A RELATIONSHIP BETWEEN D-104 VALUE AND COMPOSITION IN THE CALCITE - DISORDERED DOLOMITE SOLID SOLUTION SERIES

XRD has been a long-established technique for compositional determination in solid solution series. Differences in ionic sizes between substituting and host cations lead to systematic variations in unit cell parameters and interplanar spacings which can be detected by using XRD method. This approach has been used widely in analyzing rhombohedral Ca-Mg carbonates. For example, compositions of biogenic and inorganic (Ca,Mg)CO₃ crystals are often calculated by comparing measured d -104 value with published determinative curves.

However, the validity of these working curves has been disputed, especially for high Mg content crystals. Previous working approximation assumes that a nearly straight line connecting the respective values for pure calcite and ideal dolomite. No evidence, however, of any systematic investigation of well-characterized natural dolomites to validate this approximation. All the published working curves do not work for very high-Mg calcite (VHMC) and disordered dolomite, because Ca-Mg ordering states will also affect the unit cell size and d -104 values.

Considering the common occurrence of VHMC and disordered dolomite in natural environments, a working curve for disordered counterparts would be therefore useful to study VHMC and disordered dolomite skeletal particles and cements. Based on this idea, a new working curve is constructed based on VHMC and disordered dolomite samples synthesized from sulfide-bearing solutions at temperatures from room temperature to 45 °C. Electron diffraction and TEM observations prove that the synthesized dolomite is completely disordered. The compositions of these samples were determined using quantitative X-ray EDS analyses under TEM. There is a large difference in d -104 values between disordered dolomite and ordered dolomite. The new relationship can help quantity the MgCO₃ content in both natural and synthetic VHMC and disordered dolomite, especially for mineral mixtures that are not suitable for other study methods, e.g. atomic absorption, ICP-AES and electron microprobe.