Chemical composition of Lower Ordovician ooids from northern Öland, Sweden, and their sedimentary host matrix

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Abstract
In the Lower Ordovician of northern Öland several beds contain ooids of various mineralogical composition. At Hälludden and Horns Udde the ooids are composed of phosphate and calcite; at Hagudden both chamosite and phosphate ooids occur. At Byrum, beds with chamosite ooids have been observed, but also one bed containing goethitic and hematitic ooids. The chemical composition of the ooids has been examined, as well as their sedimentary host matrix, including phosphate intraclasts and calcite crystals. The results indicate that the goethitic ooids form an intermediate state between the chamosite and the hematitic ooids. It is suggested that the chamosite ooids are of primary origin, and that the other two types evolved from them by early diagenetic reactions. The phosphate ooids are probably also of primary origin. The carbonate fraction of all ooids contains much manganese, which might indicate that calcite filled the original ooid porosity at a later stage.