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mineral formations of spherical shape found in sedimentary rock or recent sediments. Grains of minerals, rock debris, shells, fish teeth and bones, and plant remains may serve as centers of concretion. Most prevalent among the diverse forms of concretions are globular shapes and, less frequently, elliptical, disklike, and irregular (grown-together) shapes. In terms of structure, the most common are concentrically layered (conchoidal), coarse-banded, radiate-fibrous (spherulitic), and globular concretions. They usually consist of calcium carbonates (calcite and, less frequently, aragonite), ferric oxides and iron sulfides, calcium phosphates, gypsum, and manganese compounds. In limestone they frequently consist of silicic acid (flint nodules).
Concretions are found in deposits of various geologic systems and in sediments of modern lakes, seas, and oceans. Considerable accumulations of ferromanganese concretions (nearly 10 percent of the entire area of the ocean floor), which are of practical interest, are located on the surface of the floors of the Pacific, Atlantic, and Indian oceans.