  Dolomite crystals in partially dolomitized limestone from the Platteville
Formation are both compositionally and microstructurally heterogeneous. A
single dolomite crystal usually contains three phases: the host Ca-rich
dolomite [Ca1.14Mg0.86(CO3)2], an Fe-bearing dolomite
[Ca1.06Mg0.80Fe0.14(CO3)2], and calcite inclusions. These three phases show
similar orientations. The Ca-rich dolomite exhibits modulated microstructures
with wavelength ranging from 7 to 30 nm. The modulated microstructures are not
evident in Fe-bearing dolomite. Modulations in the Ca-rich dolomite have three
predominant orientation ranges in the studied sample: from (205) to (104), from
(001) to (-101), and ~(110), which are consistent with previous studies.
Bright-field (BF) and high-angle annular dark-field (HAADF) images confirm that
these modulations are due to chemical variation rather than strain or
diffraction contrast. The Ca-rich lamellae are Mg-rich calcite with
compositions ranging from Ca0.85Mg0.15CO3 to Ca0.70Mg0.30CO3. The observed
results indicate that these Ca-rich exsolution lamellae formed during
diagenesis. In this study, three kinds of "c"-reflections, which are weak spots
in the halfway position between the principal reflections along the (104)*,
(112)*, and (110)* directions, have been found in the diffraction patterns of
some Ca-rich dolomite. Mg-Ca ordering in x-y planes was not observed directly
in Z-contrast images. FFT patterns from the Z-contrast images do not show
"c"-reflections. STEM images confirm that the "c"-reflections could result from
multiple diffraction between the host dolomite and twinned Mg-calcite
nano-lamellae under TEM imaging and diffraction modes.
