A new method for estimation of the magnesium fraction in magnesian calcite is described. It involves measuring the XRD pattern of magnesian calcite in a narrow 20 range and individual profile fitting of diffraction lines 113 and 202. The intensity ratio I-113/I-202 is linearly correlated with the Mg fraction. The method resulted from an XRD study of Adult sea urchins Sterechinus neumayeri.