The recovery of ancient or "fossil" iron meteorites from coal is the focus of a research project recently begun by geologists at the Pennsylvania State University in State College, Pennsylvania, U.S.A. Electromagnets used at coal mines to pull "tramp" iron from the raw coal stream may also be collecting fossil meteorites. By examining the output of these magnets and by encouraging coal miners and coal geologists to do likewise, Penn State geologists Andrew Sicree and David Gold hope to find what may prove to be the world's oldest falls of iron meteorites.

Upon impact with the Earth's surface most iron meteorites begin to rust away rapidly, typically surviving only a few dozen years. In desert environments they may persist for several thousand years or so. Those which have been recovered from Antarctic ice may represent falls which may have occurred as early as 300,000 to one million years ago but their terrestrial ages cannot be greater than the age of the ice sheets themselves.

Meteorites have fallen on the Earth throughout geological time, but fossil meteorites (i.e., those which have been preserved in sedimentary rocks and have geologically-old terrestrial ages) are quite rare. Only a few fossil meteorites are known and their discoveries have largely been matters of chance.

Henderson and Cooke (1942) report an extremely weathered octahedrite (Sardis) from Miocene sediments in Georgia although whether or not the meteorite fell during the Miocene is uncertain; the terrestrial age this meteorite was determined by C-14 methods to be in excess of 10,000 years (Buchwald, 1975). An iron meteorite reportedly was found during the drilling of an oil well in Eocene rocks in Texas but subsequently lost (Lovering, 1959). The terrestrial age of the Ider iron meteorite from Alabama has been estimated at 3.1 million years, and that of the Tamarugal iron meteorite from Chile at 2.7 million years (Buchwald, 1975). Yudin (1971) described relict chondrules of stony meteorites found in Mesozoic bauxites from the Ural Mountains. Thorslund and Wickman (1981) described a chondrite found in Middle Ordovician limestone from Brunflo, central Sweden, and Nyström, et al. (1988) reported a second chondrite from Ordovician limestones in the Österplana quarry at Kinnekulle, southern Sweden. These chondrites were heavily altered and were identified by means of relict chromite grains (Thorslund, et al., 1984). An iron meteorite reportedly from Carboniferous rocks in Ukraine has been determined to be a fragment of the Sikhote-Alin meteorite by virtue of its chemical composition and by use of the cosmogenic radioisotope Mn-53 which gives a terrestrial age of less than 10 million years (Petaev, 1992). Most recently, a small, nickel-bearing meteoritic fragment thought to have fallen 65 million years ago has been recovered from a sediment core from the floor of the northwest North Pacific Ocean (Kerr, 1996).

The reduced state of coal leads us to suspect that iron meteorites may be preserved in coal seams in a relatively unaltered state. Examination of the Fe-S-H20 phase diagram indicates that in the presence of sulfide an iron meteorite might become coated with a rind of pyrite which would inhibit alteration of the interior of the meteorite. Additional evidence that iron can survive in its native state exists. Although terrestrial native iron is rare in nature it has been found in large masses in basalts on Disko Island, Greenland (Palache, et al., 1944), and it has been noted in coal at Cameron, Clinton County, Missouri (Allen, 1897). More recently, native iron has been reported in a Cretaceous coal from the Dutch Creek Mine in Pitkin County, Colorado, where it occurs within the coal seam at the coke-coal interface near a felsic porphyry dike intruding through the coal seam (Thorpe, et al., in press).

By examining materials captured by large electro- magnets placed over conveyor belts at coal mines, one can search for fossil iron meteorites preserved in coal. Such equipment is quite expensive but, fortunately, many coal mines already have such magnets installed in order to remove "tramp iron" from the coal stream before it reaches their primary crushers. These electromagnets are highly efficient: some are capable of recovering a peanut-sized fragment of iron from beneath two feet of coal on a rapidly moving conveyor belt. In effect, coal mines already have the equipment in place to recover iron meteorites and have been doing so for years, but the output of these magnets has not yet been examined.

Estimates of the present-day flux of meteorites vary over four orders of magnitude, but most range from 100 to 1000 metric tons of meteorites per day for the whole of the Earth's surface, about 1% of which is recoverable "macro"- meteorites (Parkin and Tiles, 1968; Ceplecha, 1992). Using the lower figure, one can calculate an average macro-meteorite flux of 7.2 x 10-7 grams per square meter per year.

Estimates of the rate of coal accumulation indicate that a meter-thick seam of coal represents 1000 to 10,000 years of history. If a coal accumulated at the rate of 0.1 mm/year, the above macro-meteorite flux can be used to calculate amounts of meteorites in a typical coal seam, assuming that present-day fluxes applied to the past. Thus, one would expect to find about 100 grams of macro-meteorites in every 16,000 short tons of coal. If only 5% of these meteorites were strongly magnetic (i.e., iron, or stony-iron meteorites rather than much less magnetic stony meteorites), then every million short tons of coal should yield about 300 grams of recoverable magnetic macro-meteorites, assuming a 99% efficiency in recovery by the tramp iron magnets.

For example, in a large Pennsylvania, U.S.A., coal processing plant such as the Keystone Coal Processing Facility in Armstrong County which moves about 3.4 million tons of coal annually, recovery of about 1000 grams of magnetic macro-meteorites per year is possible. A large Western U.S.A. coal operation such as the Black Thunder Mine in Wyoming which moves about 36 million tons of coal each year could be expected to yield more than 10,000 grams of magnetic macro- meteorites per year. These calculations only give a rough estimate of the amounts present. Actual amounts of meteorites recoverable from coal seams could vary by as much as a factor of one hundred in either direction depending on factors such as the geochemical reactivity of iron-nickel and variations in meteorite flux throughout geologic time.