The petrology of ikaite pseudomorphs and their diagenesis

Petrographic examination of pseudomorphs after ikaite (CaCO3.6H2O) from five localities around the world, with ages ranging from Permian to Quaternary, indicates many petrographic similarities, with up to three generations of calcite, plus other 'late' infilling or replacive cements. The similarities of the first generation of calcite (type 1) developed in all samples examined suggests that the initial ikaite decomposition followed a similar pattern in all these examples. Type 2 calcite is spherulitic and type 3 is calcite spar that infills the cavities between types 1 and 2. Types 2 and 3 are present in pseudomorphs from the Black Alley Shale (Permian, Australia), Carnavon Gorge (probably Permian, Australia) and the Fur Formation (Eocene, Denmark). Gennoishi (pseudomorphs from the Miocene, Japan) and jarrowites (pseudomorphs from Quaternary muds, River Tyne, UK) contain only type 1 calcite.