BÖTTCHER, M.E. & GEHLKEN, P.-L. (1995):
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 compositions were obtained by electron microprobe measurements. Significant shifts of the 2 (12C), 2 (13C), 3 and 4 absorption band positions with the substitution of Mg2+ by Fe2+ are observed. The wave-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 shift of 29.5 ± 1.5 cm-1 is found for the stable carbon isotopes. The 2 (12C) and 4 FTIR results agree with previous measurements from the literature. To derive the wave-numbers of pure FeCO3 – MgCO3 solid-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 positions for the additonal incorporation of minor Ca2+ and Mn2+ according to BÖTTCHER et al. (1993). The results are used to construct the binary FeCO3 – MgCO3 limb in a 2 – 4 diagram for the ternary system FeCO3 – MgCO3 – CaCO3. An application of the in-plane bending mode (4) of cogenetic dolomite – ankerite and siderite – magnesite solid-solutions to geothermometry is illustrated.