While these are valid points, and I respect his work, it is painfully obvious that Marco has never duplicated the experiment as I set it out in my article of a couple of years ago. The magnetite grains that make sorting of ground level samples so difficult, because their weight selects against transport by wind, make up only a tiny percentage of the material found at roof level. The dust found at roof level is primarily silicate, and is easily minimized or removed by magnet and filter sorting. I say easily, but it's relative - it takes a couple of hours to do even this simple sorting well on a half cup of material. That's nothing compared to the challenge of sorting spherules from black sands in surface or subsurface samples I've compared to. Roof top samples contain hundreds of times more magnetite grains than spherules. Ground level soil samples contain millions of times more magnetite grains than spherules. This effect will be magnified in deflationary areas, but dramatically minimized in ice. He is absolutely correct that any assumption that a significant portion of the magnetically responsive material found at roof level is of extra-terrestrial origin is false. Of the gross matter recovered, I would estimate microspherules make up less than 1 particle in a very big number (well over a million). After extensive mechanical and magnetic sorting, I would estimate they make up 1 particle in 1000 to 10000, but are easily seen and separated, as the perfect spherules are so strikingly different in shape and sheen from the various iron bearing silicates and magnetite grains. By mass, they do not account for so large a figure, even after sorting, as 1 in 1000, or 1 in 10000, as they are among the smaller particles. I would be happy to provide samples if anyone would like to make an effort to refute or confirm. - Robert Beauford : )
Message: 13 Date: Tue, 7 Jan 2003 18:05:18 -0800 (PST) From: Robert Verish <[EMAIL PROTECTED]> To: Meteorite-list Meteoritecentral <[EMAIL PROTECTED]> Subject: [meteorite-list] re: Meteoric Dust Here's some good info' that should go into the List Archives, since this subject comes up now and then: --------------- Attached Message --------------- Date: Wed, 01 Jan 2003 12:58:24 +0100 From: Casper ter Kuile (original message from Marco) Subject: (meteorobs) re: Meteoric Dust Ed [Cannon?Majden?] bombarded me as expert on cosmic dust particles in a recent email... ;-) Indeed I did some work in this field; recovering (successfully) cosmic spherules from 400,000 yrs old sediment from an archaeological excavation. Analysis at the UNM showed at least one of these to be genuine. So at least I've seen these things and some experience with searching them under the microscope. Would not consider me an expert though. But I do have a few things to say: Unfortunately, much of the magnetic particles to be collected with a collector on rooftop-level will probably not be meteoritic. As Ed rightfully remarkes, industrial waste products will [be found] among them, and magnetic spherulic particles are a known form of these. In addition, small magnetite crystals which form a natural, often abundant, component of soils, undoubtedly will be among them. I actually feel that with such an experiment (collecting with a rooftop collector), it is most likely that the vast majority of magnetic particles collected will NOT be meteoric dust particles at all. In my own experiment I used a sediment sample collected from a sealed (and thus pristine) level predating (considerably - by 400,000 years) the onset of any form of industrial pollution. And even there, I found (after many evenings of searching behind the micro, and after first using a magnetic separation technique to extract the magnetic particles) only a handfull of possible cosmic spherules, picked out from uncountable quantities of clearly non-meteoric particles, mostly magnetite grains. (The search image was for perfect spherules, as these are not likely to be natural terrestrial products, although even here, one has to be careful, magnetite grains from soils have a crystal appearance, although this is not always clearly apparent. Industrial pollution however can be almost perfectly spherulic too). Thus, the remark from the old newspaper quoted, especially the second half, that: "Almost all the meteor dust in the bucket will contain iron; other particles will not. Thus any grains picked up by a magnet can be safely assumed to be meteor dust" is certainly NOT correct, as there are many [airborne] magnetic particles that have nothing to do with meteorites at all: not only the industrial waste products, but also magnetic soil particles blown about. If you want to find meteoric dust, the best thing to do is try your hands at a sample which is likely pristine and predating the onset of industrial pollution. This is one reason (the other is connected to maximizing collection surfaces) why scientists in this field often turn to searching in samples of Antarctic or glacier ice, or deep sea sediment cores. Hope I did not spoil anyones believes, but I have seen these kind of experiments of "collect cosmic particles yourself on your rooftop!" seen advertized many times in articles or internet pages, often even as proposed highschool or primary school science class experiments and I do not believe their optimism. I think the picture these paint of the chance of success and the level of certainty that what you collect is meteoritic dust, are much, much too optimistic. Happy 2003 to all, Marco [Langbroek] ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
