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Impact Breccias Another Visit

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Years ago I collected many more things. As a child, I collected stamps and coins. As an adult, I have collected tektites, antique balance scales, civil war artifacts, fossils, meteorites and more. I have reduced my collecting in the last couple decades to very slowly adding some additional meteorites only.

I have stopped buying anymore impact breccias or shattercones from craters around the world. I saw those rocks as an extension of meteorites and for a while was buying quite a few of them. Just because I can not resist sharing this image here is a shattercone from the Sierra Madera crater in Texas, USA.

I have not written anything about impact breccias since December 2012. So in this issue that is where we are going again. This time with a bit more photography. Most of the rocks are just chunks of material I have never cut. Some are precious favored slices collected by friends. So I took some of the chinks to the lab and cut and polished them, and a few I thin sectioned. I had no idea what I would see in the thin sections. Now I wish that I knew a great deal more about mineral identification in polarized light. I am learning along with my readers this issue.

As I went through some of the information online about these rocks, I saw the same exact slice being called different types of impact breccia so it must be difficult even for the experts to settle on what some should be called. I may get some of this wrong it is quite confusing, and I am making a few guesses based on what I observe in my chunks of rock.

When looking at images of breccias online, I thought I recognized locations just at a glance. It is surprising how many times I was fooled by a slice of rock that was not from where I thought. I am going to approach the topic by just discussing two rock types, breccias, and chunks that are completed melted glass. There are three types of breccias, and many contain some glass and melt. But, a completely melted material nearly devoid of any remaining mineral grains is found at many craters. It is clear that the rock types mix, blend, transition and even flow in a large impact event. That small particle of crushed rock would fall from the impact plume, and land in still liquid pools of melt is easy to visualize. What characteristics a particular chunk of rock has is often dependent upon where in the crater it is dug out and of course how much of the crater has eroded away.

The first of the breccias is the Monomict Impact Breccia. As the name is similar to that for meteorites, it indicates a single parent target rock is forming the clasts found in the impact breccia. Impacts being random hit all types of target rocks. Impact breccias can be made of any rock that has been crushed and mixed into a new rock. The next image is an endpiece of a granitic boulder from the Lappajärvi crater in Finland. As can be seen, it is all clasts of the same white and black granitic rock. Some clasts have almost gneiss or schist appearance. While other clasts are nearly normal granite or only deformed a little, just beginning to show an orientation of the minerals into layer like appearance. It is impossible for me to know if the clasts were already metamorphosed before being shattered and mixed to form this boulder. Since all the bits have gone through the impact, they undoubtedly show shock now, even if it did not change them much in gross appearance. If you look closely you can see the thin lines of melt that now hold all the clasts together. As a single specimen, it is monomictic. It is all black and white granitic rocks that are commonly found near each other in batholithic formations. This is not to say that another boulder found just next to the one I have might not be made of several different parent rocks and polymict in appearance. Especially since the Lappajärvi crater was formed by an impact into mixed target rocks.

I have spent considerable time hiking around the Alamo Structure in Nevada, and many of the pieces collected from there are monomictic in appearance. They have only the gray clasts. But it too formed by an impactor hitting a variety of target rocks. Even the image shown below has colored clasts in addition to the light and dark gray dolomite. At the Hancock Summit location, much of the deposit is monomict in outward appearance, but an expert would likely never say it was monomictic. I just do not have good samples of monomict impact breccias to show that type.

Since that is the best I can do as far as samples of monomict impact breccias we have to soon move on to another type. But first this little side journey. One of the lesser discussed types of impact formations is the dike breccia. It was just for fun that I crossed over to another hill at an Alamo breccia outcropping. There I found a large dike of white crystalline rock mixed loosely with angular chunks of the gray rock found in the regular Alamo breccia. The layers of the Alamo deposit had been exposed so that I could stand on top of the thick breccia layer rather than stand before a cliff face of it. Running across the ground at my feet was the white band about a foot or more wide, traveling almost perfectly straight for many feet until it disappeared at the foot of the mountain slope behind me. I was able to recover some pieces that were detached from the dike, and they are now favorite finds for me. Pictured below is a cut down piece.

Again as with meteorites the next type of breccia from impact cratering events is the Polymict type where clasts of more than one target rock have been mixed. These clasts are in a matrix of glass or melted rock. I make that distinction because the melt is not always glass. The name Suevite has been applied to many of these polymictic breccias. It originally was used for the rock found at Ries Crater in Germany but has become the generic name for impact breccias that contain bits of glass. However, many of the impact breccias do not have much or any visible glass. Instead, the melt is opaque material made from minerals containing less silica content. The following images are of slices of impact breccia out of a deposit near Chassenon, France; part of the 23-kilometer Rochechouart Crater. There are clasts of many different rocks in this material. There are also pockets where the melt shows swirls and flowing. The material shows vugs both tiny and large. But the well-fused bits of true glass are not seen as in the gray Reis suevite in the image further down.

There are a few clasts of a nice size in the Chassenon samples. The surrounding rock is almost entirely melted. The lithification process for these breccias is based on tremendous heat, not methods such as cementation, evaporation or hydrothermal processes as might be the case in other terrestrial rocks.

The Ries Crater is 24 kilometers across and contains the city of Nördlingen. The suevite was used for building material since early times. But, the gray colored suevite with the chunks of black glass is not the only type impact breccia found in the area. Somewhat rarer is a red variety of impact melt breccia. Both of my large pieces of red suevite from Ries also have black glass. They have much less of the glass than what is seen in my similar sized chunks of the gray suevite. The red suevite is also denser, not pumiceous at all as the gray is. The red is still almost completely melted rock which has surrounded the scarce clasts of broken target rock. The red suevite is quite a solid rock.

The gray suevite from Ries, in contrast, is very friable, brittle and porous rock. It is not very strong material. Embedded in the matrix of mostly frothy glass and melt are chunks of black glass that are sometimes lens and blob shaped with stretched out internal bubbles visible when the black glass is broken. This, of course, sends my mind straight to irgizites and glass impactites from many other craters. These at Ries are incorporated in the vast deposits of suevite and not deposited on the surface surrounding the crater.

During thin section production, great care had to be used not to grind away too much of the gray suevite rock. The production of the thin sections proved to be very challenging. Unlike the dozens of meteorite thin sections I have made in the last twenty years the porous nature of the suevites and the opaque nature of the melt rock made getting it thin to 30 microns a tough proposition. I did not enjoy making these thin sections as I usually do. This first batch was too much of a learning process. I was surprised by the opaqueness of the melt that refused to ever become transparent even near correct thinness.

But I was able to find some interesting features in the microscope work. Highly shocked mineral grains will sometimes show changes in the manner they become dark when rotated between polarizing filters. Normal crystal grains will simply go dark entirely in an instant at the angle where extinction occurs. But shocked grains will show other forms of extinction instead. I apologize for the poor quality of the video and the accompanying high magnification photos in this section of the article. I am pushing my microscope beyond where it is supposed to perform to do these images. The main issue is there are just not many mineral grains in most of the thin sections. They are almost exclusively made of rock melt and glass. The few clumps and isolated mineral grains are small or individual and I have had to magnify them quite a bit.

There was one nice mineral clast in one of the Rochechouart thin sections. It is shown above. Nothing else this large was in any of the other thin sections. I might have to cut up a significant amount of my material to find a few large clasts, and that is not something I am currently interested in doing.

Undulatory Extinction is one of the optical phenomena that I found in my thin sections. Several grains displayed it. Instead of the mineral grain going dark all at once a wave of darkness will move across the grain as it is rotated by turning the stage of the microscope. This wave of darkness indicates that the mineral has been changed by the shock of the impact. Undulatory extinction can not, however, be used as diagnostic of impact since it can be caused by terrestrial processes. But it is an indicator of about 5-10 Gpa of pressure. The yellow grain seen in the accompanying video is perhaps showing some additional features. Several of the grains I found in the Ries suevite showed a dark band that moved across the grain. This is typical of undulatory extinction. The changing shadow seen in the video is broader, and though it shows as a band at the upper right edge of the crystal, it has some blotchiness about it. The shadow moves not just laterally but from the center outward also. In the 1960’s as preparations to study Moon rocks were being made the term “Mosaic Undulatory Extinction” was in use. It was a higher shock effect than just Undulatory Extinction but not as high as Mosaic Extinction. I have not seen this mixed intermediate term used in more modern research writing. But it may well describe what is seen in my video. It seems to be a borderline visual effect, both blotchy as mosaicism and moving laterally across the crystal grain as undulatory extinction.



This is a detail of the grain shown in the video above that offers a somewhat better look at the parallel fractures in the crystal.

Mosaic Extinction is also an optical property that is seen in some of the mineral grains from my Ries suevite slides. It was also seen in a clast from one of my Rochechouart slides. A still image of that clast is shown below. Dark patches of mosaicism can be seen.

The blue mineral crystal at the ten o’clock position shows dark patches as does the green crystal along its left edge and in two other smaller areas. Here my microscope begins to let me down as there are some parallel fracture features. However, they do not photograph well. They can be seen a little in the tan and pink crystal at ten o’clock next to the dark blue crystal. The fine parallel fractures can also be seen in the tan colored small crystal at the bottom right corner.

Another image showing mosaicism and parallel fracturing is shown next from a slide of gray Ries suevite. Dark areas and parallel fracturing are actually quite visible in the image especially in the split lavender colored crystal at the top left. Mosaicism is an indication of higher shock but still not extreme. It is a feature that forms at pressures exceeding about 10 GPa (Carter & Officer, 1989). The fading of interference colors under polarized light is another observed shock feature. I can not say that is the cause of the pale lavender color and generally washed out colors of the image since I am not under fine enough control of the thickness of my thin sections and polarization colors change some with thickness. But such a pale lavender pink is the color that shocked grains can display.

Before leaving the thin section portion of this article, I wanted to offer one more image. It is of two tiny grains of mineral with striking colors. The meaning of the bands, zones, and fringes was inconclusive for me as I read through the materials written about them, and no image showing the features was well described in the literature I found. Some spoke of such features being the result of partial melting and others blamed shock. But in impact crater rocks it is impossible for this writer to separate the heat from the shock. The tremendous pressure is generating both effects. Once again I wish I knew more.

While the surviving mineral grains and clusters of grains showed weak shock as undulatory extinction and moderate shock with mosaicism it must be remembered that they are embedded in thin sections of rock that are dark nearly everywhere because the remainder of the slide is opaque melt and isotropic glass. It shows no colors when placed between polarizing filters. The glass is not mineral at all in the strictest sense being amorphous. But, the fact that nothing shows under polarized light does not lessen the real importance of the melt. Where the mineral grains I found were at most only moderately shocked or maybe more if it is actual Planar Deformation Features (PDFs) being seen in the one lavender crystal shown above. The melt and glass are near the highest levels of shock being completely melted material. And where nothing shows in polarized light regarding the melt a great exciting world is seen in regular light.

Above are two sliced surfaces of glassy impact melt from the 14-kilometer Zhamanshin Crater in Kazakhstan. There is a wide variety of color combinations in the Zhamanshin glassy melt. As shown in these pieces it is often flowing rivers of one color sandwiched between other rivers of differing color. But the material is very light. It often feels lighter in weight than pumice of equal size. The images express more clearly than I can the fact that this material formed from a bubbly mass of totally melted rock. There are no visible clasts of any unmelted material. The black glass irghizites that are found at the crater seem not to be embedded in this material as with black glass in the gray suevite from Ries.

There is often seen a sort of stratification in the Zhamanshin glass. It is seen in both these specimens. One edge will be nearly solid with few and tiny (almost microscopic) bubbles. But moving across the specimen, the bubbles will gradually increase in size to very large bubbles. It has all the appearance of being a poorly mixed series of flows of liquid rock full of gas. The bubbles attempted to migrate to the surface merging and enlarging but ultimately being trapped when the rock cooled, freezing them where they were. Maybe this impression is not correct, but it explains the features seen in many samples.

Some of the impact melt breccias in my collection are pale unexciting rocks with light colors and few clasts of target rock. Some are masses of clasts with little melt. Others are beautiful mixes of dense richly varied breccia containing only pockets of melt. The Popigai breccia and Kara breccia showed below are for this writer the most interesting visually. Unfortunately, I do not have pieces suited to making thin sections, and I am not ready to sacrifice any of my slices that were acquired by friends that visited the sites. But the following images will give the reader an idea of how much more interesting the material is than some of the other craters.

The image above is a slice of breccia from the 90-kilometer Popigai Crater in Russia. Popigai has been in the news the last few years because of the trillions of diamonds created by the impact. The impact breccia comes in a variety of colors and textures. Some Popigai rock is quite light, and porous other is harder, but all seem rich in clasts of many target rocks. Making this material much more the polymict type it should be and for this writer exciting to study.

The two images above are of impact melt breccia from the Kara Crater, Russia. Kara is 65 kilometers in diameter and like Popigai formed breccia from a mix of target rocks. The one image shows breccia similar to the other piece along two edges but has a large pocket of interesting melt. The layers in the melt are colorful and thin with intricate details.

Where Popigai showed more clasts and many different types of clasts by comparison to the Chassenon, for example, the Kara material takes the trend even further. The clasts in Kara breccia are packed tightly and often locked together with little space for any melt between clasts. The sharpness of the lines, corners, and angles of the Kara breccia is striking. There is often a soft roundness to the clasts in some of the breccias. There is a real freshly broken clean, sharp edge appearance to Kara breccia. As a comparison, I offer the last breccia for this article.

Ilyinets Crater in the Ukraine is an 8.5-kilometer crater thus the smallest of the ones sampled this time around. It is interesting material and also an impact into mixed target rock. It is mostly melt with some pieces having only tiny clasts. Some pieces of Ilyinets breccia are vesicular with vugs of good size, others are more solid with either no vugs or a handful that are tiny. A few pieces show clasts that are large. The material is among the least colorful in my collection. White, cream, tan, light pinkish brown with a few darker red brown clasts are about as dramatic as the color range gets. But the material is interesting under the microscope. Some of the large clasts show fine line striations running along their length.

There are so many different rocks associated with craters that one could collect them endlessly. I have not discussed the shattercones, the impactites, the microtektites, the larger tektites, the impact spherules, the flädles, or the embedded fossils that are found at craters. Guess that is material for future articles when in a couple of years I make my way back around to crater rocks.

Just because I can not resist I am going to include an image of the recent total solar eclipse that my wife and I enjoyed in Casper, Wyoming. We drove 8.5 hours from Kansas (we had planned to just jump across to Nebraska) to avoid bad weather and set up in a hotel parking lot. We both got wonderful images and saw our first total eclipse.

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About the Author

The Meteorite Exchange, Inc. was born in 1996 with meteorite.com and Meteorite Times Magazine in 2002. Still enthusiastic about meteorites and all things related to them, we hunt, collect, cut and prepare specimens. We travel to gem shows and enjoy meteorites as much now as in the beginning. Please feel free to share any comments you have on this or any of our other sites.

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