Stony meteorites

At the end of the eighteenth century, the average density of the Earth as a whole was already known to be twice that of the common cmst rocks. The original concept of an iron core was purely based on meteoritic observations. Stony meteorites provide the clue that iron is more abundant in accretion materials than in the Earth s mantle (Cox, 1989). A serious proposal of an iron core was not presented until the recognition of the two main classes of meteorites, stony and iron meteorites, in the mid-nineteenth century. The iron meteorites had been assumed to come from the core of a fragmented planet. The idea of an iron core was generally accepted once Oldham (1906) confirmed the presence of a core. 

Notice that meteorites are classified first of all into three large categories stony, iron, and stony-iron. As their name suggests, stony meteorites are similar in appearance and chemical composition to rocks found on the Earth s surface. Stony meteorites are divided into two major categories chondrites and achondrites. Chondrites are stony meteorites that contain small spheres, called chondrules, with diameters of about 1 mm, consisting of minerals that were once melted and that have now aggregated to form the meteorite. Stony meteorites lacking chondrules are called achondrites. 

Stony Irons. The stony iron meteorites are composed of substantial iron and siUcate components. The paHasites contain cm-sized ohvine crystals embedded ia a soHd FeNi metal matrix and have properties consistent with formation at the core mantle boundary of differentiated asteroids. The mesosiderites are composed of metal and siUcates that were fractured and remixed, presumably ia the near-surface regions of their parent bodies. 

Occurrence and Recovery. Rhenium is one of the least abundant of the naturally occurring elements. Various estimates of its abundance in Earth s cmst have been made. The most widely quoted figure is 0.027 atoms pet 10 atoms of silicon (0.05 ppm by wt) (3). However, this number, based on analyses for the most common rocks, ie, granites and basalts, has a high uncertainty. The abundance of rhenium in stony meteorites has been found to be approximately the same value. An average abundance in siderites is 0.5 ppm. In lunar materials, Re, when compared to Re, appears to be enriched by 1.4% to as much as 29%, relative to the terrestrial abundance. This may result from a nuclear reaction sequence beginning with neutron capture by tungsten-186, followed by p-decay of of a half-hfe of 24 h (4) (see Extraterrestrial materials). 

Tridymite. Tridymite is reported to be the siUca form stable from 870—1470°C at atmospheric pressure (44). Owing to the sluggishness of the reconstmctive tridymite—quart2 conversion, which requites minerali2ers such as sodium tungstate, alkah metal oxide, or the action of water under pressure, tridymite may persist as a metastable phase below 870°C. It occurs in volcanic rocks and stony meteorites. 

Meteorites are of two kinds stony meteorites that are rock-like in character, and metallic meteorites that consist of metallic elements. The kinds of substances in the stony meteorites are very much like the substances in the crust of the earth, if we allow for the fact that the meteors could not bring gases or liquids with them. We feel that the other type, the metallic meteors, give valuable clues about the nature of the earth s central core. Experts have long believed that these meteorites are fragments from exploded planets that, perhaps, resembled the earth. 

Chondrites Achondrites Stony iron meteorites Iron meteorites... 

Stony meteorites are similar to igneous rocks (rocks formed by fire) on earth, but they have less silicon and oxygen, and more iron, cobalt, and trace metals. A third kind of meteorite, called pallasite, is somewhere between the other two types. It is about half iron and half stone, in a mixture not normal on earth. 

Stony principally silicates or rocky meteorites. It is harder to determine the extraterrestrial origin of these meteorites and it usually requires careful laboratory analysis. 

The largest class of meteorite finds is stony meteorites, made principally of stone. The general stony classification is divided into three subclasses called chondrites, carbonaceous chondrites and achondrites, and it is at this level of distinction at which we will stop. Before looking at their mineral and isotopic structure in more detail, it is useful to hold the composition of the Earth s crust in mind here for comparison. The Earth s crust is 49 per cent oxygen, 26 per cent silicon, 7.5 per cent aluminium, 4.7 per cent iron, 3.4 per cent calcium, 2.6 per cent sodium, 2.4 per cent potassium and 1.9 per cent magnesium, which must have formed from the common origin of the solar system. 

Meteorite mineralogy (Table 6.2) is complicated and is diagnostic of the origin of the sample. Two subclasses of the stony meteorites are chosen of particular relevance to the study of the origins of life. 

Meteorites General classification into stony, stony-iron and iron, each with an interesting mineralogy, notably the carbonaceous chondrites... 

Stony-iron Class of silicate-iron-based meteorites. 

Herzog, G. F. and Anders, E., "Absolute scale for radiation aqes of stony meteorites", Geochim. Cosmochim. Acta, 1971, 35, 605-611. 

Fig. 10.1. Rb-Sr isochrone measured from separated components of the stony meteorite Guarena. The initial 87Sr/86Sr ratio is slightly higher than that inferred in basaltic achondrites (BABI) because of a period of metamorphism. After Wasserburg, Papanastassiou and Sanz (1969), with permission. Courtesy G.J. Wasserburg.

In this connection it is important that the study of the composition and probable sources of the matter now being received by the earth, in the form of stony and metallic meteorites, be continued and extended. 

Kung CC, Clayton RN (1978) Nitrogen abundances and isotopic compositions in stony meteorites. Earth Planet Sci Lett 38 421 35... 

It was indeed a pleasant surprise to discover that certain grains carrying the unmistakable signature of supernovas had survived the turbulent formation of the Solar System. In addition it proved possible to extract them from meteorites without modifying them, so that they could be studied at leisure in terrestrial laboratories. Dissolving the stony component in acid and carrying out a series... 

Tridymite is another form of crystalline sdica stable between 870 and 1,470°C at atmospheric pressure. It is found in volcanic rocks and has been identified in many stony meteorites. Tridymite also exists in various forms. It has six different modifications that undergo thermal inversions from one to another. Its density at 200°C is about 2.22 g/cm. The hexagonal unit cell contains four Si02 units. The Si—O bond distance is 1.52A. 

Figure 5-8 A Pb-Pb isochron that determined the age of the Earth to be about 4.55 Ga. Stony and iron meteorites as well as a sediment of the Earth are plotted on a Pb-Pb isochron. The sediment, as a "bulk sample of the silicate Earth in terms of Pb isotopes, plots on the same line as the meteorites, suggesting that the Earth and meteorites formed at the same time and are the same age. Erom Patterson (1956). Later studies reveal a more detailed evolution history of the Earth, including core formation (about 4.53 Ga), atmospheric formation (about 4.45 Ga), and crustal evolution.

Ganguly J. and Stimpfl M. (2000) Cation ordering in orthopyroxenes from two stony-iron meteorites implications for cooling rates and metal-silicate mixing. Geochim. Cosmo-chim. Acta 64, 1291-1297. 

Jarosewich, E. (1990) Chemical analyses of meteorites A compilation of stony and iron meteorite analyses. Meteoritics, 25, 323-337. 

Stony irons are nonchondritic meteorites that contain roughly equal proportions of silicate minerals and metal. Two types of stony irons - pallasites and mesosiderites - are distinguished. Pallasites consist of approximately equal amounts of metal and olivine (one small group contains pyroxene as well). Mesosiderates also have approximately equal proportions of metal and silicate, but the silicate fraction is basalt. 

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