Nodule

Science / Geology / Nodule: A mineral mass that has a different composition or is more weathering resistant than its surrounding rock. These are normally rounded in shape. Examples include: chert masses in a limestone rock unit, pyrite masses in a coal seam, or carbonate masses in a shale. In most cases these 'nodules' have formed within the rock unit or its former sediment mass. The term is also applied to rounded masses of manganese minerals that occur on some parts of the ocean floor.
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