47-9 Time-resolved X-ray Diffraction Studies of Dolomitization: Mechanisms and Kinetic Constraints on Ordered Dolomite Formation

Session: Minerals in Motion: Tracking Mineral Reactions Using In Situ and Synchrotron Techniques, A Celebration of the Career of Peter Heaney

Presenting Author:

Authors:

Abstract:

Dolomitization altered vast volumes of limestone during the Precambrian and Phanerozoic, but the (bio)geochemical and hydrologic pathways that governed diagenesis remain hotly debated. Although ordered dolomite is thermodynamically more stable than disordered protodolomite, efforts to crystallize long-range ordered, stoichiometric dolomite at ambient temperature and pressure have largely been unsuccessful. Therefore, the mechanisms and the kinetic constraints on ordered dolomite formation are still controversial.

The dolomitization process occurs in two steps: from calcite (S.G. R-3c) to protodolomite (S.G. R-3c), which in turn evolves to ordered dolomite (S.G. R-3). To understand mechanisms and kinetic controlling factors of each step, we conducted time-resolved X-ray diffraction (TRXRD) studies of the transformation from synthetic calcite or protodolomite to ordered dolomite at temperatures ranging from 120-220 ℃. Rietveld analysis of the TRXRD data revealed that: 1) the onset of protodolomite formation coincided with the loss of calcite peak intensity; 2) ordered dolomite diffraction peaks appeared when refined calcite abundance dropped below ~10 wt%; 3) the Ca occupancy of protodolomite plateaued at ~0.6 when there was ~60 wt % of calcite in the system; 4) With no calcite present, the precipitation rate of ordered protodolomite was ~9 times greater relative to when calcite represented ~50 wt % of the powdered solids. These results suggest that 1) the presence of calcite (limestone) may inhibit the formation of ordered dolomite; and 2) a Ca composition of ~60 mol% is the metastable composition of protodolomite when a significant amount of calcite co-exists with protodolomite.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8215

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.