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Geochemical and Mineralogical Framework |
Sedimentpetrographisches, Institut der Universitat Gottingen, D-3400 Gottingen, Goldschmidtstrassel, Federal Republic of Germany
Although mineral assemblages of ironstones vary widely, with hematite and goethite being the main components of red deposits and berthierine and siderite predominating in green deposits, the bulk major and minor elemental chemical compositions for both types of deposits are very similar. This indicates that the source material and physico — chemical conditions at the site of deposition must have been almost identical and that the different mineral associations probably developed during early diagenesis.
Data from experimental studies including iron clay mineral synthesis throw light upon the post-depositional processes. It is demonstrated that berthierine can be synthesized from iron hydroxides in solution with initial SiO2 concentrations of 5–9ppm, low concentrations of Mg and K, and with ferrous iron dominant.
Decay of organic matter causes changes of Eh towards reducing conditions, which destabilizes the iron-rich particles and leads to both increased concentration of ferrous ions in the pore solution and the release of silicic acid ions. pH and concentration of silicic and carbonic acid controls whether berthierine or siderite is formed. In red deposits the concentration of organic material is low, suggesting that the presence and amount of organic material is a controlling influence.
