• journal_title：American Mineralogist
• Contributor：Zhizhang Shen ; Hiromi Konishi ; Philip E. Brown ; Huifang Xu
• Publisher：Mineralogical Society of America
• Date：2013-04-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2013.4184
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：98
• issue：4
• firstpage：760
• section：Articles

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 [Ca_1.14Mg_0.86(CO₃)₂], an Fe-bearing dolomite [Ca_1.06Mg_0.80Fe_0.14(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 (1̄01), 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 Ca_0.85Mg_0.15CO₃ to Ca_0.70Mg_0.30CO₃. 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)*, (1̄12)*, 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.