Morphology, chemistry and growth mechanisms of calcite concretions from anindustrial water-softening process: implications for the origin of naturalooids in sediments

Low-magnesium calcite concretions, produced within a commercial, water-soft ening pellet reactor have many of the internal and external morphological f eatures of natural carbonate ooids, both ancient and modem. They form insid e the reactor through a continuous chemical process involving reaction betw een carbonate/bicarbonate ions from incoming borehole waters and added lime . Precipitation of calcite occurs around silica sand grains which act as nu cleation points. The shape of the sand grains controls the overall external morphology of the resulting ooids which are typically sub-rounded or lobat e; they vary in size from less than 0.1 mm up to 4 mm and have a high degre e of polish due to surface abrasion caused by continuous agitation inside t he reactors. Characteristic, concentric banding inside the ooids is a physi co-chemical phenomenon related to perturbations in growth conditions induce d by changes in pH inside the reactor. Individual calcite crystals grow ort hogonally from the surface of the seeds and in optical continuity across th e concentric bands. Stellate clusters of larger crystals cross-cut these ba nds. Overall, the mineralogical, textural and morphological data support th e view that radial growth of calcite crystals in natural ooids is the resul t of chemical rather than biological or diagenetic processes. Very high gro wth rates (averaging 0.1 mm per day) and the continuity of the precipitatio n process demonstrate that in certain, analogous, geochemical environments (e.g. in alkaline plays lakes) natural ooids can form in a very short time span and do not require quiescent,'rest' periods to attain their typically layered internal form. The sudden change in water chemistry resulting in pe rturbations in growth rate is the: cause of this layering.