Zur geochemie des germaniums

Abstract

The geochemistry of germanium is discussed by means of about a hundred analyses in recent literature and about a hundred own spectrochemical determinations. Studied are: 1. (1) The distribution of germanium among coexisting silicates of igneous rocks: The content of germanium in all minerals is larger in acid rocks than in basic ones. On the other hand the ratio of germanium content in a special silicate and that of the co-existing plagioclase is nearly the same in all igneous rocks and therefore typical. These enrichment factors F M / Plag = [ Ge] in mineral/[Ge] in plagioclase indicate; how many times the distribution coefficients E = [ Ge] in mineral/[Ge] in magna of the silicates are larger than those of plagio-clases, if these minerals and plagioclases have been in equilibrium with the same magna. The enrichment factors increase up to factor 10 in the following sequence: tecto-, phyllo-, ino- and nesosilicates. But exceptions of this rule are observed in nesosilicates, which still have to be studied. The results confirm Wickman's hypothesis about the intake of germanium in various types of silicates. 2. (2) The geochemical behaviour of germanium during the magnatic differentiation: Basic and acid rocks originated by crystallization of a magma contain a similar amount, about 1 ppm Ge. Because the distribution coefficient E = [ Ge] in mineral/[Ge] in magna of one mineral is nearly the same in all igneous rocks, but the mineral increases in germanium during the magmatic differentiation, the concentration of germanium in the magma must also have increased. This is possible, if the amounts of the distribution coefficients of all silicates are smaller than one. The germanium content of a rock depends on its amount of dark minerals (% D) with high distribution coefficients ( E D ), its amount of light minerals (% H) with low distribution coefficients ( E H ) and on the concentration of germanium in the magma: [ Ge] rock = (% DE D + % HE H [ Ge] magma ≈ const. Minerals formed during the pegmatitic phase contain more germanium than those formed from the magma. The enrichment factors found for magmatic minerals are not effective here. A gaseous phase released from magma could transport germanium as a germaniumhalogenide. Germanium probably exists in the hydrothermal phase as germanate—or sulfogermanate—ion as a result of the solubility of germanium(IV)hydroxide. Germanium prefers to enter sphalerites and other sulfide minerals formed in this phase.