engleski

New method for the estimation of magnesium fraction in magnesian calcite

The specimens of the adult sea urchins Sterechinus neumayeri which were collected from the bottom of McMurdo Sound, Ross Island, Antarctica, were examined by X-ray powder diffraction. Several samples were prepared from the skeleton and spines of the urchins. All the analysed samples contained magnesian calcite (Ca, Mg)CO3 as a dominant phase, only traces of feldspars were detected. The unit-cell parameters a and c of magnesian calcite in various samples were determined using Si as an internal standard, and refined by the whole-powder-pattern decomposition method [1]. The Rietveld method [2] using the PFLS program [3], and the method of line shifts in relation to pure calcite [4], were performed to determine Mg fraction in magnesian calcite. It was found that different parts of the adult sea urchin skeleton contained different Mg contents, which is in agreement with the previous results [5]. A nonlinear relation between the lengths of unit-cell parameters (a, c) and Mg fraction in magnesian calcite was found. Furthermore, a change of intensities for various reflections of magnesian calcite accompanying composition change was examined. A linear correlation was found between the Mg fraction in magnesian calcite and the intensity ratio of I113/I202, which is the basis for the quick and easy estimation of Mg fraction in magnesian calcite. [1] H. Toraya, J. Appl. Cryst. 19 (1986) 440. [2] H. M. Rietveld, J. Appl. Cryst. 2 (1969) 65. [3] H. Toraya and F. Marumo, Rep. Res. Lab. Eng. Mater. Tokyo Inst. Technol. 5 (1980) 55. [4] D. Medaković, S. Popović, N. Zavodnik, B. Gržeta, and M. Plazonić, Mar. Biol. 122 (1995) 479. [5] D. Medaković and S. Popović, Book of abstracts of the Ninth Slovenian- Croatian Crystallographic Meeting, Eds. I. Leban et al., Ljubljana, 2000, p. 47.

Magnesian calcite; X-ray diffraction; Rietveld refinement; Mg fraction

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