SUPERSTRUCTURES AND CA-MG-ORDERING IN NATURAL SEDIMENTARY DOLOMITE

Two types of super-reflections have been found in the diffraction patterns of natural dolomite. “c” reflections are those very weak and usually streaked spots in the halfway position between the principle reflections along any of the three directions of (110)*, (104)* and (012)* in the diffraction patterns of some dolomites. The “d” reflections that are present as satellites around the principle reflections with diffraction vector 1/3 000l* have also been found in a few dolomite samples. The samples for “c” and “d” reflections in this study are a Ca-rich dolomite (Ca_1.14Mg_0.86(CO₃)₂) from Platteville Formation, West Wisconsin and a slight Ca-rich (~52 mole % of CaCO₃) dolomite sample from Proterozoic carbonate rocks with “molar tooth” structure, respectively.

It was proposed that the “c” reflections can be formed in domains with an ideal composition of Ca_0.75Mg_0.25CO₃ when ordering within the basal (001) cation planes happens (Van Tendeloo et al., 1985). Our STEM images confirm that the “c”-reflections could result from multiple diffractions between the host dolomite and twinned Mg-calcite nano-lamellae under TEM imaging and diffraction modes. The “d” reflections were interpreted as indicating a stacking defect along c-axis involving periodic substitution of Ca layers for every third Mg layers (Van Tendeloo et al., 1985). Our STEM investigation of proterozoic Molar Tooth dolomite shows that “d” reflections correspond to lamellae precipitates that have cation ordering sequence of Ca-Ca-Mg-Ca-Ca-Mg along c direction with a chemical composition of Ca_0.67Mg_0.33CO₃. The crystal structure of the d superstructure calculated from density functional theory (DFT) has a space group of P31c and has a and c unit cell parameters of 4.879 and 16.26 Å, respectively, in between dolomite and calcite. The method of combining Z-contrast imaging and ab initio calculation can be used for solving structures of other nano-precipitates (such as G. P. zones) and nano-phases.