The distribution of trace quantities of germanium between metal, silicate and sulflde phases

Abstract

The distribution of trace quantities of Ge between metal, silicate and sulfide phases has been studied hydrothermally at controlled P O 2 by means of oxygen buffer techniques. Radioactive Ge 68-labeled olivine of varying Mg and Fe composition and Ge 68-labeled iron were synthesized for this study. At 900°C and 500 bars, Ge is strongly siderophile in the iron-olivine system even at the equilibrium P O 2 of iron-wüstite. Similar strongly siderophilic behavior of Ge was also observed in the Fe-FeS system at this temperature and pressure. Thermodynamic data indicate that the free energy of formation of germanium dioxide lies between that of the iron oxide and nickel oxide at all temperatures. In the presence of metallic iron, the reduction of GeO 2 to Ge and the formation of Fe-Ge alloy are spontaneous reactions. If nickel is added to the system, the metal phase (Fe-Ni alloy) can exist at a higher P O 2 than pure iron. At the low Ni content of about 10 per cent, as found in most iron meteorites, the change in P O 2 Of metal-wüstite from iron-wüstite is small and is unlikely to affect the siderophilic behavior of Ge. A discussion is given to indicate that the diffusion of Ge out of the metal phase is possible if the Ni content of the metal is very high. From these results, it seems unlikely that a simple one-stage separation of metal from silicate would cause the extremely low Ge content observed in some iron meteorites e.g. iron meteorites of chemical groups IVA and IVB. Other possible fractionation processes involving Ge are also discussed.