MORPHOLOGICAL AND GENETIC RELATIONS BETWEEN SIDERITE, CALCITE AND GOETHITE IN A LOW MOOR PEAT FROM SOUTHERN GERMANY

Siderite, calcite and goethite occur in a Low Moor Feat near Freising, southern Germany. Siderite is present mainly as microcrystalline ovoi ds which have a lamellar structure and are less than 5 mu m long. Sele cted area electron diffraction and X-ray diffraction data suggest a cr ystal size less than that of the ovoids, probably as small as 20-50 nm . Siderite occurs interstitially between plant materials, predominantl y of the genus Equisetum. Calcite, which is subordinate to siderite, o ccurs as pseudomorphs of plant cells. This suggests an earlier crystal lization for calcite predating that of the siderite. The distribution of mineral morphologies and pH and Eh data suggest that siderite weath ers by oxidation to goethite under mildly alkaline conditions, despite the probable production of protons. This process initially involves a pseudomorphism of the siderite ovoids by goethite. However, the goeth ite, comprising poorly acicular crystals with a mean coherence length of 11 nm normal to the 111 plane, eventually develops into a massive f abric. Calcite occurs with goethite in oxidized samples in the same lo cation as in the unoxidized samples, but in decreased amount. Thus, ca lcite dissolution may act as a buffer to siderite oxidation.