Redox topotactic reactions in Fe~(Ⅱ-Ⅲ) (oxy)hydroxycarbonate new minerals related to fougerite in gleysols: 'trebeurdenite and moessbauerite'

摘要

Fougerite mineral responsible for the bluish-green shade of gleysols in aquifers was identified as Fe~(Ⅱ-Ⅲ) oxyhydroxycarbonate, [Fe_(6(1-x)~ⅡFe_(6x)~ⅢO_(12)H_(2(7-3x))]~(2+) • [CO_3~(2-)• 3H_2O]~(2-) where the average ferric molar fraction x = [Fe~Ⅲ/Fe_(total)] was restricted to the [1/3-2/3] range, up till now. In this paper, Moessbauer spectra of gleys extracted from the schorre of maritime marshes have values of x in the [2/3-1] range. Magnetic properties of homologous chemical compounds studied by Moessbauer spectroscopy are explained with long range order of Fe~Ⅲ ions within Fe cation layers for x = 1/3,2/3 and 1. Observed values are mixtures of topotactic domains that are in fact minerals with names proposed to IMA: fougerite for x = 1/3, trebeurdenite for x = 2/3 and mossbauerite for x = 1.