Carbonate sand from Bermuda. Many of the grains in this sand are tests of benthic foraminifera, single-celled amoeboid organisms that are commonly abundant on the seafloor. The pink forams are of the genus and species Homotrema rubrum. The microscopic image linked at right also shows a squarish peneroplid foram grain at the upper center of the image.         The diverse grain sizes seen here are not unusual in carbonate sands, and they probably result from the variation in density between grains of nearly solid CaCO3 and more porous grains.

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