Dolomite cation order in the geological record

Abstract

Dolomite is a carbonate mineral frequently found in sedimentary rocks from Proterozoic to Holocene. In the Iberian Peninsula, dolomites appear in a wide number of geological formations and they can be considered as representative of dolomites crystallised in most (post)sedimentary environments on Earth. In this paper, we present a first systematic study of the cation order of dolomites formed from Neoproterozoic to late Holocene. We found that the lowest values of cation order (quantified by measuring I01.5/I11.0 intensity ratios on diffractograms) mainly correspond to dolomites formed in about the last 30 Myr. In contrast, older dolomites usually reach maximum I01.5/I11.0 intensity ratios. Furthermore, higher values of cation order seem to be related to higher values of the full width half maximum of 10.4 diffraction peaks (FWHM10.4). Assuming that a decrease in FWHM10.4 (i.e. an increase in the crystallite size) in sedimentary environments indicates mineral ripening, our results show that Mg-Ca ordering in dolomites might take place mainly by a dissolution-(re)crystallisation ageing process operating over large geological periods.