1911 Encyclopædia Britannica/Meteorite
METEORITE, a mass of mineral matter which has reached the earth’s surface from outer space. Observation teaches that the fall of a meteorite is often preceded by the flight of a fireball (see Meteor) through the sky, and by one or more loud detonations. It was inferred by Chladni (1794) that the fireball and the detonations result from the quick passage of the meteorite through the earth’s atmosphere.
The fall of stones from the sky, though not credited by scientific men till the end of the 18th century, had been again and again placed on record. One of the most famous of meteorites fell in Phrygia and was worshipped there for many generations under the name of Cybele, the mother of the gods. After an oracle had declared that possession of the stone would secure to the Romans a continual increase of prosperity, it was demanded by them from King Attalus about the year 204 B.C., and taken with great ceremony to Rome. It is described by the historian as “a black stone, in the figure of a cone, circular below and ending in an apex above.” Plutarch relates the fall of a stone in Thrace about 470 B.C., during the time of Pindar, and according to Pliny the stone was still preserved in his day, 500 years afterwards. Both Diana of the Ephesians “which fell down from Jupiter,” and the image of Venus at Cyprus, appear to have been conical or pyramidal stones. One of the holiest relics of the Moslems is preserved at Mecca, built into a corner of the Kaaba; its history goes back far beyond the 7th century; the description of it given to Dr Partsch suggests that the stone had fallen from the sky. The oldest existing meteorite of which the fall is known to have been observed is that which fell at Ensisheim in Elsass on the 10th of November 1492. It was seen to strike the ground and was immediately dug out; it had penetrated to a depth of 5 ft. and was found to weigh 260 ℔. It was long suspended by a chain from the roof of the parish church, and is now kept in the Rathhaus of the town.
It was not till scientific men gave credence to the reports of the fall of heavy bodies from the sky that steps were taken for the formation of meteorite collections. The British Museum (Natural History) at South Kensington now contains specimens belonging to 566 distinct falls; of these falls 325 have been actually observed; the remaining specimens are inferred to have come from outer space, because their characters are similar to those of the masses which have been seen to fall. Of these meteorites the following twelve have fallen within the British Isles:—
|In England.||Wold Cottage, Thwing, Yorkshire||Dec. 13, 1795.|
|Launton, Oxfordshire||Feb. 15, 1830.|
|Aldsworth, Gloucestershire||Aug. 4, 1835.|
|Rowton, Shropshire||April 20, 1876.|
|Middlesbrough, Yorkshire||March 4, 1881.|
|In Scotland.||High Possil, Glasgow||April 5, 1804.|
|Perth||May 17, 1830.|
|In Ireland.||Mooresfort, Tipperary||Aug. 1810.|
|Adare, Limerick||Sept. 10, 1813.|
|Killeter, Tyrone||April 29, 1844.|
|Dundrum, Tipperary||Aug. 12, 1865.|
|Crumlin, Antrim||Sept. 13, 1902.|
Meteoritic falls are independent of thunderstorms and all other terrestrial circumstances; they occur at all hours of the day and night, and at all seasons of the year; they favour no particular latitudes. The number of stones which reach the ground from one fireball is very variable. In each of the two Yorkshire falls only one stone was found; the Guernsey County meteor yielded 30; at Toulouse, as many as 350 are estimated to have fallen; at Hessle, over 500; at Knyahinya, more than 1000; at L’Aigle, from 1000 to 2000; at both Pultusk and Mocs no fewer than 100,000 are estimated to have reached the earth’s surface. The largest single mass seen to fall is one of those which came down at Knyahinya, Hungary, in 1866, and weighed 547 ℔; but far larger masses, inferred from their characters to be meteorites, have been met with. The larger of the Cranbourne masses, now in the British Museum (Natural History), before rusting weighed 31 tons; the largest of the masses brought by Lieut. Peary from western Greenland weighs 361 tons. A mass found at Bacubirito in Mexico is 13 ft. long, 6 ft. wide and 5 ft. thick, and is estimated to weigh 50 tons.
From observations of the path and time of flight of the luminous meteor it is calculated that meteorites enter the earth’s atmosphere with absolute velocities ranging from 10 to 45 m. a second; but the speed of a meteorite after the whole of the resisting atmosphere has been traversed is extremely small and comparable with that of an ordinary falling body. According to Professor A. S. Herschel’s experiments, the meteorite which fell at Middlesbrough must have struck the ground with a velocity of only 412 ft. a second. In the case of the Hessle fall, several stones fell on the ice, which was only a few inches thick, and rebounded without breaking the ice or being broken themselves. The depth to which a meteorite penetrates depends on the speed, form, weight and density of the meteorite and on the nature of the ground. At Stannern a meteoric stone weighing 2 ℔ entered to a depth of only 4 in.; the large Knyahinya stone already mentioned made a hole 11 ft. deep.
The area of the earth’s surface occupied by towns and villages being comparatively small, the probability of a shower of stones falling within a town is extremely minute; the likelihood of a living creature being struck is still more remote. The first Yorkshire stone, that of Wold Cottage, struck the ground only 10 yds. from a labourer; the second, that of Middlesbrough, fell on the railroad only 40 yds. away from some platelayers at work; a stone completely buried itself in the highway at Kaba; one fell between two carters on the road at Charsonville, throwing the ground up to a height of 6 ft.; the Tourinnes-la-Grosse meteorite broke the pavement and was broken itself; the Krühenberg stone fell within a few paces of a little girl; the Angers stone fell close to a lady standing in her garden; the Braunau mass went through the roof of a cottage; at Macao, in Brazil, where there was a shower of stones, some oxen are said to have been killed; at Nedagolla, in India, a man was so near that he was stunned by the shock; while at Mhow, also in India, a man was killed in 1827 by a stone which is a true meteorite, and is represented by fragments in museum collections,
Though the surface of a meteoric stone becomes very hot during the early part of the flight through the air, it is cooled again during the later and slower part of the flight. Meteorites are generally found to be warm to the touch if immediately dug out; at the moment of their impact they are not hot enough to char woody fibre on which they chance to fall, nor is the surface then soft, for terrestrial matter with which the surface comes into contact makes no impression upon the meteorite. Where many stones fall at the same time they are generally distributed over a large area elongated in the direction of the flight of the luminous meteor, and the largest stones generally travel farthest. At Hessle, for instance, the stones were distributed over an area of 10 m. long and 3 m. broad.
Meteorites are almost invariably found to be completely covered with a thin crust such as would be caused by intense heating of the material for a short time; its thinness shows the slight depth to which the heat has had time to penetrate. They are presumably cold and invisible when they enter the earth’s atmosphere, and become heated and visible during their passage through the air; doubtless the greater part of the superficial material flicks off as the result of the sudden heating and is left behind floating in the air as the trail of the meteor. The crust varies in aspect with the mineral composition of the meteorite; it is generally black; it is in most cases dull but is sometimes lustrous; more rarely it is dark-grey in colour. Each stone of a shower is in general completely covered with crust; but occasionally, as in the case of the Butsura fall, stones found some miles apart fit each other closely and the fitting surfaces are uncrusted, showing that a meteorite may break up during a late and cool stage of the flight through the atmosphere. A meteorite is generally covered with pittings which have been compared in size and form to thumbmarks; the pittings are probably caused by the unequal conductivity, fusibility and frangibility of the superficial material. As picked up, complete and covered with crust, meteorites are always irregularly-shaped fragments, such as would be obtained on breaking up a rock presenting no regularity of structure.
About one-third, and those the most common, of the chemical elements at present recognized as constituents of the earth’s crust have been met with in meteorites; no new chemical element has been discovered. The most frequent or plentiful in their occurrence are: aluminium, calcium, carbon, iron, magnesium, nickel, oxygen, phosphorus, silicon and sulphur; while less frequently or in smaller quantities are found antimony, arsenic, chlorine, chromium, cobalt, copper, hydrogen, lithium, manganese, nitrogen, potassium, sodium, strontium, tin, titanium, vanadium. The existence of minute traces of several other elements has been announced; of these special mention may be made of gallium, gold, iridium, lead, platinum and silver. Iron occurs chiefly in combination with nickel, and phosphorus almost always in combination with both nickel and iron (schreibersite); carbon occurs both as indistinctly crystallized diamond and as graphitic carbon, the latter generally being amorphous, but occasionally having the forms of cubic crystals (cliftonite); free phosphorus has been found in one meteorite; free sulphur has also been observed but may have resulted from the decomposition of a sulphide since the fall of the stone.
Of the mineral constituents of meteorites, the following are by many mineralogists regarded as still unrepresented among native terrestrial products: cliftonite, a cubic form of graphitic carbon; phosphorus; various alloys of nickel and iron; moissanite, silicide of carbon; cohenite, carbide of iron and nickel (corresponding to cementite, carbide of iron, found in artificial iron); schreibersite, phosphide of iron and nickel; troilite, protosulphide of iron; oldhamite, sulphide of calcium: osbornite, oxysulphide of calcium and titanium or zirconium; daubréelite, sulphide of iron and chromium; lawrencite, protochloride of iron; asmanite, a species of silica; maskelynite, a singly refractive mineral with the chemical composition of labradorite; weinbergerite, a silicate intermediate in chemical composition to pyroxene and nepheline.
Of these troilite is perhaps identical with some varieties of terrestrial pyrrhotite; asmanite has characters which approach very closely to those of terrestrial tridymite; maskelynite, according to one view, is the result of fusion of labradorite, according to another view, is an independent species chemically related to leucite. Other compounds are present corresponding to the following terrestrial minerals: olivine and forsterite; enstatite and bronzite; diopside and augite; anorthite, labradorite and oligoclase; magnetite and chromite; pyrites; pyrrhotite; breunnerite. Quartz (silica), the most common of terrestrial minerals, is absent from the stony meteorites; but from the Toluca meteoric iron microscopic crystals have been obtained of which some have certain resemblances to quartz, and others to zircon. Free silica is present in the Breitenbach meteorite but as asmanite. In addition to the above there are several compounds or mixtures of which the nature has not yet been satisfactorily ascertained.
Meteorites are conveniently distributed into three classes, which pass more or less gradually into each other: the first (siderites or meteoric irons) includes all those which consist mainly of metallic iron alloyed with nickel; only nine of them have been actually seen to fall; the second (siderolites) includes those in which metallic iron (alloyed with nickel) and stony matter are present in large proportion; few of them have been seen to fall; those of the third class (aerolites or meteoric stones) consist almost entirely of stony matter; nearly all have been seen to fall.
In the meteoric irons the iron generally varies from 80 to 95% and the nickel from 6 to 10%; the latter is generally alloyed with the iron, and several alloys or mixtures have been distinguished by special names (kamacite, taenite, plessite). Troilite is frequently present as plates, veins or large nodules, sometimes surrounded by graphite; schreibersite is almost always present, and occasionally also daubréelite. The compositeness and the structure of meteoric iron are well shown by the figures generally called into existence when a polished surface is etched by means of acids or bromine-water; they are due to the inequality of the etching action on thick and thin plates of various constituents, the plates being composed chiefly of two nickel-iron materials (kamacite and taenite). A third nickel-iron material (plessite) fills up the spaces formed by the intersection of the joint plates of kamacite and taenite; it is probably not an independent substance but an intimate intergrowth of kamacite and taenite. The figures were first observed in 1808 and are generally termed “Widmanstätten figures” in honour of their discoverer; the plates which give rise to them are parallel to the faces of the regular octahedron, and such masses have therefore an octahedral structure. A small number of the remaining masses have cubic cleavage; instead of Widmanstätten figures they yield fine linear furrows when etched; the furrows were found by Neumann in 1848 to have directions such as would result from twinning of the cube about an octahedral face; they are known as “Neumann lines.” For meteoric irons of cubic structure the percentage of nickel is lower than 6 or 7; for those of octahedral structure it is higher than 6 or 7; the plates of kamacite are thinner, and the structure therefore finer the higher the percentage of that metal. A considerable number of meteoric irons, however, show no crystalline structure at all, and have percentages of nickel both below and above 7; it has been suggested that each of these masses may once have had crystalline structure and that it has disappeared as a result of prolonged heating throughout the mass while the meteorite has been passing near a star.
An investigation of the changes of the magnetic permeability of the Sacramento meteoric iron with changing temperature led Dr S. W. J. Smith to infer that the magnetic behaviour can only be explained by imagining the meteorite to consist largely of plates of nickel-iron containing about 7% of nickel (kamacite), separated from each other by thin plates of a nickel-iron constituent (taenite), containing about 27% of nickel and having different thermomagnetic characters from those of kamacite; he suggests, however, that taenite is not a definite chemical compound but a eutectic mixture of kamacite and a nickel-iron compound containing not less than 37% of nickel.
About eleven out of every twelve of the known meteoric stones belong to a division to which Rose gave the name “chondritic” (χόνδρος, a grain); they present a very fine-grained but crystalline matrix or paste, consisting of olivine and enstatite or bronzite, with more or less nickel-iron, troilite, chromite, augite and triclinic feldspar; through this paste are disseminated round chondrules of various sizes and generally with the same mineral composition as the matrix; in some cases the chondrules consist wholly or in great part of glass. Some meteorites consist almost solely of chondrules; others contain only few; in some cases the chondrules are easily separable from the surrounding material. In mineral composition chondritic meteorites approximate more or less to terrestrial lherzolites.
A few meteorites belonging to the chondritic division are remarkable as containing carbon in combination with hydrogen and oxygen; those of Alais and Cold Bokkeveld are good examples.
The remaining meteoric stones are without chondrules and contain little or no nickel-iron; of these the following may be mentioned as illustrative of the varieties of mineral composition: Juvinas, consisting essentially of anorthite and augite; Petersburg, of anorthite, augite and olivine, with a little chromite and nickel-iron (both Juvinas and Petersburg may be compared to terrestrial basalt); Sherghotty, chiefly of augite and maskelynite; Angra dos Reis, almost wholly of augite, but olivine is present in small proportion; Bustee, of diopside, enstatite and a little triclinic feldspar, with some nickel-iron, oldhamite and osbornite; Bishopville, of enstatite and triclinic feldspar, with occasional augite, nickel-iron, troilite and chromite; Roda, of olivine and bronzite; and Chassigny, consisting of olivine with enclosed chromite, and thus mineralogically identical with terrestrial dunite.
Almost all meteoric stones appear to be made up of irregular angular fragments, and some of them bear a close resemblance to volcanic tuffs. In the large group of chondritic stones, chondrules or spherules, some of which can only be seen under the microscope while others reach the size of a walnut, are embedded in a matrix apparently made up of minute splinters such as might result from the fracture of the chondrules themselves. In fact, until recently it was thought by some mineralogists that the chondrules owe their form, not to crystallization, but to friction, and that the matrix was actually produced by the wearing down of the chondrules through frequent collision with each other as oscillating components of a comet or during repeated ejection from a volcanic vent of some small celestial body. Chondrules have been observed, however, presenting forms and crystalline surfaces incompatible with such a mode of formation, and others have been described which exhibit features resulting from mutual interference during their growth. The chondritic structure is different from anything which has yet been observed in terrestrial rocks, and the chondrules are distinct in character from those observed in perlite and obsidian. It is now generally believed that the structural features of meteoric stones are the result of hurried crystallization.
No organized matter has been found in meteorites and they have brought us, therefore, no evidence of the existence of living beings outside our own world.
Authorities.—The literature consists chiefly of memoirs dispersed through the journals of scientific societies. The following separate works may be consulted: A. Brezina, Die Meteoriten-Sammlung d. k-k. min. Hofkabinetes in Wien (Vienna, 1896); A. Brezina u. E. Cohen, Die Structur und die Zusammensetzung der Meteoriten (Stuttgart, 1886–1887); P. S. Bigot de Morogues, Mémoire historique et physique sur les chutes des pierres (Orléans, 1812); Chladni, Ueber den Ursprung der von Pallas gefundenen und anderer ihr ähnlicher Eisenmassen (Riga, 1794), and Ueber Feuer-Meteore, und über die mit denselben herabgefallenen Massen (Vienna, 1819); E. Cohen, Meteoritenkunde (Stuttgart, 1894–1905); L. Fletcher, An Introduction to the Study of Meteorites, 10th ed. (London, 1908); E. King. Remarks concerning Stones said to have fallen from the Clouds both in these Days and in Ancient Times (London, 1796); S. Meunier, Météorites (Paris, 1884); C. Rammelsberg, Die chemische Natur der Meteoriten (Berlin, 1870–1879); G. Rose, Beschreibung und Eintheilung der Meteoriten (Berlin, 1864); G. Tschermak, Die mikroskopische Beschaffenheit der Meteoriten (Stuttgart, 1883–1885); E. A. Wülfing, Die Meteoriten in Sammlungen und ihre Literatur (Tübingen, 1897). (L. F.)