Two stages of authigenic growth are recognized in pyrite concretions which occur in thinly laminated Lower Ordovician shales of the Oslo region, Norway. At an early stage growth took place by the precipitation of framboids and tiny, single crystals in the pore spaces of the sediments. During a subsequent stage space was provided by displacement of sediment by dendritic growth. The development of cone-in-cone structures is related to the dendritic growth mechanism which is assumed to be due to the establishment of supersaturation gradients ahead of the growing crystals.

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