An alternative model for the formation of iron meteorites

Abstract

The formation of iron meteorites is, at present, considered to have been a high temperature process. It seems generally accepted that these bodies are derived from molten cores of small planets with some 10 z km in diameter. They are thought to have been broken up later. This hypothesis implies a heating up of the rather small bodies and a subsequent separation of liquid metal from a silicate phase. For this hypothesis it is necessary that in the beginning short-living radioactive nuclides acted as a heat source. The separation mechanism of metals and silicates, however, is difficult to imagine because the gravitational forces in such small bodies appear to be insufficient. Even the moon has been found to have no or only a small metal core. Moreover, the disruption of a body of asteroidal size up to its core presents severe difficulties. Problems also arise regarding the structure of certain iron meteorites. Many features are incompatible with a fused metallic phase at the time of enveloping materials such as crystals of Laurencite (FeC12) and broken silicates with sharp corners. Neither of them could have survived temperatures near the melting point of the iron phase in which they have been found (P. Ramdohr [1] ). All this suggests that the existing models of iron meteorite formation are not quite satisfactory. As an alternative method for iron meteorite formation, a mechanism working primarily at low temperatures could have great advantages. We propose as a mechanism the thermal decomposit ion of metal carbonyls (Mex(CO)y). It is well known that comets contain carbon monoxide, iron and nickel, among other constituents and therefore a reaction leading to carbonyls is not at all unlikely. The formation of Fe(CO)s and Ni(CO)4 is now performed industrially (Mond [2] ). This type of reaction is exothermic (50 kcal/mole). It can be performed at atmospheric pressure and below 100°C, although the BASF uses several hundred atmospheres in order to gain time (Zimmermann [3] ), and iron sulfide as a catalyst. If ironand nickel-carbonyls are heated to about IO0°C, decomposit ion takes place and the free metals are formed together with carbon monoxide. 100°C Me (CO)x ~ Me + xCO