Carbon in natural goethites

Abstract

Carbon, as a possibly “indigenous” minor element impurity in goethites, appears to exist in two principal forms: 1.(1) CO2 trapped in the mineral structure and2.(2) organic matter. With one exception the total carbon content of the samples in this study ranges between 0.2 and 2.0 weight percent (reported as CO2). Total carbon δ13C values have a range from −27.2 to −8.1 per mil. Values of the mole fraction of trapped CO2 in the goethite total carbon as determined for seven samples vary from 0.11 to 0.50. Much of the variation of total carbon δ13C values in these samples can be attributed to different trapped CO2/organic matter ratios in the different goethites. By analogy with speleothems and soil carbonates, δ13C values of trapped CO2 could indicate whether the environment of goethite formation was relatively open or closed to mixing with CO2 from the Earth's atmosphere.In goethite samples subjected to H2O2 treatment, calculated organic matter δ13C values range from about −35 to −24 per mil. These values probably reflect a predominance of organic matter derived from C3 plants in this particular sample population. 13C12C ratios in organic matter associated with goethite could be a source of information on the Earth's ancient biosphere.