NEW HORIZONS — Hunting for meteorites

Countless meteorites have fallen throughout history, but fossilized meteorites, which fell millions of years ago and have been preserved in sedimentary rocks, are quite rare.

Upon impact with the earth’s surface, most iron meteorites typically survive only a few dozen years. In desert environments, they may persist for several thousand years. Meteorites recovered from Antarctic ice may have fallen as long ago as 1 million years, but their terrestrial ages cannot be greater than the age of the ice sheets themselves.

Very few fossil meteorites have been found, and their discoveries have been sheer happenstance. An extremely weathered octahedrite was recovered from Miocene sediments in Georgia. Another iron meteorite was reportedly found in Texas during the drilling of an oil well in Eocene rocks. Two heavily altered chondrites were found in Middle Ordovician limestones in Sweden and identified by means of relict chromite grains.

An iron meteorite falling into a coal swamp will develop a rind of pyrite, which inhibits alteration of the interior of the meteorite. After coalification, an iron meteorite encased in coal will be preserved from further corrosion by the reduced state of the coal, particularly in a coal seam with a methane-dominated vapor base. This reduced state of coal leads us to suspect that iron meteorites may be preserved in coal seams in a relatively unaltered state.

Recently, native iron has been reported in Cretaceous coal from the Dutch Creek mine in Pitkin Cty., Colo., where it occurs at the coke-coal interface near a felsic porphyry dyke intruding through the coal seam.

A search is now under way for fossil iron meteorites preserved in coal.

Fossil meteorites may be found by examining magnetic materials captured by large electromagnets placed over conveyor belts at coal mines. Many coal mines already are equipped with such magnets, which serve the purpose of removing “tramp iron” (such as bulldozer teeth that have broken off) from the coal before it reaches the primary crushers.

Finding such meteorites is difficult, but a nation-wide search for them is under way, through the examination of magnetic materials pulled from coal by tramp iron electromagnets. In order to protect their crushers, many coal mines and processing plants suspend these electromagnets over conveyor belts; this makes it possible to remove tramp iron from the coal. Some of these magnets are also ideally designed for picking up any fossil iron meteorites that might be in the coal. Coal operators might already be, unknowingly, recovering iron meteorites and disposing of them in their scrap metal. Such meteorites may be difficult to recognize; they may be rusty, or coated with pyrite or an iron carbonate mineral. In some cases, the original iron-nickel metallic structure of the meteorite may be almost completely corroded away.

Enlisting coal miners and processors is important to the success of recovering these meteorites. Coal miners and processors can help by examining the output of their tramp iron magnets when cleaning the magnets, or when emptying their scrap iron bins.

One would expect to find about 100 grams of macro-meteorites in every 66,000 short tons of coal. If only 5% of these meteorites were magnetite (that is, iron or stony iron meteorites rather than non-magnetic chondrites) and the tramp iron magnets had a 99% recovery efficiency, then every 1 million short tons of coal would yield about 75 grams of recoverable magnetic macro-meteorites.

A large western U.S. coal operation, such as the Black Thunder mine in Wyoming, which moves nearly 34 million tons of coal per year, could be expected to yield more than 2,500 grams of magnetic macro-meteorites per year.

— The authors are with the Department of Geosciences at Pennsylvania State University.