Simple X-ray diffraction techniques to identify mg calcite, dolomite, and magnesite in tropical coralline algae and assess peak asymmetry

Understanding of dolomite development in reef environments has been frustrated by the failure to identify dolomite forming in abundant tropical reef algae. The analytical and sampling techniques presented here will allow fundamental questions about dolomite development in reef environments to be addressed using the physical evidence available but not previously identifiable. In this paper we describe techniques to determine the mineralogy of living tropical coralline algae, and specifically to identify dolomite and magnesite. Methods based on X-ray diffraction peak position have attracted criticism in the past because the resulting Mg contents in tropical coralline algae were in some cases lower than those based on chemical analyses of the bulk sample (in solution). The recent discovery of dolomite and magnesite in living coralline algae skeletons explains this issue, and this study demonstrates that the use of X-ray diffraction (XRD) is a reliable, quick, and affordable method for the study of the mineral make-up of coralline algae. Building on existing XRD methods to develop simple sampling and analytical methods to identify the presence of dolomite and magnesite, this study presents techniques to numerically assess (104) peak asymmetry caused by the overlapping reflections of calcite, dolomite, and magnesite. This technique allows differences between many samples and representative types to be found quickly and accurately, without relying on visual descriptions of peak shapes or mineral phase quantification. The information on the additional mineral phases, dolomite and magnesite, can be used to study intracellular calcification and/or mineral alteration processes. The results illustrate that XRD can be an integral part of any mineralogical analysis of coralline algae skeletons, which may be composed of not only Mg calcite (Ca_0.8-0.9Mg_0.2-0.1)CO₃, but also dolomite (Ca_0.5Mg_0.5)CO₃, magnesite (MgCO₃), and aragonite (CaCO₃). Furthermore, the XRD-subsampling and analytical techniques could be utilized to reanalyze carbonate reef cores to improve our understanding of dolomite distribution in reefs and transition from primary biomineralized dolomite to fabrics observed in ancient dolomites.