Dozens of iron concretions are trapped in a sandstone pothole in the Grand Staircase Escalante in southern Utah. These iron concretions formed naturally between 6 and 25 million years ago as water dissolved the iron pigment in the red sandstone in the area. The pigment flowed down through the now bleached sandstone and then solidified when it came in contact with oxygenated water, forming a new iron mineral called hematite between the grains of sandstone. Over time, the sandstone eroded away, leaving the more durable iron concretions behind. These largely spherical balls are composed of a hard outer layer of hematite covering a ball of pink sandstone. By volume, the sandstone makes up the majority of these iron concretions, though those found elsewhere in the Colorado Plateau may contain much more hematite. Scientists aren't sure why they form in spheres or if they need something in particular as a nucleus to start growing.
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