By Jim Straight
The initial recognition of meteorites as being rocks from space was not fully recognized by the scientific community until about 1803. At 1 p.m. on April 26, 1803 near the French village of L’Aigle, 75 miles west of Paris, the appearance of a fireball followed by detonations and a single shower of ordinary stony-chondrites (estimated as being as many as 2,000 to 3,000 small stones) fell within a 6 x 2.5-mile elliptical strewn-field with the major axis to the northwest.
The fall was witnessed by hundreds of peasants; many were frightened by reported near misses. Hundreds of stones were collected and studied. The details of this scientific investigation established beyond any pre-existing European scientific doubt that stones were indeed falling from outer space. It wasn’t until about 1808, however, that any lingering doubt remained in America.
It is easy to understand why the fall of meteorites as being from outer space was not accepted by
scientists until 200 years ago. How many of our ancestors even witnessed first-hand such an event and then were unable to report the phenomena to the known scientific community?
Classification, Description of Meteorites
The following information is compiled from Grady, Nininger, Norton, and Shand. For more detailed information, “Rocks From Space,” by Norton is an excellent single reference. By definition, a meteorite is the remaining product of a larger meteor, or meteoroid, that survived the intense heat in passing through the earth’s atmosphere and is still able to retain its earlier identity upon impact.
A Brief Classification: There are basically three types (or classes) of meteorites: stony-iron, iron, and stony. They share common similarities as all contain iron and are subject to chemical and physical weathering as does the terrestrial rocks within the earth’s atmosphere. Their iron content makes them slightly-to-strongly magnetic and reactive to a metal detector. However; some types may have a smooth rusty surface or be heavier than their surrounding terrestrial rock environment. Others may have a darker surface fusion-crust with characteristic flow banding resulting from the intense frictional heat generated while passing through the
atmosphere.
A Brief Description: Stony-iron
meteorites account for less than five percent of known finds. While the rarest, they
are also the easiest to be recognized. They are a mixture of silicate and free metal (nickel-iron) in nearly equal proportions. The stony-irons are divided into three main groups: pallasites, mesosiderites and lodranites. Pallasites are a mixture of olivine crystals (a silicate of iron
and magnesium), embedded in a free nickel-iron alloy matrix. When cut and polished they can be strikingly magnificent. Mesosiderites are a mixture of the silicates of pyroxene and plagioclase feldspar; the nickel-iron alloy matrix is not free but a mix. Lodranites are made of olivine, pyroxene, and nickel-iron metal.
Iron meteorites account for about 20 percent of the known finds. As Shand noted: “Even a small lump of iron is more likely to attract notice than a lump of stone.” Thus, they are what most everyone mentally thinks of when they hear the word “meteorite.” They are composed of a nickel-iron alloy, but the nickel content is used to determine the three main groups: hexahedrites, octahedrites and ataxites.
Hexahedrites have a low nickel content (4-6 percent by weight). Polished surfaces may have fine striations known as Neumann lines which formed by shock during a violent impact.
Octahedrites have a nickel content (about 1-17 percent by weight). The characteristic Widmanstatten pattern is spectacular when the meteorite is polished and etched in weak nitric
acid. Ataxites are the richest in nickel content (plus 16 percent by weight).
Stony meteorites fall much more frequently than do the iron ones, making up to about 75 percent of all known finds, but being more brittle they are usually found as smaller fragments.
Achondrites are stony meteorites lacking chondrules. They most closely resemble terrestrial igneous rocks and make up only about 7 percent of the sum-total of known meteorite finds.
Chondrites are common stony meteorites containing re-melted spherical grain-sized particles known as chondrules. The stony chondrites are represented by three groups: The first are the enstatite chondrites, having other letter designations, such as “E” (for containing enstatite, a pyroxene), as well as their metal content. The second are the carboniferous chondrites, which are cataloged by their increasing degree of alteration and the six different mineral and chemical content (of six different but well documented) carboniferous “prototype” meteorites designated by; CI, CM, CV, CO, CR, and CK. As an example, the “CI” designation stands for carboniferous meteorites of the (“I” for Ivuna) Ivuna meteorite that fell in Tarzana in 1938. The third are the ordinary stony chondrites which are similar looking to many common terrestrial rocks; thereby making their “space-rock” identification a more accidental discovery. They are cataloged by their degree (i.e. grade) of alteration, ranging from a numerical designation of 1 through 6; and also their free nickel-iron content, such as H (high iron), L (low-iron) and LL (low-low iron).
Early History of Meteorite Hunting
Dr. H. H. Nininger (1887-1986) was an early pioneer in the scientific pursuit of meteorites. His
career, which spanned over a half century, began Nov. 9, 1923 when he witnessed a blazing stream of fire passing southwest above McPherson, Kansas. As a scientist, teaching both biology and geology at McPherson College, Dr. Nininger had a keen sharp analytic mind. Using a scientific approach, and with the aid of cooperating statewide newspaper editors, he was able to compile reports from startled witnesses. Although he was looking for the proverbial “needle
in the haystack,” his publicity efforts in alerting the public and offering a reward of a dollar a pound led directly to the recovery of other meteorites from earlier falls. Thus, as a pathfinder, he self-launched his successful career as a “meteorite chaser,” and was able to survive financially during the Depression years followed by gas and tire rationing during WWII.
As early as 1932, Dr. Nininger used a simple hand held electromagnet that was mounted on a pole with a power cord attached to a battery. With it he collected about eight pounds of meteorite fragments from around the rim of the Odessa crater in Ector County, Texas.
Metal Detecting for Meteorites
About 1995 metal detectors were proven to be an effective tool in identifying the parameters of the Gold Basin, Arizona strewn-field. Thousands of stony chondrites, first discarded by the nugget shooters as terrestrial ‘hot rocks,” have since been recovered by knowledgeable detectorists while out hunting for gold in Gold Basin.
Using a metal detector for “meteorite-shooting” is not a new science. Dr. Nininger was a pathfinder in applying this now-blossoming technology over 65 years ago. In 1935, G.L. Barrett of Oklahoma invented a large, cumbersome, primitive, magnetic induction balance detector that was awkwardly attached to a battery by a long extension cord. It was Mr. Barrett’s intent to use it to work old Spanish trails, camps and battlefields, and to search for buried treasure throughout the Southwest. However, with financial help from a Mr. Dean Gillespie, Dr. Nininger was able to visit Mr. Barrett who demonstrated his machine’s ability to detect even a stony meteorite. Dr. Nininger was able to borrow the machine and with his son, Bob, returned to detect around the perimeter of the Odessa meteorite crater where he had earlier found the meteorite fragments with his electromagnet. On this second trip with the detector, they recovered 27 iron-octahedrite
meteorites, weighing the total of 34 pounds from around the crater rim, the deepest being buried seven inches.
In 1947, H.O. Stockwell successfully used a wheelbarrow-mounted metal detector to find
meteorites. The detector was so bulky that he transported it on top of his car; however, it worked and he recovered a total of one-and-a-half tons more of the Brenham stony-iron pallasite fall (1887) in Kiowa County, Kansas. Later Stockwell went to Trenton, Wisconsin where an iron octahedrite fall had been first discovered in 1858. Once there, he recovered two more masses, weighing 413 and 527 pounds, respectively.
After WWII, surplus mine detectors became available to civilians. While the detectors were
sensitive to trash such as tin cans, pieces of wire, bottle caps and nails; they were found to be sensitive to an average half-pound metallic meteorite at a depth of up to 10 inches and a two-pounder as deep as two feet.
Unfortunately, according to Dr.Nininger, ubiquitous magnetite, a common terrestrial accessory rock forming mineral “would sound persistently in the earphones just like a meteorite.” Thus, it was noted as an early reference to the terrestrial “hot rocks” that nugget hunters still find
troublesome while detecting for gold.
Meteorite “Prospecting” Tips
For those of you who “coinshoot” and/or “nugget-shoot,” some of these hunting techniques can be transferred to “meteorite-shooting.” Basic tools in addition to your detector and digging tools could include maps, a GPS, and a quality 10X loop. Also a container, such as a backpack, suitable to protect and carry any finds; additionally, various magnets are recommended, such as the strong magnet on a pole used by carpenters to clean up nails from construction sites or, better yet, make one yourself using a super-magnet. The deflection of an inexpensive North Pole seeking compass needle, by holding the compass against the specimen, is useful to identify any weak magnetic property.
As any successful nugget shooter knows, gold can be found in various disguises; such as being totally encrusted with dirt, caliche, or even having a black coating. As with a nugget, the crust of a meteorite can have a rust coating, or be coated with dirt or caliche or stained by terrestrial weathering to a brownish appearance.
Since meteorites are randomly scattered throughout the earth’s surface, it would be possible to deliberately spend years in fruitless searching without knowing what a stony meteorite may look like. However, the irons can usually be identified by sight, and are usually found by accident. They can be confused with hematite or magnetite terrestrial rocks; however a fusion crust is often a giveaway. Like gold, where one nugget has been found, there are often more nuggets to be found. It is believed by meteoriticists that in particular, the stony falls, upon penetrating our atmosphere shatter into many smaller particles; therefore, it is theoretically possible that
pea-sized or larger stones can still be found in the vicinity of a known fall.
While the common stony meteorites are the most numerous fall, they are mostly small-sized and closely resemble terrestrial mantle rocks. This makes them harder to identify. For those of you who enjoy rock hounding, hiking, fishing, hunting, bird watching, gold-panning, dredging, sluicing, dry-washing, detecting for gold or coin-shooting in a park, or even working in your yard or walking across an open field, be aware of their existence and don’t carelessly toss a stone away.
I will close this article by adding, as I know it is not needed to remind any of my fellow GPAA brethren, do not trespass or destroy any vegetation, injure the critters, or leave unsightly dig-holes. If you should happen to find what might be identified as a rare meteorite, remember it is an item of great scientific interest and of value in our understanding of the solar system.