going to be a problem if it is 1/1000 of a second or 1/500 of a second or for most people with a digital who have steady hands even 1/250 of a second exposure time. The automatic aperture control will pull closed the lens to the correct setting automatically to make a well exposed unblurred image.
But, at a sporting event at night with poor light and fast action you want to use that 1/1000 or even 1/2000 of a second to stop the action and there is not enough light. The images are under exposed or the camera keeps trying to use a flash. Which is a joke and nuisance since you are far from the sports action and you are using a telephoto lens. The solution is always more light. A bigger lens or a telescope, or higher ISO setting that makes the sensor seem more sensitive and let you use a shorter exposure to get that blur free stop action well lit image. Thus you have the huge diameter lens that you see at such events. For macrophotography the issues are similar to the last paragraph. You stop down the aperture to a tiny opening so you can get great focus. Now you need a tremendous light source or a long exposure. Remember that relationship between the length of the time the light is going in the lens, to the amount of actual light allowed through the lens opening. You are constantly trying to balance these two factors to get the correct exposure in all types of photography. But for macro the lens opening has been made tiny to get depth of field forcing longer exposure time and brighter lights.
Again nothing beats seeing what the camera will do before you take the shot. The LCD screen will show you what you are going to get. You put the camera on Manual Mode set a high f-stop number and see what is on the screen. Frame the picture and focus (usually) on center of field. Use the aperture preview button if you have one. You will see the depth of field focus but maybe not see the way the exposure will be. The image may be dark since it is stopped down in preview. If you want a nice clean non grainy shot you need to keep the ISO down a little. Newer digital cameras are much better as far as high ISO noise than they used to be. But generally speaking I would stay as low as I could on the ISO setting. It is always better to work with higher photon quantities than to increase the amplifier gain and work with fewer photons. We are going to make that possible by using a vibration free sturdy tripod, and a remote shutter cable or the delay timer for the shots. Now you can make the exposure time longer to get back to a well exposed image. We are going to eliminate vibration during the long exposure by being hands free. Now that 1/2 second or 1/4 second exposure if you have to use it is fine. With really bright studio type lights I am often around a 1/20 second exposure. But it is not unheard of to be at an exposure time of as long as 1 second.
As far as artistic possibilities for macrophotography they are still there. The framing of the meteorite is going to be in the eye of the person on the shutter button. The choice of what feature you think is cool and want to shoot is still all yours. The background you place under your specimen is an area you can have fun with. The arrangement of extra lights to accentuate some features are yours to decide about. For example, I often hand hold a pure white LED flashlight to use as a fill light to lessen shadows on one side or to illuminate and reflect the brightness of metal grains into the camera. Lighting placement is going to be something to learn and even if you choose a light box for smooth even illumination it is not going to do everything you want. You will likely find yourself wishing that you could shoot at an angle to get some reflection or want to backlight through a thin slice of pallasite or tektite. You will need several options for lighting.
This is an image of a portion of a slice of Springwater pallasite. The light was positioned to get the reflection that I liked and to show off the etching of the metal. The settings for this shot were f22 and 1/20 second with the 50mm Macro at about 2-3 inches from the slice. This is NWA 725 a slice of Acapulcoite/Winonaite. The metal of meteorites can be a challenge to photograph in a way that does not overwhelm the rest of the shot. I have made a regular set up with the 50 mm Macro lens and the regular lighting that would give me a flat no metal reflection image. I have taken a second less bright light and set it so that it will reflect the metal up into the lens. By controlling this light I can get well defined metal grains and still get sharp surface details of the meteorites. Settings were f22 and 1/20th second at about 2-3 inches from slice. Sometimes there are meteorites where going in closer just does not really get you that return for the trouble. This slice of NWA 6488 a polymict eucrite is a variety of grays and has a very porous texture that when shot close up become quite distracting. This meteorite slice looks much better when shot at a distance and with less magnification factor. This was shot to get the entire slice in the frame at f22 and 1/20 of a second but at a lens to subject distance that makes it no longer a macrophoto. In general though the definitions are not universal, for it to be macrophotography the object has to be smaller than 2 inches or 5 cm. There are a great many “bug” photographers doing macro work with lens that let them be two or three meters from the insect. In a more studio type setting like for meteorites the equipment can be very different and the work done much closer to the subject.
All the images in this article were taken with a Canon T4i using macro photography equipment. All were taken with a remote cable release switch. The camera was on a tripod pointing down at the specimens. Nothing fancy for these images they were done deliberately without much fluff. Some were taken using a lifesize extension ring, others were taken with macro lenses alone. No microscopic images are in this article. Someday I may take on that topic. Much of the discussion is similar though the problems are just even more intense with a microscope. This is by no means the last word. Hundreds of whole books have been written on this. For the creative there is a big DIY area available. Stages and stands to hold specimens, or magnet holders that will lift specimens and diminish or eliminate shadows are all fun projects for handy imagers. Hope you go and have fun. Meteorite Times Magazine
Meteorite Market Trends Michael Blood
This Month’s Meteorite Market Trends
by Michael Blood Meteorite Times Magazine
Bob’s Bulletin – A newsletter for “orphaned” meteorites from the USA. Robert Verish
Here in the United States of America, if you want to get a meteorite classified, you have to pay some professional to get it done. It does sound like the American-way, but there was a time not too long ago that you could get a classification done on your meteorite just for the price of a type-specimen. But even back then, the cost for doing that analysis was coming out of somebody’s pocket. So, it should come as no surprise that the free-ride has come to an end.
It’s usually at this point that a U.S. taxpayer will disagree with my statements and try to convince me that their tax-dollars are paying for thousands of Antarctic meteorites to get cataloged. But the truth is ONLY Antarctic meteorites are funded by U.S. tax dollars. Which means that there are no funds budgeted by the NSF for NASA to catalog any other meteorites (not to mention their recovery or classification). What most U.S. taxpayers don’t realize is that none of their tax dollars are spent on the recovery or classification of meteorites found in the USA.
What this means is that, if you should be lucky enough to find a meteorite here in the USA, you’ll have to make a serious donation in order to entice a researcher to study your find, and in the end you will be paying for its classification. Worse, what appears to becoming common practice, is that researchers are turning down Ordinary Chondrites and are refusing to classify them, even when they are getting paid to do the classifications. And since the vast majority of meteorites that are found here in the USA are Ordinary Chondrites, the vast majority of future USA meteorite finds will never get classified or cataloged.
I call these meteorites, “orphans”, because they will soon be lost to the ages.
But, I’m not writing here to bemoan this new fact-of-life. The end of one era is always followed by another.
Instead, this edition of Bob’s Findings is introducing a new format. The new format will be more like a newsletter that announces the latest list of these “orphaned” meteorites.
Newsletter for Orphaned Meteorites from USA – Volume 1 No. 1 — January 2015 Petrographic Descriptions Meteorite-Recovery Information
Meteorite Specimen Petrographic Descriptions:
N130912 N130929A N130929B N130930A N130930B
Example Petrographic Description
Field ID Number N130912 Newsletter 01-1 Nevada, Location USA Thin-section ID Number V007 3.0cm x Dimensions 2.25cm x 2.25cm 14.68 Weight grams Ordinary Class Chondrite
It may even come to pass that these orphans will go overseas to an obliging institution to get classified. It should be noted that many countries, such as Japan and China, classify and catalog 100% of the meteorites found in their country, and in fact, in the majority of countries, the notion that some meteorites are not worthy of being cataloged would be foreign to them. A future where meteorites, found here in the United States, may end up going out of the country permanently is very unsettling. But as each year goes by, it becomes more and more unlikely that funding will ever be budgeted for these orphaned meteorites.
References:
Meteoritical Bulletin: the search results for all provisional meteorites found in “USA” – Published by Meteoritical Society – Meteoritical Bulletin, Database.
Meteorites of California the list of formally-recognized California meteorite falls and finds.
My previous articles can be found *HERE*
For for more information, please contact me by email: Bolide*chaser Meteorite Times Magazine
Symplectites in NWA 5784 diogenite John Kashuba
The wavy black forms signal a symplectitic texture. Symplectites appear in metals, minerals and other materials. They are intergrowths of two or more constituents and appear in a variety of configurations. They may be considered a disequilibrium textural feature. The different phases may form from: a single phase that becomes unstable from a pressure or temperature change (e.g. exsolution); a reaction between adjacent materials; the introduction of a reactive fluid.
In this dunitic diogenite the dark mineral is very likely chromite, according to a researcher who has worked on similar meteorites.
Cross-polarized light. Field of view is 0.4mm wide. Cross-polarized light. Field of view is 3mm wide. Plane-polarized light. Field of view is 3mm wide. Incident light. Field of view is 3mm wide. Cross-polarized light. Cross-polarized light. Cross-polarized light. Norm’s Tektite Teasers: Aouelloul Glass, Adrar, Mauritania
The recent announcement of “tektites” in the Atacama forced me to revisit the criteria that distinguish tektites from other impact glasses. At both extremes are examples where there is solid consensus. This one is a tektite, but that one isn’t. Somewhere in between is a poorly described definitional boundary. I don’t intend to fight that battle in this column. Instead, I’d like to take you to an impact glass near the bottom edge of glass-producing events, a case where most everyone can agree that the glassy splashforms are not tektites: Aouelloul (wah LOOL) glass. (Atacamaites may prove to be a yet smaller event, but that’s a story for next time).
Journey to a howling sand sea in the desert band of northwest Africa, an exceptionally hostile place. The Aouelloul crater is about 380 m in diameter and was nearly 80 m deep (now about a third filled with drifting sand). It formed in the Pliocene, some 3.1 my ago, blasted into a bedrock of Ordovician sandstone. The village of Chinguetti, Mauritania swelters 41 km to the northeast as the crow would fly, were it not too hot for them here.
You might note that this crater is quite similar in diameter to Monturaqui, Chile, which reports in at about 350 m diameter. There, the impactites include no splashforms, but are represented by breccias set in a matrix of glass. Between that limit, where glass is formed but not ejected, and monster events like that of the Australasian tektites---between those boundaries is the known spectrum of glass-ejecting encounters with space rocks.
We begin our tour of Aouelloul crawling on hands and knees in “world’s hottest places” sorts of conditions. (One of the collectors that we bought inventory from spent time in a Mauritanian hospital recovering from heatstroke!) Scorpions are sheltered under the rocks we check out. The bits of brownish-gray glass we seek are mostly scattered around the outside rim of the crater (wind-blown sand-fill prevents examination of the crater floor). Bits of glass can be found as much as 1 km eastward from the crater. (I don’t know if that distribution is purely primary or if it may have been modified by secondary processes, but it provides an upper limit for the horizontal flight distance of the glass).
Figure 1: An assortment of fine Aouelloul Glass fragments.
Most of the glass pieces weigh only a few grams, but the biggest known (broken into 3 jigsaw puzzle pieces) totaled over a kilo. Figure 1 illustrates an assortment of nicer pieces (like anywhere, there is an abundance of broken bits that don’t tell much of a story).
The glass is rough surfaced and crude, like a sandy brownish-gray paste. There are no shiny dings along the edges from bouncing home in a bucket across rough terrain. It’s just not that glassy. But don’t be fooled. This isn’t a “just barely melted, almost but not quite” glass. It contains lechatelierite, testifying of temperatures of formation in excess of 2200 degrees C. I suspect it may be that the target rocks lacked the necessary constituents or proportions for quality glass-making, but that’s beyond my scope of knowledge. (That is, however, an idea worth considering when comparing and contrasting assorted impactites and impact phenomena).
However that may be, the rough, ribbony ropes and twists of crude, pasty glass apparently squirted from the impact interface, and may have been further processed within the rising mushroom cloud and fireball where temperatures can exceed those at the surface of the sun. In your mind’s eye, picture the explosion of a one megaton hydrogen bomb and you will have a reasonable approximation of the Aouelloul impact scene. For a bit of perspective, consider this: the energy release associated with the moldavite-producing Ries, Germany impact has been estimated as equivalent to 87,000 one megaton bombs exploding simultaneously! (Bevan, 1998)
The glass remained significantly plastic when it fell to the ground surface. In figure 2 we see a fragment of a ribbon with a flap that flopped over and welded. An impact-deformed (“splatted”) teardrop is visible in figure 1.
Most pieces are fragments--- but even these often have a slightly flattened impact-face. The material was evidently pretty brittle but still a bit gooey inside when it hit. It froze within instants of impact, recording the ground on which it fell, and the angle of landing. Most fell within a few hundred meters of the crater. There is no reported evidence of ejecta beyond a kilometer distant (and even that instance is Figure 2: Ropy 7.3 gm specimen showing a drooped limited to a rather narrow eastern sector). and welded flap of glass Aouelloul glass geochemistry also has a story to tell. When first studied (early 1950s), the origin of the Aouelloul crater was uncertain, but investigations soon disclosed that the glass composition very closely matched that of the sandstone target rocks---except for elevated iron, nickel, cobalt, iridium, and such like. Hmmm??? Meteorite anyone? Subsequently, tiny (sometimes sub-micron) Ni-Fe spherules have been observed in the glass.
Reference cited:
French, B.M. (1998) Traces of Catastrophe: A handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures. LPI Contribution No. 954, Lunar and Planetary Institute, Houston. 120 pp.
Meteorite Times Magazine
Kamil Crater (Egypt) a natural laboratory to study shock metamorphism and impact melting Agnese Fazio
Kamil Crater is one of the most recent discovered impact craters. It was incidentally identified in 2008 by Vincenzo De Michele (Italy) during a Google Earth survey while he was searching for ruins of prehistoric settlements in a rocky desert area of the southwestern part of Egypt (22°01’06’’N, 26°05’16’’E; Figure 1). In 2010, an Italian-Egyptian geophysical campaign was organized with the aims to carry out the meteorite systematic sampling and geophysical surveys (e.g., radar and geomagnetic surveys).
Fig. 1. Enhanced true color QuickBird satellite scene (22 October 2005; courtesy of Telespazio) of the Kamil area (Egypt; see inset).
Kamil is a simple crater of only 45 m in diameter. It was generated by the hypervelocity impact of the Gebel Kamil iron meteorite into sandstone rocks of the Cretaceous Gilf Kebir Formation. On the basis of archeological evidence the impact occurred likely < 5000 yr ago.
Kamil can be considered a natural laboratory to study the cratering process of small impactors (about 1-m- in diameter) on Earth and the consequences produced by them for three main reasons:
1. Geological setting. The cratering involved horizontal bedded quartz-dominated sedimentary rocks, i.e. sandstones. Shock effects of quartz are the best studied because of its mineral structure and its abundance on Earth’s surface (e.g., Langenhorst and Deutsch, 2012, Elements – http://elements.geoscienceworld.org/content/8/1/31.abstract). 2. Small crater. The Kamil Crater has a diameter of 45 m. On Earth, small impact craters (< 300 m in diameter) are rare, only 17 out of 184 impact craters already known (Earth impact Database - http://www.passc.net/AboutUs/index.html). Statistics estimate that bodies able to form small impact craters (< 300 m in diameter) occur on Earth with a decadal to secular time scale (Bland and Artemieva, 2006, MAPS, – http://onlinelibrary.wiley.com/doi/10.1111/j.1945- 5100.2006.tb00485.x/abstract). This discrepancy is due to the fact that small impact structures are more subject to weathering and geological processes such as tectonic, erosion, sedimentation. 3. State of preservation. Contrary to the great majority of Earth impact craters Kamil is very well preserved, as mainly indicated by the rayed ejecta deposit (Figure 1). The good state of preservation allows the possibility to report and to study shock effects never associated at small impact craters.
In the framework of my PhD I am working on the shock metamorphism and impact melting of Kamil. At last Annual Meeting of the Meteoritical Society in Casablanca (8th-12th September 2014) I presented two abstracts (one as oral presentation and another as poster). Thanks to the abstract entitled “Shock metamorphism and impact melting in small impact craters on Earth: Evidence from Kamil Crater, Egypt”, authored by A. Fazio, L. Folco, M. D’Orazio, M. Frezzotti, and C. Cordier, I was selected by the program committee for the meeting to win the travel grant Brian Mason Award funded by the International Meteorite Collectors Association (IMCA). The paper related to this abstract was published in the December issue of the journal Meteoritics & Planetary Science (http://onlinelibrary.wiley.com/doi/10.1111/maps.12385/abstract).
This work is a detailed report of the petrography and some chemical observations of samples from the crater wall and ejecta deposits. Samples from the crater wall do not show any evidence of shock; hence, their features reflect those of the target rocks. They are mainly made of quartz (up to 99 vol%). The accessory phases constitute ~ 2 vol% and the most common are Fe-Ti oxides, zircon, and tourmaline. The matrix is composed by kaoline and minor by iron oxides. The porosity is generally < 4 vol%.
Shock features were found only in sandstone fragments from the ejecta. Sandstone fragments show an almost complete set of shock metamorphic features including fracturing (Figure 2), planar deformation 1 features (PDFs) in quartz (Figure 3) and tourmaline, zircon decomposition (Figure 3), SiO2 polymorphs (coesite (Figure 4) and stishovite), diamond, melt veins (Figure 5) and melt films in shatter cones2 (Figure 6). Figure 2. Backscattered Scanning Electron Microscope (BSE-SEM) image showing concussion fractures in an ejected sandstone fragment.
Figure 3. BSE-SEM images showing (a) a portion of a quartz grain with four sets of PDFs and (b) a quartz grain with two sets of enlarged PDFs in an ejected sandstone fragments. The bright aggregate on the left- side of the image is a fine intergrowth of baddeleyite (ZrO2) and a SiO2 phase, resulting from the decomposition of a zircon crystal. Figure 4. Coesite occurring in an ejected sandstone fragments. a) Raman spectra for coesite and for coesite + diaplectic glass/ SiO2 melt. b) Photomicrograph of intergranular colorless SiO2 melt surrounded by brownish cryptocrystalline and amorphous material (optical microscope plane polarized light image). c) BSE-SEM image of the area of photomicrograph (b). The arrows in (b) and (c) indicate the same vesicles within the colorless SiO2 melt. d) Detail of the outer zone (white rectangle in (c)) made up of sub- micrometric coesite grains (C) embedded in a glassy matrix (G). Similar structures were described in shocked sandstone from Barringer Crater and they are also known as symplectic regions (Kieffer et al., 1976, CMP -http://link.springer.com/article/10.1007/BF00375110).
Figure 5. BSE-SEM images of a melt vein occurring in an ejected sandstone fragments. a) Finely vesicular melt vein. The bright material is enriched in Fe and Ti. Note the straight contact with the undeformed host rock, and the arrowed injection vein on the right side of the vein. b) Close-up view of the white rectangular area in (a) showing a finely vesicular portion with schlieren and relict quartz grains planar amorphous lamellae (similar to PDFs).
Figure 6. Mesoscopic and microscopic features of a sample showing shatter cones. a) Shatter cone structures with striae arranged in a horse-tail patterns. b) Close-up view of the rectangular area in image (a). Striations on the shatter cone surface radiate from a common apex. They are discontinuously coated with by a white film (100s of µm thick) of silica-rich glass (black arrows). The white arrow indicates where images (c) were taken. c) A cross sectional view of the shatter cone surface coated by silica rich glass. d) BSE-SEM image of the silica-rich glass film on shatter cone striae. Vesicles in the melt are coherently stretched and oriented indicating that frictional melting contributed, at least in part, to shatter cones formation.
In addition to sandstone fragments, impact melt lapilli and bombs also occur in the ejecta deposit. Two types of glasses constitute the impact melt lapilli and bombs: a white glass and a dark glass (Figure 7). The white glass is highly vesicular and almost exclusively made of SiO2 (lechatelierite). The dark glass is a silicate melt with variable content of Al2O3 (0.84-18.7 wt%), FeO (1.83-61.5 wt%) and NiO (<0.01-10.2 wt%). The dark glass typically includes fragments (from few μm to several mm in size) of shocked sandstone, lechatelierite, and diaplectic glass3 and Ni-Fe metal spherules. The occurrence of two type of glass indicates that the white glass experienced a negligible interaction with the projectile and, conversely, the dark glass experienced an extensive interaction with the projectile. Figure 7. Impact melt bombs (cut surfaces). a) White glass. b) Dark glass with inclusions of sandstone clasts, lechatelierite clasts, and meteorite fragments. Abbreviations: MF = meteorite fragment; SC = shocked sandstone clast; LG = lechatelierite.
Shock features found at Kamil are classified into two categories: 1- pervasive shock features and 2- localized shock features. Pervasive shock features include fracturing, PDFs, and impact melt lapilli and bombs and occupy ~100 vol% of sample. They reflect the shock pressure suffered by the target rock: fracturing, PDFs, and impact melting indicate a maximum shock pressure of 5 GPa, 20-25 GPa, and 30-60 GPa, respectively. Localized shock features include high-pressure phases and localized impact melts occurring as intergranular melt, melt veins and melt films enveloping shatter cones. They occupy less than 1 vol% of the sample. They are a consequence of a local enhancement of shock pressure and temperature corresponding to heterogeneities of the target rock.
The maximum shock pressures recorded at Kamil can be achieved through face-on impact velocities between 5.0 km s-1 (30 GPa) and 7.5 km s-1 (60 GPa), assuming an impact angle of 45°.
In conclusion, from Kamil we learnt that the hypervelocity impact of meter-sized iron meteorite projectiles can produce shock effects similar to those observed in high velocity, larger impacts. The young age of the crater (most likely < 5000 yr), the mechanical strength of target rocks and the low erosion rates of the hot- desert area played a crucial role in the preservation of all these shock features. Moreover, Kamil is the smallest impact structure where shatter cones, coesite, stishovite, diamond, and impact melt (target and projectile) have been reported.
1Planar deformation features = Submicroscopic amorphous lamellae occurring in shocked minerals as multiple sets of planar lamellae (optical discontinuities under the petrographic microscope) parallel to rational crystallographic planes; they are indicative of shock metamorphism. (From Glossary of Geology – American Geological Institute).
2Shatter Cones = A distinctively striated conical structure in rocks, ranging in length from less than a centimeter to several meters, along which fracturing has occurred. It is generally found in nested or composite groups in the rocks of impact structures and formed by shock waves generated by impact (Dietz, 1959). Shatter cones superficially resemble cone-in-cone structure in sedimentary rocks. They are most common in fine-grained homogeneous rocks such as limestone and dolomite, but are also known in shale, sandstone, quartzite, and granite. The striated surfaces radiate outward from the apex in horsetail fashion; the apical angle varies but is close to 90°. (From Glossary of Geology – AGI).
3Diaplectic Glass = Amorphous form of crystals, “solid state glass”, resulting from shock wave compression and subsequent pressure release of single crystals or polycrystalline rocks; most commonly observed for tectosilicates. (From Glossary of Geology – AGI).
Kamil Bibliography – D’Orazio M., Folco L., Zeoli A. and Cordier C. (2011) Gebel Kamil: The iron meteorite that formed the Kamil crater (Egypt). Meteoritics & Planetary Science vol. 46, pp. 1179–1196. – http://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2011.01222.x/abstract
– Fazio A., Folco L., D’Orazio M., Frezzotti M. L. and Cordier C. (2014) Shock metamorphism and impact melting in small impact craters on Earth: Evidence from Kamil Crater, Egypt. Meteoritics & Planetary Science vol. 49, pp. 2175-2200.
– Folco L., Di Martino M., El Barkooky A., D’Orazio M., Lethy A., Urbini S., Nicolosi I., Hafez M., Cordier C., van Ginneken M., Zeoli A., Radwan A. M., El Khrepy S., El Gabry M., Gomaa M., Barakat A. A., Serra R. and El Sharkawi M. (2010) The Kamil Crater in Egypt. Science vol. 329, pp. 804. – http://www.sciencemag.org/content/329/5993/804.full
– Folco L., Di Martino M., El Barkooky A., D’Orazio M., Lethy A., Urbini S., Nicolosi I., Hafez M., Cordier C., van Ginneken M., Zeoli A., Radwan A. M., El Khrepy S., El Gabry M., Gomaa M., Barakat A. A., Serra R. and El Sharkawi M. (2011) Kamil Crater (Egypt): Ground truth for small-scale meteorite impacts on Earth. Geology vol. 39, pp. 179–182. – http://geology.gsapubs.org/content/39/2/179.abstract
– Urbini S., Nicolosi I., Zeoli A., El Khrepy S., Lethy A., Hafez M., El Gabry M., El Barkooky A., Barakat A., Gomaa M., Randwan A. M., El Sharkawi M., D’Orazio M. and Folco L. (2012) Geological and geophysical investigation of Kamil Crater, Egypt. Meteoritics & Planetary Science vol. 47, pp. 1842–1868. – http://onlinelibrary.wiley.com/doi/10.1111/maps.12023/abstract Meteorite Times Magazine
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