Meteorites metallic iron

The nuclei of iron are especially stable, giving it a comparatively high cosmic abundance (Chap. 1, p. 11), and it is thought to be the main constituent of the earth s core (which has a radius of approximately 3500 km, i.e. 2150 miles) as well as being the major component of siderite meteorites. About 0.5% of the lunar soil is now known to be metallic iron and, since on average this soil is 10 m deep, there must be 10 tonnes of iron on the moon s surface. In the earth s crustal rocks (6.2%, i.e. 62000ppm) it is the fourth most abundant element (after oxygen, silicon and aluminium) and the second most abundant metal. It is also widely distributed. 

We were quite elated, and it appeared that it was a rich field. Now, fifty years later, I must say that it wasn t as rich as we thought. But we have over the years discovered half a dozen natural radioactive elements, and two of these, the samarium-147 with its decay to neodymium-143 and rhenium-187 with its decay to osmium-187, prove to be of use in Nuclear Dating. The importance of rhenium is that it is iron soluble while the other radioactivities are insoluble in metallic iron. In fact, the best half life we have for rhenium-187 was obtained by measuring the osmium-187 to rhenium-187 ratio in iron meteorites which had been dated by other methods. This work was started many years ago by Dr. Herr and others in Germany. The half life is 43,000,000,000 years. 

Irons are non-chondritic meteorites that are predominantly metal. Iron meteorites formed by melting of, most likely, chondritic material and segregation of metal melt from silicate. Many apparently represent asteroidal cores, although some may have formed as dispersed metal pockets in the parent asteroids. 

Kelly, W. R. and Larimer, J. W. (1977) Chemical fractionations in meteorites, VIIL Iron meteorites and the cosmochemical history of the metal phase. Geochimica et Cosmochimica Acta, 41, 93—111. 

Jarosewich, E. (1990) Chemical analyses of meteorites A compilation of stony and iron meteorite analyses. Meteoritics, 25, 323-337. A compilation of many years of painstaking wet-chemical analyses of representative chondrite powders. These analyses are especially useful because they distinguish the amounts of metallic iron and Fe2+. 

Irons. Approximately Tr of meieorile falls are irons, Because they are distinctive rocks and weather relatively slowly, most meleoriies that were not seen lo fall, bur were found accidentally, are irons. Iron meteorites are composed of metallic iron and sideraphile elements Ihut fractionated from molten parent bodies. They may have been cores of asteroids nr they may have only been localised inelal accumulations. 

The stony-iron meteorites are intermediate between chondrites and irons. These very rare meteorites are equal mixtures of iron/nickel alloys and silicate minerals. Pallasites are striking examples of this type of meteorites, consisting of green olivine crystals in a matrix of metallic iron. Another type of stony-iron meteorite, called mesosiderites, contain pyroxene and plagioclase feldspars, minerals that are common on Earth. 

Fig. 13. MnO/FeO correlation first noted by Laul et al. 10°) for lunar samples. A similar correlation seems to hold for terrestrial and meteoritic samples, too. By oxidation of metallic iron the data point of a material with the composition an ordinary H-group chondrite (marked H) will move along the horizontal dashed line. The location of the point of maximum oxidation (Femetai = 0) is also indicated...

Meteorites consisting mainly of metallic iron alloyed with nickel are termed siderites. Siderolites differ from the above in contaimng stony matter as well as native metal. Aerolites consist essentially of stony matter. 

An answer to the first question was suggested by Lancet and Anders (1970). The principal meteoritic phases stable above 350-400 K (olivine, pyroxene, Fe, FeS) are not effective catalysts for the Fischer-Tropsch reaction, whereas the phases forming below this temperature (hydrated silicates, magnetite) are. P hough metallic iron is often regarded as a catalyst for this synthesis, the catalytically active phase actually is a thin coating of FCjO formed on the surface of the metal (Anderson, 1956)]. Thus CO may have survived metastably until catalysts became available by reactions such as ... 

Burbine et al. (2002) tested an extreme case of a possible composition for the surface of Mercury. They made spectral observations of enstatite achondrites (igneous meteorites composed almost entirely of pure MgSi03, with some accessory minerals and essentially no FeO). The spectral features of enstatite achondrites (aubrites see Chapter 1.05) are similar to those for Mercury, but lack the spectral reddening observed in spectra of Mercury and have an additional feature at 0.5 p.m caused by troilite (FeS). This reddening (visible to UV ratio) is the result of space weathering, in which FeO is reduced to very small grains of metallic iron. Thus, the reddening indicates that some FeO must be present on Mercury to produce the nanophase iron. Alternatively,... 

Noble and Pieters (2001) suggest that metallic iron might have been added to a FeO-free surface by meteorite impact. 

The rock reservoirs on the modem Earth show a very narrow range of Mn/Fe ratios, ranging only from 0.016 to 0.019, which demonstrates how similar the two elements are in the normal terrestrial rock cycle. Carbonaceous chondrites, which are meteorites that presumably represent the primordial composition of the Earth as a whole, are enriched in iron relative to manganese compared to the Earth s cmst and mantle. This difference reflects the concentration of metallic iron, but not of manganese, in the Earth s core. Another variation in composition from the normal cmstal value of 0.017 is seen in Archean cmst, which averages 0.023, a value that is higher than any common igneous rocks. 

Lithophile elements are those which have a preference for a silicate host, whereas chalcophile elements have an affinity for sulfur and so will most frequently be found in sulfides. Siderophile elements are those which will partition preferentially into a metallic iron phase and so are enriched in the Earth s core and in iron meteorites. Atmophile elements prefer the gaseous phases of the Earth atmosphere. This classification is discussed more fully in Chapter 2, Section 2.3.2. 

Types of meteorite Meteorites may be subdivided into two main categories - unmelted meteorites, that is those which come from a parent body which has not been fractionated since its aggregation early in the history of the solar system, and melted, or differentiated meteorites (Fig. 2.8). Unmelted meteorites are stony meteorites, the chondrites, and are made up of the same silicate minerals that are found on Earth. Melted meteorites are of three types. They include some stony meteorites (the achondrites), the iron meteorites, whose composition is dominated by a metallic iron-nickel alloy, and stony-iron meteorites, meteorites which are made up of approximately equal proportions of... 

Stony irons. Stony-iron meteorites are those which contain equal proportions of silicate minerals and metallic iron. Pallasites are made up of olivine and Fe-Ni metal and are thought to represent samples from the core-mantle boundary of their parent body. Mesosiderites are brecciated mixtures of silicates and Fe-Ni metal. 

Geochemical constraints The siderophile elements provide an important control on the processes of core formation. Siderophile elements are those elements which have a chemical affinity for metallic iron (Section 2.3.2) and so might be expected to concentrate in the core. This is in fact what is observed, and siderophile element concentrations in the mantle are depleted relative to concentrations in chondritic meteorites, with... 

These known facts seem to fit well with the assumption that the core has the same composition as metallic meteorites, namely, iron and nickel. 

The approximately 50 known classes of meteorite, excluding those known to have originated from the Moon or Mars, span a wide range of compositions. Most meteorites are stones. The remaining classes of meteorites, the irons and the stony-irons, are products of melting and differentiation. They consist mainly of metallic iron-nickel alloys, sulfides, carbides, phosphides, and igneous silicates. (See the chapter by M. Lipschutz in this volume). 

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