9. THE NATURE OF METEORITES
So far in this book we have dealt with meteorites indirectly, chiefly in connection with their fall, distribution, and recovery. In this chapter, however, we are shifting our attention to the meteorites themselves, and will tell what the main types of meteorites are, what meteorites are made of, what they look like, and how to tell them from ordinary rocks.
First of all, meteorites neither all look alike nor have the same composition. The general term “meteorite” applies to any mass that reaches the earth from space. Such masses are made up of metals and minerals in varying proportions. The term “meteorite” is nearly as general in meaning as the word “rock,” which geologists apply to bodies, large and small, that are formed by earth processes and are composed of various kinds of minerals. Actually, there are almost as many different kinds of meteorites as there are kinds of rocks; so you can see that in meteorites a wide range of composition and appearance is possible.
All recognized meteorites belong to one of three main divisions,[7] irons, stones, and stony-irons.
The irons are composed of an alloy of iron and nickel which may contain small inclusions of nonmetallic minerals.
Internal structure revealed when the “etching” process is applied to that type of meteorite known as a “granular hexahedrite.” See [p. 120].
After a cut section of an iron meteorite has been polished, the flat surface, except for possible inclusions, is mirror-like and resembles stainless steel. It appears to be remarkably uniform and uninteresting, but this appearance is misleading. A characteristic and beautiful structural pattern develops when such a polished nickel-iron surface is treated with, for example, a special mixture of nitric acid, alcohol, and Arabol glue.
This process of treatment is known as “etching.” The different structural patterns brought out by such etching give us the basis for classifying the iron meteorites.
If the etching process reveals certain features from which we can infer a cubic, or 6-faced, crystalline structure, we classify the iron meteorite as a hexahedrite.
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.
A. BREZINA & E. COHEN PHOTO Widmanstätten pattern which emerges when the carefully polished surface of that type of iron meteorite technically known as a “fine octahedrite” is “etched.”
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.
A. BREZINA & E. COHEN PHOTO Enlarged section of a stony-iron meteorite showing rounded olivine grains (dark in color) set in a network of nickel-iron alloy (light in color).
A. BREZINA & E. COHEN PHOTO Polished and etched section of a silicate-siderite showing angular fragments of silicate minerals (dark in color) imbedded in a metallic matrix.
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.
COURTESY OF AMERICAN MUSEUM OF NATURAL HISTORY Microphotograph of a thin section of a chondrite, showing the circular, or nearly circular, cross sections of a number of chondrules, including one of large size at the upper edge of the section.
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.
CHICAGO MUSEUM OF NATURAL HISTORY PHOTOS (BOTTOM RIGHT) INSTITUTE OF METEORITICS PHOTO A few of the many shapes exhibited by meteorites: ring-shaped, perforated and highly irregular, pear-shaped, jaw shaped, needle-shaped.
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!
COURTESY OF AMERICAN MUSEUM OF NATURAL HISTORY The Willamette iron, famous for its great size and weight (14 tons), on exhibit at the Hayden Planetarium, New York City. See pp. [36], [39].
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.
Piezoglyphs (oval pits resembling thumb-prints) in a stone meteorite, found at Belly River, Canada. See [p. 132].
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.
If the specimen you have found already has a corner knocked off, do you see specks and grains of metal on the broken surface? Such scattered bits of nickel-iron (not to be confused with the shiny mica flakes often seen in igneous rocks) characteristically occur in the grayish or brownish groundmass of stony meteorites. If your specimen is unbroken, hold it lightly against a spinning carborundum wheel or use a file to grind a small flat surface upon it, and then examine this surface for specks of metal.
If the answers to these questions are yes, then there is a good possibility that you have found a genuine meteorite.
If meteorites remain buried in the ground for a long period of time, their characteristic surface-features may weather away. Under such conditions, iron meteorites develop heavy-layered coatings of rust (iron oxide) as much as several inches in thickness. If irons stay in the ground long enough, they may rust away almost completely and turn into shale balls, like those found near the ancient Wolf Creek, Australia meteorite crater. (See [Chapter 4].) Stone meteorites buried in the ground for any great length of time may disintegrate and become completely unrecognizable as meteorites.
The fact that meteorites of all kinds are attacked by weathering has always argued strongly in favor of their prompt recovery. In the case of witnessed falls, prompt recovery is even more important, for only thus can specimens still retaining measurable amounts of various short-lived radioactivities be made available to physicists eager to investigate them with the most modern radiometric equipment.
10. TEKTITES, IMPACTITES & “FOSSIL” METEORITES
Before southern Australia was occupied by the white man, the native tribesmen of that region treasured certain small rounded pieces of black glass as medicine stones, rainmaking stones, and message stones. The Wadikali tribe referred to these objects as mindjimindjilpara, a word meaning “eyes that look at you like a man staring hard.” The early European settlers of the area called the same black glassy masses “blackfellows’ buttons.” Both phrases applied to objects that modern scientists call “australites,” which are now one of the best known types of tektites (Greek: tēktos, molten).
These Australian tektites and the tektites from many other countries around the world are a problem to meteoriticists. The question is, are they really meteorites? Many investigators believe that the answer is yes, and they are inclined to add to the three main divisions of true meteorites listed in the preceding chapter, a fourth: the tektites.
These mysterious glassy objects occur in such widely separated localities as Czechoslovakia, the Philippine Islands, Borneo, the Ivory Coast of Africa, Australia, Indo-China, Texas, Malaya, and Java. In these and still other areas, they have been found by the thousands in surface deposits of sand, clay, and gravel.
(left) “Flanged buttons” from Australia. (right) Several sizes of “dumbbells” from Australia. See [p. 136].
Tektites have never been seen to fall. In spite of this fact, as we noted above, a number of scientists believe that, like the meteorites, the tektites really did come from outer space but, that they fell to earth before man was here to see them come down—or at least before he had acquired the means and skill to make lasting records of such an occurrence.
Tektites are usually quite small, weighing between 1 and 100 grams, although a few of much larger size have been found. One large specimen from the Philippines weighed about ½ pound. Two giant tektites, one weighing ¾ pound and the other over 1 pound, are in the collection of the British Museum. In composition, tektites are an impure silica-glass containing low percentages of the oxides of such elements as iron, magnesium, calcium, and titanium.
If tektite fragments are held under a lamp and observed by reflected light, their thicker parts generally appear to be jet-black. If, however, these same specimens are held up between the observer and the light, then their thin razor-sharp edges are seen to be bottle-green, yellow-green, brownish, or even colorless.
In shape, many tektites are roundish or oval. Others are shaped like dumbbells, ladles, canoes, and teardrops. So they are known by those descriptive terms. One particularly interesting example is the unusual “flanged button” of Australia. Tektites of this type look like miniature South American gold-pans, the bateas, heaped high with pay dirt. Australian gold-field workers regarded these tektites as magical, and used them as good-luck charms. Superstitious American gold-seekers brought them into the United States all the way from Australia!
(above) Rounded tektite from Texas. (below) Deeply grooved bediasite from Texas.
Some tektites (for example, many of the “bediasites” from Texas) are deeply grooved and channeled, and have a very jagged and irregular appearance. Even the smoother tektite surfaces are characterized by flow lines, flow ridges, and bubble pits.
Many weathered pebbles and fragments of obsidian somewhat resemble the tektites superficially. There is a very simple test by which you can distinguish true tektites from obsidian. If you hold a thin splinter of tektite glass in a blowpipe flame, the glass melts quietly but only with the greatest difficulty. On the contrary, when you test in the same flame the terrestrial glass, obsidian, it froths up much more easily, into a bubbly, whitish mass.
Although the question of where the tektites came from is still not entirely settled, most scientists agree that all tektites did have a common origin. For example, tektites from widely scattered localities on the earth’s surface show not only similar queer shapes and surface markings (technically known as “sculpturing”), but also have very much the same chemical composition and, in particular, the same content of radioactive elements.
Because the tektites chemically resemble certain terrestrial rocks, scientists at first believed that some kind of earth process must have created them. One suggestion was that lightning had fused dust particles suspended in the air to form them; another, that they had come from volcanoes; still another, that the tektites were simply inclusions that had weathered out of terrestrial rocks. A few scientists once took seriously the possibility that tektites were refuse from primitive glass factories!
Tektite vs. obsidian, after blowpipe test.
While such theories have not yet been completely discarded, most scientists now feel that the tektites had their origin somewhere outside the earth. There are several reasons for this belief. First, the shape of such unusually symmetrical forms as are found, for example, among the australites, indicates that these small bodies at one time were members of a swarm of freely-spinning liquid masses. Again, flow features observed on the surfaces of certain tektites (and the fusion crust definitely identified on one specimen) show that these bodies at some time must have traveled through the earth’s atmosphere at high velocity.
If, then, the tektites were not produced by earth processes, where did they come from? According to primitive legends, they were “rocks” or “pebbles” from the moon. Indeed, one of the earliest scientific theories as to their origin (proposed by the Dutch authority Verbeek in 1897) likewise attributes them to debris jetted out from the moon. Another holds that tektites are fragments of the outermost glassy layers of some so-called “meteorite-planet,” or planets.[8] Still another idea is that tektites are what is left of a comet when it passes so close to the blazing-hot sun that the “ices” which make up most of the cometary nucleus (head) are all distilled away.
These theories of the origin of the tektites are based primarily on their observed shapes, surface features, and compositions. The senior author of this book has suggested still another possible theory based on the very unusual nature of the observed distribution of the tektites on the face of the earth.
To explain this theory, we first recall that the planet on which we live is more nearly a true sphere than are such familiar “spherical” objects as baseballs or basketballs. Consequently, any plane through the center of the earth cuts its surface in a curve that to all intents and purposes is what geometers refer to as a great circle.
Every plane passing through the center of a sphere intersects the surface in a great circle. In this figure, only the front half of the great circle cut out by the plane is shown.
Now the significant fact is that all the tektite deposits known at present are located on or very near to three great circles on the earth’s surface. Mathematics shows that if some earth process had created the tektites at random over the surface of the earth, then the odds would be very strongly against the existence of this peculiar “great-circle distribution.” But such distribution along great circles would be expected if the tektites had resulted from what might be likened to “chain-falls” upon the earth of objects like nearby satellites moving in orbits encircling our globe.
This notion brings up the interesting possibility that at some time in the remote past, the earth may have been the proud possessor of a set of equatorial rings. These rings would have been similar to those at present circling in the plane of Saturn’s equator. (Jupiter, too, may once have had its own set of equatorial rings.) The rings of Saturn are known to be made up of countless very small meteorites. In the same way, the “earth rings” of prehistory could have consisted of swarms of tiny nearby meteoritic satellites—the tektites—moving about the earth in the plane of its then-existing equator.
Eventually, the innermost of these small natural satellites collapsed onto the earth’s surface, falling along the old equator. At least twice thereafter, this process was repeated, the points of impact of the later tektite falls again lining up along whatever great circle of the earth happened to be the equator at the time of fall.
As the geologists and other investigators have shown, major shifts have occurred in the position of the earth’s equator during past geologic ages. This fact is well-substantiated by discoveries of fossil shells and plants on the cold Antarctic continent and of glacial deposits in hot South Africa. Therefore, we could hardly expect the tektite deposits, which are believed to have fallen at widely separated intervals of time, to have all occurred along a single great circle on the earth’s surface.
As you can see, the so-called “tektite-puzzle” is a complex one. As if this were not bad enough, Mother Nature has added to the confusion by creating in addition to the tektites another type of silica-glass not only found along the very same three great circles sprinkled with true tektites, but also having other features in common with the tektite glasses.
At Mount Darwin in Tasmania and at Wabar in the Rub’ al Khali desert of Arabia, large and small fragments of this curious silica-glass have been collected. At Wabar the masses of silica-glass were found in and about the rims of a series of meteorite craters formed in nearly pure sand, as we pointed out in [Chapter 4]. These meteorite craters are known to have resulted from the high-speed impact of iron meteorites upon the sand dunes of the Wabar site. Since the silica-glasses of Wabar have been found to contain countless spherules of nickel-iron of the same composition as the iron meteorites discovered about the Wabar meteorite craters, it seems quite certain that both the sand of the earth target and the nickel-iron of the falling meteorites were vaporized by the intense heat generated at impact. Consequently, it is natural that these Wabar masses of congealed silica-glass and nickel-iron be called impactites. They are silica-glasses, created chiefly from terrestrial materials by the impact of large crater-forming meteorites. This same name is now applied to all silica-glasses believed to have the same origin as those at Wabar.
As regards size if not composition, the crater-forming meteorites responsible for the Wabar and other impactites may have been big brothers of the small-fry responsible for the showers of true tektites. Or these big ones may have moved about the earth in orbits distinct from those followed by the tektite swarms but lying in the same plane as one of these swarms.
In addition to the curious puzzle of the tektites, meteoriticists have also run up against the problem of “fossil” meteorites or, more exactly, the problem of the lack of “fossil” meteorites. As we have already mentioned, no positively identified meteorite has ever been found in other than the most recent rock layers. With all the mining—particularly coal mining—that has gone on throughout the world in historic times, this fact does seem astonishing.
A number of explanations can be suggested for this absence of ancient meteorites. In the geologic past, meteorite falls may not have occurred as often as they do today. For example, the primeval atmosphere of the earth may have been so much denser than at present that even quite large meteorites were totally vaporized as they passed through it and therefore never reached the ground. Again, even if the rate of infall of meteorites was the same in the remote past as now, still various weathering processes active ever since the earliest meteorites fell may have so changed them in appearance and composition that they are no longer recognizable for what they are.
Several unusual lumps of rock from England and a mass of iron from Austria, all found at some depth by coal miners, have been tentatively put forward as “fossil” meteorites. But studies of these masses have so far produced no conclusive results. Still, we should not ignore the possibility that someday meteorites may be found and identified in rocks of considerable age.
L. J. SPENCER PHOTO COURTESY OF AMERICAN MUSEUM OF NATURAL HISTORY Mysterious glass objects found in the Libyan Desert. (right) Cut and polished specimens.
Does it seem as if we have posed more problems than we have solved in this chapter? It is very true that we have done just that. In speaking briefly about the tektites, the impactites, and the absence of “fossil” meteorites, we have by no means tried to present the last word on the troublesome but highly interesting problems connected with these objects—problems that admittedly may take scientists years or even decades of further research to solve. Perhaps you will find here the kind of unusual and thought-provoking problems that make the study of meteorites a rather special challenge. If so, you may wish to take an active part someday in unraveling these puzzles.