Surface structural controls on dolomite composition: Evidence from sectoral zoning

Abstract

Sectoral zoning in calcium-rich, sedimentary dolomite crystals involves differences in CaCO 3 content of up to 3 mol% between coeval 101̄4 and 101̄1 growth sectors. Cathodoluminescence microscopy shows that these nonequivalent sectors exhibit different overall luminescence, suggesting that consistent differences in Mn and/or Fe concentrations also exist between them. Surface structural models predict differences in microtopography and in detailed atomic configuration between these growth faces, which are interpreted as the cause of differential incorporation. Structural models also constrain possible growth mechanisms, and differences are expected for {101̄4} and {101̄1}. Transmission electron microscopy reveals that growth microstructures within 101̄4 and 101̄1 sectors differ, indicating that growth mechanisms on their respective faces differed in some manner. Analysis of concentric zoning patterns demonstrates that linear growth rates on {101̄4} and {101̄1} faces changed relative to one another during crystal growth. However, CaCO 3 content shows no corresponding change, which supports the importance of surface structural controls on element incorporation in dolomite. Surface-dependent and mechanistic influences on element incorporation during crystal growth demonstrate disequilibrium, which may be more important than previously realized.