Saddle dolomite is a common product of late-stage diagenesis and hydrothermal activity. It has been suggested that during growth Ca enrichment occurs towards crystal edges leading to a lattice expansion relative to face centres. However, backscatter scanning electron microscopy of eight samples has revealed that, instead of edge associated Ca enrichment, saddle dolomites have a series of edge associated Mg enriched wedges, between 5 and 20 µm thick and 300 µm long. Wedge geometry implies development of extra lattice layers at edges relative to face centres. It is suggested that the wedges develop during rapid, transport-controlled, crystal growth. The wedges possibly reflect a switching from continuous growth across the face to edge-nucleated growth as the boundary layer solution becomes progressively depleted. Continuous growth might be reinstigated through convective turnover of the boundary layer presenting fresh solution to the growing crystal.
