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 sea urchins. All the analyzed samples contained magnesian calcite (Ca, Mg)CO3 as a dominant phase, and 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 employed to determine the Mg fraction in magnesian calcite. It was found that different parts of the adult sea urchin skeleton had different Mg contents, which is in agreement with previous results [5]. A nonlinear relation between the lengths of unit-cell parameters (a, c) and the Mg fraction in magnesian calcite was found. Changes of intensities for various reflections of magnesian calcite accompanying composition change were also examined. A linear correlation between the Mg fraction in magnesian calcite and the intensity ratio of I113/I202 was found, which is the basis for the quick and easy estimation of the 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. Medakovi , S. Popovi , N. Zavodnik, B. Gr eta and M. Plazoni , Mar. Biol. 122 (1995) 479. [5] D. Medakovi and S. Popovi , Book of abstracts of the Ninth Slovenian- Croatian Crystallographic Meeting, Eds. I. Leban et al., Ljubljana, 2000, p. 47.

Magnesian Calcite; Sea Urchins; Xrd; Rietveld Refinement; Magnesium Fraction in (CA; Mg)CO3

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