Meteorite hunting in a Wisconsin Gravel Pit
Last Updated: 17th Feb 2015By Bill Cordua
Note: This first appeared in a Midwestern Federation Newsletter
As a geology professor in Wisconsin I am often called upon to identify rocks and minerals for the general public. Most often what they want to know if what they found is a meteorite. Most often I have to tell them “meteorwrong”. One youngster asked me “Why can’t you sometimes find meteorites in Wisconsin gravel pits?” After all, the glaciers and rivers depositing the gravels certainly came across and transported meteorites on occasion, so why not look for them in gravel pits? He’s right – you should be able to, but the problem is how to recognize them.
Meteorites can be spotted in environments where few other rocks are accumulating, or where they look substantially different from what is ordinarily there. In the Sahara, for example, where there are miles and miles of light-colored sand, a black meteorite cobble will stand out. So will an odd rock perched by itself on top of an Antarctic glacier.
But Wisconsin gravel pits are tough environments for finding meteorites. The rocks are deposited dominantly by rivers and glaciers, where millions of rocks from many different environments are jumbled together. Only the hardiest of rocks will survive the mechanical grinding of centuries in the ice and fast-moving rivers. Plus meteorites, especially stony ones, weather fast, and won’t stand up to the harsh earth environment very long. In addition, Wisconsin gravel pits contain many earth rocks that are magnetic - primarily banded iron ores from local iron ranges and gabbroic rocks with significant magnetic ilmenite and magnetite. These are often mistaken for meteorites. So there is a lot of what you might call “noise” to “signal” in seeking out the few magnetic extraterrestrial visitors that survived.
Still, every once in a while, there ought to be a glacially transported meteorite in a gravel pit. After all, float copper nuggets turn up from time to time. Of course they weather a nice green color, so are easy to spot. I decided to see what it would take to find a meteorite in the challenging environment of a gravel pit.
I was invited to visit a gravel pit in St. Croix County Wisconsin in early summer, 2013. This is not far from where a documented 53-pound iron meteorite, the Hammond meteorite, was found in a farmer’s field in 1884. I came armed with ToolShop magnetic pickup – a strong magnet on a four-foot long wooden handle, which I got it on sale at the local Home Depot. I wrapped a cloth cover on the magnet so that magnetic minerals would be easier to remove. I tested my tool against both iron-nickel and stony meteorites, and found both stuck well to it.
In the pit I focused on washed piles of pebble-sized materials, thinking my chances were better with smaller objects. Sure enough, I picked up quite a few pieces that stuck to the magnet. I carefully put these into a small sample bag to wash and examine at home. Once cleaned it was easy to see than many were banded iron ore and others were magnetic pieces of gabbro.
I did a have small set of pieces that were not clearly so easy to identify. I chipped off the ends, and found these were all massive magnetite – no iron-nickel metal. Had I failed in my search? Not so fast! Meteoritic iron (the iron-nickel minerals taenite and kammacite) does alter to magnetite in fusion crusts that form as coatings on meteorites as they partly melt coming through the earth’s atmosphere. Also iron-nickel metal can weather to magnetite when exposed to earth conditions. There was still a chance one or more of my little nondescript chunks were weathered meteorite. The key was determining if any of them had significant nickel in them.
Terrestrial magnetite commonly includes only trace amounts of nickel. Iron minerals in meteorites usually contains form 5% to over 7% nickel. I reasoned that, even after conversion to magnetite, weathered meteoritic material should have higher than trace amounts of nickel still there.
To test my remaining unknowns, I bought a little meteorite test kit from a company called Meteorites Plus. This allowed me to make a quick qualitative wet chemical test for nickel using the chemical dimethylglyoxime. Note 1: this test requires using concentrate ammonia and hydrochloric (or muriatic) acid and is not for children or for those disposed to ignore printed instructions and MSD sheets! Following the instructions I mixed the appropriate solutions and tested them on the powders of a few meteorites – one from my collection and one sent as part of the kit. (Note 2: grinding up meteorites to test makes me cry, but fortunately you don’t need much sample). Then I tested my unknowns in the solutions.
The two known meteorites I tried did stain the solution a lovely purplish-pink, as they should with significant nickel. Sad to say most of my gravel pit suspects tested negative for significant nickel. Apparently they were massive magnetite of earth origin, weathered free of matrix so their source wasn’t obvious upon examination. BUT one of my samples tested positive for nickel This piece seems otherwise indistinguishable from similar pieces showing a negative nickel test.
Is this a weathered meteorite fragment? I did prove it was a nickel-bearing magnetite. Some library research on magnetite geochemistry revealed that nickel in terrestrial magnetite is low, only in rare cases approaching 1%. The ID as a meteorite fragment remains the best theory. So, I think it is possible, with determination, to find a meteorite fragment in a local gravel pit. All my collecting buddies got that day in the pit were a few measly Lake Superior agates!
As a geology professor in Wisconsin I am often called upon to identify rocks and minerals for the general public. Most often what they want to know if what they found is a meteorite. Most often I have to tell them “meteorwrong”. One youngster asked me “Why can’t you sometimes find meteorites in Wisconsin gravel pits?” After all, the glaciers and rivers depositing the gravels certainly came across and transported meteorites on occasion, so why not look for them in gravel pits? He’s right – you should be able to, but the problem is how to recognize them.
Meteorites can be spotted in environments where few other rocks are accumulating, or where they look substantially different from what is ordinarily there. In the Sahara, for example, where there are miles and miles of light-colored sand, a black meteorite cobble will stand out. So will an odd rock perched by itself on top of an Antarctic glacier.
But Wisconsin gravel pits are tough environments for finding meteorites. The rocks are deposited dominantly by rivers and glaciers, where millions of rocks from many different environments are jumbled together. Only the hardiest of rocks will survive the mechanical grinding of centuries in the ice and fast-moving rivers. Plus meteorites, especially stony ones, weather fast, and won’t stand up to the harsh earth environment very long. In addition, Wisconsin gravel pits contain many earth rocks that are magnetic - primarily banded iron ores from local iron ranges and gabbroic rocks with significant magnetic ilmenite and magnetite. These are often mistaken for meteorites. So there is a lot of what you might call “noise” to “signal” in seeking out the few magnetic extraterrestrial visitors that survived.
Still, every once in a while, there ought to be a glacially transported meteorite in a gravel pit. After all, float copper nuggets turn up from time to time. Of course they weather a nice green color, so are easy to spot. I decided to see what it would take to find a meteorite in the challenging environment of a gravel pit.
I was invited to visit a gravel pit in St. Croix County Wisconsin in early summer, 2013. This is not far from where a documented 53-pound iron meteorite, the Hammond meteorite, was found in a farmer’s field in 1884. I came armed with ToolShop magnetic pickup – a strong magnet on a four-foot long wooden handle, which I got it on sale at the local Home Depot. I wrapped a cloth cover on the magnet so that magnetic minerals would be easier to remove. I tested my tool against both iron-nickel and stony meteorites, and found both stuck well to it.
In the pit I focused on washed piles of pebble-sized materials, thinking my chances were better with smaller objects. Sure enough, I picked up quite a few pieces that stuck to the magnet. I carefully put these into a small sample bag to wash and examine at home. Once cleaned it was easy to see than many were banded iron ore and others were magnetic pieces of gabbro.
I did a have small set of pieces that were not clearly so easy to identify. I chipped off the ends, and found these were all massive magnetite – no iron-nickel metal. Had I failed in my search? Not so fast! Meteoritic iron (the iron-nickel minerals taenite and kammacite) does alter to magnetite in fusion crusts that form as coatings on meteorites as they partly melt coming through the earth’s atmosphere. Also iron-nickel metal can weather to magnetite when exposed to earth conditions. There was still a chance one or more of my little nondescript chunks were weathered meteorite. The key was determining if any of them had significant nickel in them.
Terrestrial magnetite commonly includes only trace amounts of nickel. Iron minerals in meteorites usually contains form 5% to over 7% nickel. I reasoned that, even after conversion to magnetite, weathered meteoritic material should have higher than trace amounts of nickel still there.
To test my remaining unknowns, I bought a little meteorite test kit from a company called Meteorites Plus. This allowed me to make a quick qualitative wet chemical test for nickel using the chemical dimethylglyoxime. Note 1: this test requires using concentrate ammonia and hydrochloric (or muriatic) acid and is not for children or for those disposed to ignore printed instructions and MSD sheets! Following the instructions I mixed the appropriate solutions and tested them on the powders of a few meteorites – one from my collection and one sent as part of the kit. (Note 2: grinding up meteorites to test makes me cry, but fortunately you don’t need much sample). Then I tested my unknowns in the solutions.
The two known meteorites I tried did stain the solution a lovely purplish-pink, as they should with significant nickel. Sad to say most of my gravel pit suspects tested negative for significant nickel. Apparently they were massive magnetite of earth origin, weathered free of matrix so their source wasn’t obvious upon examination. BUT one of my samples tested positive for nickel This piece seems otherwise indistinguishable from similar pieces showing a negative nickel test.
Is this a weathered meteorite fragment? I did prove it was a nickel-bearing magnetite. Some library research on magnetite geochemistry revealed that nickel in terrestrial magnetite is low, only in rare cases approaching 1%. The ID as a meteorite fragment remains the best theory. So, I think it is possible, with determination, to find a meteorite fragment in a local gravel pit. All my collecting buddies got that day in the pit were a few measly Lake Superior agates!
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