Concretions from Cretaceo-Tertiary sediments from the Kaipara harbour, New Zealand, exhibit a complex suite of mineral and textural relationships. Spherulitic barite occurs as an exsolution product from calcium carbonate, as does rhombohedral rhodochrosite. Spherulitic calcite is common, and spherulitic rhodochrosite also occurs. From a comparison of these textures with experimental evidence on the inhibition of metastable carbonates by manganese ions, a complex diagenetic history is inferred. It is suggested that solid solutions of the calcite and aragonite structure and also calcium carbonate monohydrate were present in the early diagenetic history of the rock.