MICHAEL SZOENYI / SCIENCE PHOTO LIBRARY MICHAEL SZOENYI / SCIENCE PHOTO LIBRARY
Calcite rock concretion. This round rock (lower left) formed as the mineral calcite (calcium carbonate) precipitated out of water close to the water-sediment border at the bottom of an ancient lake. This concretion is one of several at Kettle Point, on Lake Huron, Ontario, Canada. Lake Huron is seen in the background.
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