  Nano-precipitates with tripled periodicity along the c-axis are observed in a
Ca-rich dolomite sample from Proterozoic carbonate rocks with "molar tooth"
structure. This observation is consistent with previous description of d
reflections. High-angle annular dark-field STEM imaging (or Z-contrast imaging)
that avoids dynamic diffraction as seen in electron diffraction and
high-resolution TEM imaging modes, confirms that d reflections correspond to
nanoscale precipitates aligned parallel to (001) of the host dolomite. The
lamellae precipitates have a cation ordering sequence of Ca-Ca-Mg-Ca-Ca- Mg
along the c direction resulting in a chemical composition of Ca0.67Mg0.33CO3.
This superstructure is attributed to the extra or d reflections, thus is
referred to as the d superstructure in this study. The structure can be simply
described as interstratified calcite/dolomite. 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 16.260 {\AA},
respectively, values between those of dolomite and calcite. The detailed
structural characteristics and parameters obtained from ab initio calculations
are also reported in this paper. The method of combining Z-contrast imaging and
ab initio calculations can be used for solving structures of other
nano-precipitates and nano-phases.
