engleski

The fraction of magnesium in magnesium calcite from the adult Antarctic Sea urchins Sterechinus neumayeri

The adult sea urchins Sterechinus neumayeri sampled from the bottom of McMurdo Sound, Ross Island, Antarctica, were examined by X-ray powder diffraction. Several samples were prepared from a sceleton and spines of the urchines. All the analyzed samples contained magnesium calcite (Ca, Mg)CO3, which was a dominant phase. Only traces of feldspars and amphiboles were detected. In this work our interest has been focused on determination of Mg content in magnesium calcite. The unit-cell parameters of magnesium calcite in various samples were determined using Si as an internal standard, and refined by the whole-powder-pattern decomposition method [WPPF program, H. Toraya, J. Appl. Cryst. 19 (1986) 440]. The Rietveld method was performed to determine Mg content substituting for Ca in magnesium calcite. It was found that different parts of the adult sea urchin skeleton contained more Mg than spines, which is in agreement with the previous results [D. Medaković and S. Popović, Book of abstracts of the Ninth Slovenian-Croatian Crystallographic Meeting, Eds. I. Leban et al., Ljubljana, 2000, p. 47]. The obtained Mg content was correlated with the integrated intensities and shifts of diffraction lines of magnesium calcite in relation to pure calcite (CaCO3) and pure magnesite (MgCO3), and also with the unit-cell parameters. A technique for easy estimation of Mg content in magnesium calcite from the integrated intensities of selected diffraction lines will be proposed.

Sterechinus neumayeri; XRD

nije evidentirano