CATIONIC SUBSTITUTION IN NATURAL SIDERITE-MAGNESITE (FECO3-MGCO3) SOLID-SOLUTIONS - A FTIR SPECTROSCOPIC STUDY

The mid-infrared spectra of natural siderite-magnesite solid-solutions (magnesium indices Mg2+/(Mg2++Fe2+) between 0.02 and 0.22; less than 28 mol% (Ca,Mn)CO3) were analyzed by FTIR spectroscopy. The chemical c ompositions were obtained by electron microprobe measurements. signifi cant shifts of the nu 2(C-12), nu 2(C-13), nu 3 and nu 4 absorption ba nd positions with the substitution of Mg2+ by Fe2+ are observed. The w ave-numbers increase with increasing Mg2+ contents, in agreement with the trends predicted from the synthetic pure end-members. For the out- of-plane bending mode of the carbonate ion group, an average isotope s hift of 29.5 +/- 1.5 cm(-1) is found for the stable carbon isotopes. T he nu 2(C-12) and nu 4 FTIR results agree with previous measurements f rom the literature. To derive the wave-numbers of pure FeCO3-MgCO3 sol id-solutions, the results of this study are combined with 56 published mid-infrared spectra of synthetic siderite and magnesite, and natural breunnerites, and evaluated by correcting the absorption band positio ns for the additional incorporation of minor Ca2+ and Mn2+ according t o BOTTCHER et al. (1993 b). The results are used to construct the bina ry FeCO3-MgCO3 limb in a nu 2-nu 4 diagram for the ternary system FeCO 3-MgCO3-CaCO3. An application of the in-plane bending mode (nu 4) of c ogenetic dolomite-ankerite and siderite-magnesite solid-solutions to g eothermometry is illustrated.