An X-ray diffraction signal common to a wide range of clay minerals can mimic the 015-ordering reflection of dolomite

The precipitate dolomite under Earth surface conditions has been a longstanding problem in geology. Many experiments have been performed under different conditions using a wide range of additives, including different precursor minerals, such as aragonite, organic matter, bacteria, and more recently also sulphide, microbial expopolymeric substances, or clay minerals. At the same time, a study by Gregg et al. (2015) revealed that many of these experiments exhibit no ordering peaks (c-reflections) characteristic of ordered dolomite. The c-reflections are specific for the R-3 symmetry of dolomite showing cation ordering. If the ordering reflections are missing, the mineral exhibits an R-3c symmetry typical of calcite, even if the cations Ca²⁺ and Mg²⁺ occur in a near to 1:1 stoichiometric ratio – this mineral is informally called “Very high Mg-calcite” or “protodolomite”. Gregg et al. (2015) revealed that the ordering peaks have been misinterpreted in several experimental studies, and that they may in fact represent peaks of other phases, such as phosphates. Here we revisit the discussion initiated by Gregg et al. (2015), suggesting an alternative origin for the reflection at 34.7° 2theta, i.e. at the position where the 015-ordering reflection of dolomite would be expected.

A diffraction peak occurs around 34.6° 2theta in a wide range of clay minerals, such as illite, smectites, and kaolinite. While clay minerals usually exhibit only very broad baseline elevations rather than distinct peaks at higher 2theta angles, the peak seems to amplify by superposition of diffraction patterns if multiple clay minerals are present, giving rise to a sharp peak. This has been recognised in natural shale samples from Pierre Shale (South Dakota, USA; Schultz, 1964) containing a variety of different clay minerals.

In conclusion, caution must be taken in dolomite precipitation experiments if clay-rich sediment is added as a carbonate-free matrix or nucleation substrate, where the XRD reflections of clay minerals may indeed mimic the 015-ordering reflection of dolomite within 0.1° 2theta. This essentially would leave the finding of ordered dolomite unconfirmed.

Gregg, J.M., Bish, D.L., Kaczmarek, S.E. and Machel, H.G. (2015) Mineralogy, nucleation and growth of dolomite in the laboratory and sedimentary environment: a review. Sedimentology, 62, 1749–1769.

Schultz, L.G. (1964) Quantitative interpretation of mineralogical composition from x-ray and chemical data for the pierre shale. Geological Survey Professional Paper 391-C. U.S. Government Printing Office, Washington, D.C. 20402.