Part 11

If etching produces a certain special pattern from which we can infer an 8-faced, or octahedral, crystalline structure, we recognize the second subdivision of iron meteorites: the octahedrites. This remarkable pattern was discovered and first described by Alois von Widmanstätten, of Vienna, in 1808.

The third subdivision of iron meteorites consists of the “structureless” ataxites. (From the Greek for “without arrangement.”) On an ataxite, etching brings out only a finely granular pattern with a stippled appearance.

The stones are composed chiefly of minerals that are combinations of various elements with silicon and oxygen—for example, olivine (Mg, Fe)₂SiO₄. Meteorites belonging to this division also contain combinations of elements with oxygen—such as magnesium oxide (MgO) and aluminum oxide (Al₂O₃). Usually, the stony groundmass contains scattered specks, grains, and thin veins of the same shiny nickel-iron alloy that makes up the iron meteorites almost in their entirety.

The stony-irons, as the name indicates, are an “in-between” division. Some of the stony-irons, called pallasites, are sponge-like but rigid networks of nickel-iron alloy in which the smoothly rounded openings in the sponge enclose small gemlike masses of olivine. A cut and polished section of a pallasite showing round and oval gems of yellow-green olivine set in a silvery mesh of nickel-iron is a beautiful museum specimen indeed!

In the silicate-siderites, another type of stony-iron, a nickel-iron matrix is studded with angular fragments, shreds, and splinters of silicate minerals of all sizes. In the photograph, we can see that each of the various areas of the nickel-iron matrix (lighter in color) exhibits its own distinct crystallographic orientation, as is clearly indicated by the different Widmanstätten patterns.

Even a hasty comparison of polished sections of silicate-siderites and pallasites will leave no doubt that two quite distinct modes of formation were required to produce stony-irons of such different types.

Meteoritic nickel-iron has the following average chemical composition. To the nearest tenth, this alloy contains: Iron (Fe), 90.9%; nickel (Ni), 8.5%; cobalt (Co), 0.6%. This alloy gave scientists the key to the development of commercial stainless steels. It may also contain small amounts of phosphorous, sulfur, copper, chromium, and carbon.

The average chemical composition of stony meteoritic material is somewhat more complicated. To the nearest tenth, the “stones” contain: oxygen (O), 41.0%; silicon (Si), 21.0%; iron (Fe), 15.5%; magnesium (Mg), 14.3%; aluminum (Al), 1.6%; calcium (Ca), 1.8%; sulfur (S), 1.8%. The stony material may also contain smaller percentages of nickel, cobalt, copper, carbon, chromium, and titanium.

In the stony-iron meteorites, we analyze the nickel-iron and stony portions separately. On the average, each of these portions has about the chemical composition that is given for it above.

Mineralogists have identified a variety of familiar minerals in meteorites. These include olivine, the plagioclase feldspars, magnetite, quartz, chromite, and, rarely, microscopic diamonds. All of these minerals are found here on earth in such igneous rocks as basalts and peridotites.

On the other hand, the meteoritic nickel-iron alloys (kamacite, taenite, and plessite, for example) and such meteoritic minerals as schreibersite (nickel-iron phosphide) and daubreelite (iron chromium sulfide) do not occur naturally on the earth.

We should stress here that although unusual combinations of known elements are present in meteorites, no new elements have been discovered during the increasingly intensive study of these masses during the last 150 years.

The majority of stony meteorites show a structure not found in terrestrial rocks. These meteorites are made up of rounded, shot-like bodies called chondrules (from the Greek word for “grain”). The individual chondrules may vary in size from those as large or even larger than a walnut down to dust-sized grains. The most common size is about that of peppercorns. The chondrules are often composed of the same material as the groundmass in which they are imbedded and unless the meteorite containing them is a very fragile one, they will break with the rest of the mass, as will sand grains in a quartzite. If the meteorite is fragile, however, the individual chondrules can generally be broken out whole. Meteorites containing chondrules are called chondrites.

A small percentage of stony meteorites have no chondrules. These meteorites are known as achondrites (meaning “not chondrites”) and they resemble terrestrial rocks more closely than the chondrites do. Some achondrites contain almost no trace whatever of metal, although in others (for example, the Norton County meteorite, of Chapter 2) small lumps and specks of nickel-iron are sparsely distributed through the stony groundmass.

Meteorites are as variable in shape as they are in composition and structure. Many are cone-shaped; others shield-, bell-, or ring-shaped; still others pear-shaped. One iron fragment recently recovered from the Glorieta, New Mexico, fall has been described as “macro-spicular,” meaning needle-shaped on a very large scale. The photographs opposite illustrate a number of the commoner forms known. The Glorieta specimen has been nicknamed “Alley Oop’s shillelagh,” for only a person of great strength could wield this 13-pounder with ease!

In general, the shape of meteorites depends upon the amount of mass lost by “evaporation” during passage through the earth’s atmosphere. This factor, in turn, depends not only upon the speed of transit, but also on such physical characteristics of the meteorite as its tensile strength and whether or not it contains certain alloys and minerals that vaporize more easily than the rest of the meteorite. The ring-shape of the Tucson, Arizona, iron is believed to have resulted from the “melting away” of a huge inclusion of stony material during the descent of the meteorite.

When meteorites are recovered and taken to the laboratory for study, one of the first things scientists do is to weigh them. If a meteorite is very large, special scales sometimes have to be constructed for this purpose. Such was the case for the largest meteorite so far weighed: the giant Ahnighito, Greenland, meteorite, which Peary brought to New York City by ship. (See Chapter 3.) A specially constructed scale on which this huge mass is now mounted gives for its weight about 68,000 pounds. Other meteorites famous for their great size are: the Bacubirito, Mexico, 27 tons; Willamette, Oregon, 14 tons; Morito, Mexico, 11 tons; and the Bendego, Brazil, 5 tons. All of these are irons.

The largest stone meteorite so far recovered as one mass is the so-called Furnas County, Nebraska, stone, which is the principal fragment of the Norton, Kansas, fall, and weighs about 2,360 pounds.

At the other end of the size-range, investigators have recovered meteoritic masses weighing no more than a small fraction of a gram. From a stone shower that occurred at Holbrook, Arizona, field searchers have found some of the very smallest specimens in anthills. The insects had carried these tiny meteorites along with sand and garnet grains in building their hills!

The only sure way to determine whether or not an object is a meteorite is to have a small piece of it (say, a fragment the size of an egg) tested chemically and microscopically by an expert on meteorites. Nevertheless, there are several questions whose answers will help you to decide whether or not you are on the right track in suspecting that a “rock” you have found may be a meteorite:

Is your specimen especially heavy?

Does your specimen show a thin blackish or brownish crust on its outer surface?

Does your “rock” have shallow, oval pits on its outer surface?

If the specimen has a corner knocked off, do you see specks and grains of metal on the broken surface?

Is your specimen especially heavy? The iron and stony-iron meteorites are very heavy. A 1-inch cube of iron meteorite weighs approximately 8 times as much as a 1-inch cube of ice. Even the stones, which are only about half as dense as the irons, are much heavier than ordinary rocks.

Does your specimen show a thin blackish or brownish crust on its outer surfaces? You will recall that specimens of both the Ussuri and Norton meteorites showed a “glaze” of fused material which we call fusion crust. Most freshly-fallen meteorites are covered with such a crust. To illustrate how this crust forms, consider a snowball that you bravely hold in your freezing hand until the outer surface melts. If you then were to leave the snowball outside overnight, the melted outer surface would freeze into a hard crust.

In similar fashion, the surface of a meteorite melts during the blazing-hot part of its flight through the air, only to “freeze” into a hard, firm coating in the lower, cooler portions of its path. This hardened coating, the fusion crust, is of much importance. Its presence is one of the best indications that a “rock” is really a meteorite. From the character of the fusion crust, experts can piece together a good deal about what happened to a meteorite on its way down to earth. If you should be lucky enough to find a meteorite, don’t break off the fusion crust. A whole encrusted specimen in the hand is worth 200 crustless fragments scattered at your feet!

Does your “rock” have shallow, oval pits or depressions on its outer surface? Such features are known technically as piezoglyphs (Greek piezein, to press + glyph, to carve) and popularly as “thumb-prints.” They were formed during the meteorite’s flight through the atmosphere when the softer portions of its outer shell were “eroded” away, leaving small scooped-out places. These pittings are very similar to the prints that would be made by the human hand in a lump of modeling clay or bread dough. In one case, they gave rise to the false idea that the meteorite had fallen in a plastic state and that the imprints had been formed when its finders first pulled the mass out of the ground by hand.