Determination and distinction of Ca-dolomites, Fe-dolomites, ankerite or stoichiometric dolomite by X-ray diffraction using a profile fitting

The X-ray diffraction study of dolomites from various sedimentary rocks shows that the dolomite is rarely a stoichiometric one. The substitution of some Mg 2+ (ionic radius: 0.66 A) by larger ions like Fe 2+ (0.74 A) or Mn (0.80A) or Ca 2+ (0.99A) increases the size of the unit cell, which leads to an increase of the d 10.4 spacing. The d 10.4 spacing varies from 2.888 A (dolomite) to 2.901 A (ankerite). The distinction of the dolomite from ferroan dolomite and ankerite in carbonate mixtures is obtained by a profile fitting of X-ray patterns which permits the precise localization of individual peaks even in cases where several distinct carbonates are present. The X-ray determination is confirmed by electron microprobe analysis and the structural data of the dolomites is calculated.