BÖTTCHER, M.E. & GEHLKEN, P.-L. (1996):
Characterization of inorganic and biogenic magnesian calcites by Fourier transform infrared spectroscopy.
The position of the 2 seems to be related to the stoichiometric solubility product results of BUNSENBERG & PLUMMER (1989).
The spectra of some biogenic magnesian calcites display specific absorption bands of organic material (e.g. Echinus esculentus spine: 1003, 950, 626 cm-1) disappearing due to H2O2 treatment of the carbonate. A band near 3700 cm-1 indicates brucite (Mg(OH)2) in Goniolithon high-magnesian-calcite.
In the present study, natural inorganic and biogenic, and synthetic CaCO3 – MgCO3 (calcite -magnesite) solid-solutions with space group Rc are investigated by Fourier Transform infrared spectroscopy , between 4000 and 400 cm-1. The compositions cover the range between 0 and 23 mol % MgCO3. They include samples which have been used in the experimental solubility and dissolution kinetics studies of BUNSENBERG & PLUMMER (1989) and SVENSSON & DREYBRODT (1992). The FTIR spectra are compared to those obtained for ordered dolomite (Ca0.5Mg0.5CO3; space group R), huntite (Ca0.25Mg0.75CO3; space group R32) and magnesite (MgCO3; space group Rc).
In the FTIR spectra of all magnesian calcites the out-of-plane bending (2), the asymmetric stretching (3), and the in-plane bending modes of the carbonate ion group are found to be infrared-active. Besides the first order internal modes, the (1+ 3) and ( 1+ 4) combination modes are observed. Although theoretically forbidden in the Rc structure, the symmetric stretching mode 1 is observed between 1083 and 1088 cm-1, probably due to some positional disorder of the carbonate ion group (BISCHOFF et al. 1985). A small satellite band which is shifted to lower wave-numbers with respect to the main 2 band results from the out-of-plane bending mode of 13CO32- (e.g. BÖTTCHER & GEHLKEN 1995). The average carbon-isotopic-shift coefficients for 2 (0.969) agrees well with theoretical predictions (GOLYSHEV et al. 1981).
Increasing substitution of Ca2+ by Mg2+ leads to increasing wave-numbers of the 1, 3, 4, (1+3) and (1+4) modes. The Mg2+ content of all magnesian calcites correlates linearely with the position of the very sharp 4 mode, independend from the origin of the Mg-calcites, according to: MgCO3 (mol %) = 2.4508 x 4 (cm-1) – 1744.7 (r = 0.98, n = 33).
The position of the 2 seems to be related to the stoichiometric solubility product results of BUNSENBERG & PLUMMER (1989).
The spectra of some biogenic magnesian calcites display specific absorption bands of organic material (e.g. Echinus esculentus spine: 1003, 950, 626 cm-1) disappearing due to H2O2 treatment of the carbonate. A band near 3700 cm-1 indicates brucite (Mg(OH)2) in Goniolithon high-magnesian-calcite.