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Meteorite Detecting Articles
Posted on this website January 6, 2002
The following is a series of articles by Ron Smith, hope you enjoy them, if so, give Ron an e-mail!
Part 1: A little background, the story of my own meteorite
Posted by Ron(TX) (used with permission) December 27, 2001 at 20:50:11
Originally posted here: http://www.treasurenet.com/whites/forum/
There's no such thing as an "ordinary" rock anymore. I once paid little attention to rocks, assuming they were all just broken pieces of hills, mountains, lava flows and ancient ocean beds, as indeed most of them are. They come in all sizes, shapes and colors, and though once in a while a particularly interesting rock would seize my attention, I was generally unimpressed. That was before.
On August 15th, 1994 I was driving slowly down a graded dirt road. As is usually the case in this part of Texas, various rocks lined the high edges of the road, most of them dark brown with sharply contrasting dull white caliche coating the recently exposed undersides. Some of the rocks sported a shiny, black film called desert patina or desert varnish that gave them a dull luster from my point of view, looking away from the sun. It was a hot afternoon, I had the window open with the air conditioner on (wasteful, I know), and I was traveling between 15 and 20 miles per hour; any faster and I'd be raising huge clouds of dust that could obscure my vision if I drove through a cross-draft or hit one of the frequent dust devils.
On this trip, as with several before, I was looking at rocks with a purpose. Six weeks before, I had begun studying pictures of meteorites in catalogs and books at the local library. I had seen a few in museum displays, and had read accounts of others finding meteorites in various places. One fact had dominated the stories: For the most part, these rare rocks had been found by accident, and some surface detail or combination of features had drawn the discoverer's attention to that rock over all others in the area. Some subtle detail had made it stand out long enough to warrant a second and more deliberate look.
Unlike the others around it, the rock I saw had a smooth, irregular surface with no sharp edges. It was much broader at its base, and on the top had a trace of dull black where most of it was brown. I stopped, backed up and got out, walked up to the rock and kneeled to examine it closer; its mostly brown surface had a mottled appearance with a network of fine cracks. I reached for my magnet, a small 1-ounce horseshoe magnet with a four-pound pull suspended from a string which I wore around my neck. I hung the magnet close to it; when I moved it to within an inch of the rock, it snapped audibly and stuck to the surface. I got very excited, because obviously this was no ordinary rock. This one had iron in it; not oxidized iron, but real metal.
I turned the rock over, and its base was covered with light-colored caliche and the red oxide color of rust. I picked it up, feeling the greater-than-normal weight, tucked it under my arm and brought it home with me. I didn't yet know what kind, but I was certain I had found a meteorite.
On my front porch, I took a coarse grinding stone made for sharpening axe heads and ground a small spot close to the base. Once I had worked a dime-sized flat spot, I used successively smoother grades of sandpaper to polish the spot to about 400 grit. With a small 6x magnifier I could see what I now knew were chondrules, tiny spherical inclusions that do not occur in terrestrial rocks. I could see several small flakes of bright shiny metal, more numerous as distance increased from the flash-melted surface, called fusion crust, covered with an intricate interlocking system of tiny contraction cracks, the inevitable result of a glassy silicate surface rapidly cooled by the deep-frozen interior.......
Part 2: Introduction to iron meteorites
Posted by Ron(TX) December 27, 2001 at 22:39:41
My meteorite was eventually classified and described by Dr. Rhian Jones of the University of New Mexico in Albuquerque. It was an H5 Chondrite, a common type of stone meteorite, yet still a relatively rare find because of the many similarities with terrestrial stones. The first "myth" about meteorites is that they're all solid iron; of the total number of meteorites that survive their passage through the Earth's atmosphere each year, it is estimated that more than 94 percent are stone. The remaining portion are either solid iron or stony-iron.
There are, however, some very good reasons for the perception of the general public leaning more strongly toward meteorites being made of solid iron, so none of us should bear the blame for this one. The most prominent meteorite displays in major museums are usually large solid irons. Most meteorites found in the field are discovered by ordinary people, and a solid piece of iron is much more obvious than a stone which at a glance may be almost identical to the surrounding rocks. The most universally recognizable of meteorite surface features, the classic regmaglypts or "thumbprints," are far more common in irons than stones and tend to be much more pronounced. These are caused by the more rapid ablation of internal nodules of lower-melting-point minerals, and can dramatically alter the appearance of a meteorite. As the outer surface of an iron rusts and scales off, the once-dull ridges between these regmaglypts sharpen and the depressions themselves become wider. Being more durable, irons weather much more slowly and gram-for-gram will outlast their stony counterparts.
A person encountering a fresh iron meteorite in the field within a few hours of its fall would see a bluish-black shiny mass resembling a freshly welded piece of steel. The extremely thin fusion crust quickly rusts, and it's far more realistic to expect a rusted lump of iron, very heavy and possibly of wildly irregular shapes from the enormous stress of its arrival. Digressing a bit in order to further clarify this point, all the classic meteoritic craters were made by large irons. Unlike stones that shatter and melt relatively easily the irons are often unable to shed sufficient mass before reaching Earth. Though they strike at considerably less than cosmic velocity, the largest irons tend to explode, releasing all their remaining energy at once. It's of particular interest here to note that no iron fragments have ever been found in the famous Meteor Crater in Arizona, all being confined to the plains surrounding the rim. The crater is 600 feet deep and over 4,000 feet in diameter.
Irons are crystalline in structure, being composed primarily of two basic alloys of iron and nickel called kamacite and taenite. A very old and extensively weathered iron may show this crystalline structure on exposed edges, since the alloys are of different hardness and weather at different rates. Normally, irons are cut and polished, then etched with a weak (~5%) nitric acid solution to yield the familiar cross-hatched pattern we see in smaller museum specimens, called Widmanstatten patterns. Varying amounts of nickel will cause various sized crystals, with some Widmanstatten patterns being finer than others. The appearance of the patterns depends largely on how the polished and etched surface was cut in relation to the internal crystalline structure. There can, however, be so much or so little nickel (>30% or <5%) that crystals are prevented from forming, and when etched the polished face will be virtually featureless except for nodules of graphite or troilite (iron sulfide)..........
Part 3: "This doesn't come from around here, does it?"
Posted by Ron(TX) December 28, 2001 at 08:38:58
Because of the general public's idea of the classic iron meteorite (and they most certainly DO exist, no doubt about that!), more pieces of ordinary iron are mistakenly thought to be meteorites than any other artifacts. I've made this same mistake myself, recently, and in the end it's always best to be sure before you throw the piece away. However, there are some natural substances which mimic meteorites even better than unusual terrestrial rocks or rusted scrap iron. The stone variety I'll try to cover in more detail later because of its critical importance, but on the subject of irons, those in the northwest should be aware of an additional complication.
Josephinite is a rare terrestrial nickel-iron found in Josephine County, Oregon, and can understandably be confused with iron meteorites. I've never seen any, but as closely as this substance is reported to resemble meteoritic iron, I seriously doubt it contains the mineral inclusions common in iron meteorites. Still, it apparently does satisfy a chemical test for nickel and is said to have crystalline structural characteristics, so, as with the stony types, never assume a specimen is genuine, have it checked professionally to make sure.
A brief additional caution is appropriate here. Since the science of meteoritics is not generally taught past an introductory level in the study of geology, taking a rock you suspect to be a meteorite to the local university for identification isn't always a good choice. Meteorite identification is often much more complicated than a simple visual inspection, and I know of at least one incident in which a local rock-hound was told he had a basket full of small meteorites; to me, they were obviously hematite. The man insisted they were genuine, and rather than needlessly antagonize him I simply suggested he donate one to the university as a meteorite. I think he got the message, and I sincerely hope he avails himself of the proper resources in the future. I brought mine to a local rock shop where the owners had many years of experience identifying rocks and minerals; they had no idea what it was and politely refused to cut it with their rock saw. I had to suppress a chuckle when one asked me with a curious look, "This doesn't come from around here, does it?" Normally no meteorite dealer or collector worth his salt will buy or trade for a suspected meteorite without authentication from an expert source, so always leave final judgement on authenticity to experts.
Back on the subject, field recognition of stony meteorites obviously can be more difficult. Many of them may look so similar to any surrounding rocks (assuming your area has rocks) that a glance just isn't enough to draw your attention. Still, some physical properties of meteorites in general and stone meteorites in particular will make them stand out in a crowd.
Genuine stone meteorites usually have smooth surfaces, and except for obvious breaks will have no sharp edges. They will usually have a smooth, hard melted coating called fusion crust, which is most often black or brown but depending on the type of meteorite can be lighter colors as well. Fusion crust is never a thick coating, normally only 1mm-2mm thick, though the ablation process in which melted material is pushed along the meteorite's surface and falls away behind it in flight can deposit fusion crust somewhat thicker in some areas. Fusion crust is often fragile, and every effort must be made to preserve it because its presence and the percentage of its coverage will affect the value of a meteorite or fragment. Underneath the thin coating is the stony matrix, unaltered by the heat of its melted surface but possibly affected to some depth by external weathering from water and terrestrial minerals.
Fusion crust is often broken by a network of contraction cracks caused by rapid cooling of the silicate or glassy material by the deep cold of its interior as soon as passage through the Earth's upper atmosphere slows to the point where friction no longer causes it to melt. In the field, meteorites can sometimes be spotted by the contrast between the darker color of this crust and the somewhat lighter color of the stone's interior. Sometimes fusion crust will be roughly the same color as the stone's matrix, as was the one I found, but I'm aware of no case where the interior of a stone meteorite is actually darker than its fusion crust. Effects of weathering turning what was once black crust to brown does seem to be probable, resulting in fusion crust being lighter in appearance than the material underneath, though I haven't personally seen such a stone..........
Part 4: Shapes, surface features and simple tests
Posted by Ron(TX) December 28, 2001 at 14:42:15
Aside from fusion crust, other surface features that can be used to recognize stone meteorites in the field are the presence of rust, pitting (which is not typical of the majority of stone meteorites and is never true of the smaller specimens), and flight orientation. Meteorites are rare, and flight-oriented meteorites are rarer still, but can be recognized in fresher falls by lines and tiny ridges radiating from a central point on the stone. Most meteors apparently tumble during their fiery flight to Earth, but a few achieve stable flight and therefore bear the marks of ablated material flowing down the sides toward its base and depositing there. Moderately weathered oriented stones will still retain their basic shape, but many of the finer flight markings will disappear as their fusion crust weathers slowly away.
Flight-oriented meteorites are much closer to our everyday experiences than most of us may realize. When our space program was in its infancy and the problems of extreme stress on re-entering spacecraft were being considered, the curious conical shape of many well-oriented meteorites manifest itself in our basic hardware. The shapes of the Mercury, Gemini and Apollo manned spacecraft were no accident; scientists and engineers roughly copied the blunt aerodynamic shape of these objects which had already survived successful stable flights through our upper atmosphere and landed on Earth.
One of the best indicators of a stone's iron-bearing interior is a simple magnet on a string to check for the presence of iron. Mine is a one-ounce horseshoe magnet with a four-pound pull that I once wore religiously around my neck, and still do at times (rust stains in the middle of a white T-shirt can be difficult to explain). Suspended from a string, there is usually nothing subtle about the magnetic attraction when one applies this simple test to a suspect stone. Weak magnetic attraction may be due to common minerals in some terrestrial rocks, so this test is by no means conclusive, but it's a good initial indicator. I once found a dark green stone which exhibited a noticeable magnetic attraction despite its small size and sent it to a laboratory to have it tested. It took almost a month and recalibration of the lab's electron microprobe to eliminate it as a possible meteorite; they were aware, as I was, that there are genuine meteorites (achondrites) that look almost identical to the one I found, but the test was conclusive, it was terrestrial. Meteorites are made of the same minerals as are terrestrial rocks; only the concentrations, combinations and mechanics of formation are different.
If you suspect you have a stone meteorite and it passes the magnet-on-a-string test, the next step is a visual inspection of its interior. Before you run for your hammer, all you need is to grind a small dime-sized spot on its surface and polish it to the point where you can examine the matrix with a low-power magnifier. Small spherical inclusions known as chondrules are a dead giveaway, as are specks of shiny metallic iron; free iron oxidizes rapidly and terrestrial stones containing iron are probably more rare than meteorites, and chondrules are definitive of certain types of meteoritic stone. They do not occur in nature on Earth, and no one has ever been able to duplicate them.
Chondrules can be as small as 1mm in diameter or as large as 7-8mm, and are indicative of the class of stone meteorites called chondrites. There are several subtypes, reflecting not only their percentage of iron by weight but the degree of alteration of their chondrules due to heat, stress and chemical action as well. My meteorite, for example, was an H5 chondrite, meaning it has relatively high iron content and the chondrules it contain had been significantly altered since the formation of its stone matrix. The least altered, and therefore most "primitive" of meteorites have subtypes 1 and 2 on the petrologic scale, and subtype 7 is the most altered, the chondrules being totally destroyed or absent from re-heating and melting of the stone. As you can begin to see, the meteorite I found was nothing unusual among its companions.........
Part 5: If I find it, what do I do with it?
Posted by Ron(TX) December 28, 2001 at 17:02:06
A number of rare types of meteorites comprise the achondrites, originally understood to be those without chondrules, though this basic distinction isn't always the case. Achondrites account for somewhat less than 10 percent of all known meteorites, and can be much more difficult to identify. They do, however, normally have the same range of shapes and surface features as the more common chondrites, and it's these shapes and surface features which cause us to give any unusual stone a second and more critical glance. How well one can learn basic meteorite shapes and surface features will ultimately determine whether he or she recognizes a genuine meteorite in the field or walks over it as the vast majority of others will do (pictures are wonderful teachers in this regard, but are still a poor substitute for seeing actual meteorites with your own eyes, so try to see as many collections as possible in museums).
Chemically, achondrites are grouped according to their total calcium content; the only distinction calcium content will have to us as hunters of these stones is that generally the higher the calcium content, the darker the meteorite's fusion crust will be. Among achondrites, it's common to see not only glossy black fusion crust but light-colored as well; I have a small specimen of a larger stone whose fusion crust was a milky gray color, unweathered because it was a witnessed fall and was recovered almost immediately. Many achondrites have jet-black fusion crust, which is a tremendous aid to anyone searching for them in the open, and if even slightly weathered will show rust on their black surface. Studying the interiors of these stones for clues to resolving any doubt about their extraterrestrial origin is even more importantly left to experts. Among meteorites, achondrites are the ones that most closely resemble common terrestrial rocks, and conclusive identification is often dependent on careful analysis that most of us cannot provide. Even an educated guess just isn't good enough.
Fortunately, there are laboratories that can take care of that for us. The one listed below is the only one I know of presently that will do this testing free of charge; I don't think they'd appreciate everyone here sending them a box full of meteorite "possibles" (it's estimated that only one or two of every two thousand possible meteorites will turn out to be the real thing), but if you have a stone or piece of iron you just can't seem to throw away because you genuinely believe it may be a meteorite, by all means, make sure. Here's how.
If the stone or iron is unusually large or you don't want to part with the whole mass (things DO get lost and damaged in shipping), send a small piece (dime-sized) to the lab for analysis. Detaching the piece will vary according to type; with stones, breaking off a piece is always risky because you risk shattering the specimen and decreasing its potential value. However, though cutting it in a rock saw is much safer, doing so not only wastes material (the thickness of the blade x the dimensions of the stone at the location of the cut will be lost) but the various materials used as lubricants may seriously damage a meteorite. Water-based lubricants are common, and aren't good to apply to meteoritic stone. A friend and fellow collector let me use his rock saw, which was used a special lightweight non-flammable type of oil for lubrication.
Cutting a piece of an iron meteorite presents a different problem. The alloys and crystalline nature of iron meteorites give them the durability of an anvil, and cutting with a hacksaw is difficult at best. NEVER use an oxy-acetylene torch to cut an iron meteorite or cut it with a band-saw without adequate lubrication; the resulting heat will destroy its internal crystalline structure, rendering it practically worthless. Drilling is an option if one can control the heat generated, but the waste of valuable metal would hardly justify doing so. Still, options are limited, so in extreme cases seek help from an expert or utilize secure shipping methods, or if you live within driving distance, hand-carry the piece.
Send suspected meteorites to:
Dr. Carleton B. Moore, Director
Arizona State University
Center For Meteorite Studies
Main Campus, P.O. Box 872504
Tempe, AZ 85287-2504
Part 6: What's in a name?
Posted by Ron(TX) December 28, 2001 at 18:57:40
Getting back to general information, meteorites are given place names which must be approved by the nomenclature committee of the international Meteoritical Society. I chose the name "Barrilla" for mine, after the nearby Barrilla Mountains, (no, you can't name them after yourself, though the stony-iron subtype Pallasite was named for an 18th century German named Peter Simon Pallas) and since there was no conflict with other existing names it was accepted. Meteorites are named for the closest major geological feature or location, and many of them have been named for the cities or towns closest to the site of the actual discovery. Most dealers and collectors will not buy, sell or trade even authentic meteorites unless they have an accepted location name, and there are good reasons for this, chief among them being the desire to avoid being fooled into dealing what appears to be a one-of-a-kind stone which later turns out to be one from a known fall. Occasionally, duplications will result; recently, there was a brisk business in hundreds of individual stones sold and traded under two different accepted names. The Meteoritical Society eventually determined that all the stones came from the same fall and gave them all the same location name.
Location names can be a real problem for those collecting hundreds of meteorites on the Antarctic ice, and a detailed system of nomenclature had to be devised for that purpose. For example, one of the more famous is Allan Hills 84001, an achondrite with apparent Martian origin. I once heard a lecture at the local university by a scientist who had collected meteorites in Antarctica during one of its short field seasons in which he referred to Allan Hills 84001 as "...the first meteorite found in the 1984 field season...", a claim which I had heard many times before. My suspicions were aroused, particularly since my information said it was recovered close to the end of the collecting season. I sent a letter to the editor of "Meteorite!" magazine in New Zealand, who did an investigation of his own and replied (in the magazine and in a separate letter to me) that Roberta Score, who actually found the meteorite, related that over a hundred meteorites were recovered that season before ALH 84001. It was collected in the Allan Hills area of Antarctica during the 1984 field season, and because of its obviously rare nature it was catalogued first so that it could be studied without delay.
Meteorites of Martian origin and a few of lunar origin have been catalogued, and when available at all can sell for exorbitant prices. I once saw an advertisement for a small vial of dust (total weight about 0.7 grams) selling for an even $50,000.00. Not in my budget for this year either. With my luck, it would turn out to be floor sweepings.
I knew exactly where to start this little series, but as odd as it may sound I don't know quite where to stop. There's so much good information available that it's hard to formulate an ending. So, let me make this offer: Keeping in mind that I'm most definitely not an expert, at this point if anyone has any questions, ask them and I'll try my best to either provide an answer or point you in what I believe is the right direction.
The very best all-around introductory work I've ever found for those with further interest is a book titled "Rocks From Space" by O. Richard Norton (available in paperback when I bought mine a couple of years ago for $20.00, may be more now), Mountain Press Publishing Company, P.O. Box 2399, Missoula, Montana, 59806, 1994 (ISBN 0-87842-302-8). The book even has a section on searching for meteorites with metal detectors, and though the information it gives is all basic, it quite prominently showcases the White's Spectrum XLT............Ron
Posted on this website January 6, 2002
An article by Ron Smith, hope you enjoy it, if so give Ron an e-mail!
XLT meteorite readings (long, dry and boring)
Posted by Ron(TX) (used with permission) December 26, 2001 at 14:46:15
Original forum post link: http://www.treasurenet.com/whites/forum/
These are the results of some simple tests I conducted with a few representative samples of meteorites from my small collection. My XLT was ground-balanced to a predetermined "clean" spot and each specimen was subsequently placed within that spot, lying on the surface. I used the standard 9.5" coil and kept the XLT factory relic program as close to the factory settings as possible in an attempt to establish a baseline useful for comparison among different machines.
The overwhelming majority of the readings noted agree with those earlier posted by Ed in SD (12-17-2001, 18:59:57), although those here were taken with a different machine and will be more recognizable to many White's users. Types selected were solid iron, Pallasite (even mix of iron and stone), Mesosiderite (uneven mix of iron and stone), high-iron stone and low-iron stone. With the few samples available to me, I had no opportunity to compare the effects of different sizes and shapes, but the information may be of some use to anyone considering such a venture. Thanks again to Ed for the idea of testing meteorites directly for a comparison with known targets; though I have no plans to hunt meteorites with my XLT, his post raised some valid questions, especially considering the wide range of values (from .25/gram to as high as $50,000.00/gram) of meteorites in the present market, and at least I have temporarily satisfied my own curiosity.
GIBEONIron, fine octahedrite with small troilite inclusions, 49.3 gram irregular piece with ground, polished and etched surface, measuring 41mm x 28mm x 3-14mm: VDI @ -73, single high bar on Signagraph with adjacent low bars on either side, audible to 8 inches above target, XLT relic program with discriminator set to accept 95 to +95.
ESQUELPallasite, small polished partial slice measuring 43mm x 24mm x 2mm, continuous nickel-iron matrix with ~50% olivine (peridot) crystals: VDI @-67, single high bar on Signagraph with no adjacent bars, audible to 6 inches over target, XLT relic program with discriminator set to accept 95 to +95.
VACA MUERTAMesosiderite, uneven mix of nickel-iron and stone, end piece measuring 52mm x 39mm x 14mm, one flat surface ground and polished: VDI @-87, one high bar with one adjacent half bar on Spectragraph, audible to 4 inches over target, XLT relic program with discriminator set to accept 95 to +95.
BARRILLAStone, H5 Chondrite (high nickel-iron content, max. 27% total iron by weight), full 633.7 gram slice measuring 189mm x 114mm x 12mm, ground and polished: VDI @-93, single bar 75% height, audible to 5 inches over target, XLT relic program with preamp gain increased from 2 to 8, discriminator set to accept 95 to +95.
ALLENDEStone, CV3 Carbonaceous Chondrite (very low nickel-iron, high carbon content), complete individual stone, 21 grams: No consistent VDI reading, irregular scatter of very low bars on Signagraph, audible to 4 inches over target, XLT relic program with preamp gain increased from 2 to 8, discriminator set to accept 95 to +95.
With two of these specimens (Vaca Muerta, Allende) a characteristic decrease in VDI number with increasing distance from the target, regardless of preamp gain setting, suggests that a meteorite of lesser mass may be barely noticed in audio and perhaps not at all in Signagraph or VDI. This factor alone would preclude searches for stone or smaller iron meteorites at even moderate depths in all but the most iron-free areas.
Though there is at least one indicator of the possible presence of a specific type of meteorite (Esquel or other unweathered and minimally oxidized Pallasite, which in test yielded the highest VDI reading that did not decrease with distance from the target and displayed a single high bar on the Signagraph), there is probably no reliable method of distinguishing a meteorite from other buried targets of varying iron or mineral content.
Mineralization of buried meteorites would undoubtedly alter the readings described above, and this would be further complicated by the scaling effect of meteorites in general as they decompose (depending on variables such as humidity and type of soil) and solid iron meteorites in particular, as I'm sure anyone who has noted the difference in readings of relatively fresh and badly rusted bottlecaps can imagine.
The overall mass of stone meteorites of even high iron content would seem to be extremely important, due to the fact that their total iron content is not only low but usually spread evenly throughout the meteorite's matrix, most in the form of free iron flakes but some at the molecular level. It should be noted also that the fusion crust of fresh meteorites, though very thin, contains some magnetite, which may also affect the final reading. Increases in preamp gain will apparently lead to a corresponding increase in detection, but the resulting loss of stability may render the exercise useless.
To further complicate the possibility of finding meteorites with a metal detector, many terrestrial stones will without a doubt read similar to meteorites. Though they contain no elemental iron, certain iron oxides may still fool a metal detector. I still have an obviously ordinary rock that I saved for one specific reason: It's the only terrestrial rock I've ever encountered that actually causes a "hit" on my Bullseye pinpointing probe............Ron
Great Post, Ron
Posted by Ed in SD December 27, 2001 at 22:47:02
In Reply to: XLT meteorite readings (long, dry and boring)....posted by Ron(TX) on December 26, 2001 at 14:46:15
I too noted the similarities between the way our different brands of detectors responded. The White's appeared to have more depth in your tests, but since the samples and conditions were totally different, that conclusion might be unfounded.
Since the signals fade so quickly with increasing depth or decreasing size, it might almost be easier to hunt stonys by sight and then use the detector to check them individually. Or the detector could be used to search for any signal coming from "mere" rocks, then you'd have to apply your own battery of tests. Digging all iron and all hot rock signals would be a must. Fortunately, the more meteorite samples you see in person, the easier it is to recognize many types. Others are just so much like an ordinary rock you might need a molecular analysis to determine the origin.
I'm glad someone with an XLT was able to participate, we weren't able to ground balance the one we tried indoors at the museum.
What the tests reveal is that most any detector would probably give some sort of response to many common meteorites. One needn't get a top-of-the-line model, in fact an obsolete, inexpensive unit that is more prone to reading ground disturbances might be more responsive to meteorites in the field.
Did you notice any audio nulls when testing the stonys? Or the sample behaving differently when passed under the coil, such as responding on the edges rather than the center of the coil? Were you able to read any nickle in your samples, or was it predominately an iron signal?
Thanks again for a great post!
Re: Great Post, Ron
Posted by Ron(TX) December 27, 2001 at 23:57:42
In Reply to: Great Post, Ron posted by Ed in SD on December 27, 2001 at 22:47:02
Thanks, Ed, it was an interesting (though superficial) study, and I learned some things.
I agree about the visual search, and a detector with known responses would be a valuable tool in certain circumstances. For instance, because of the incoming angle of a meteorite, strewnfields are elliptical in shape, with the smaller stones at one end of the major axis and the largest fragments with the straightest trajectories at the opposite end. If one could visually locate a couple of small stones on the surface, it would be practical to use a detector to search the immediate area for any buried stones. A little windblown dust is all it would take to obscure a small meteorite from sight, but a detector could possibly uncover dozens more.
No audio nulls that I noticed, but I wasn't really expecting any, may be a good reason to go back and re-test to some extent, with the question of coil position in mind as well. Might also be interesting to see what effects different sized coils generated.
As for the nickel content, I can't prove it but I believe the Esquel Pallasite read different from the others for one of two reasons: Either the nickel content caused it to be more consistent (not too likely because the Gibeon is a fine octahedrite with possibly even higher nickel by weight) or the XLT coil's RF field found the even distribution of the iron and silicate matrix appealing for whatever unknown reason. After all, a thin slice like that is essentially holes linked like chain mail to the detector, which would ignore the minerals, and maybe it was reacting to what it saw as a series of small rings. A larger piece of the same Pallasite, more 3-dimensional in appearance, might therefore read totally different, more like a solid iron.........Ron
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