Pisolite

One of the most fantastic type of rocks I had the pleasure of seeing during field camp was pisolite. These pisolites in particular were formed in the back reef of the Permian aged Capitan Reef complex in modern-day southern New Mexico when CaCO3 precipitated around a nucleus. The motion of the tide in these shallow environments allowed a repeated and somewhat even coating of calcium carbonate to surround the original nucleus.

 Pisoids (the circular, layered grains) are most easily differentiated from ooids, another common type of coated grain, by their size: pisoids are greater than 2mm in diameter.

We did much of our stratigraphy studies in the back reef and came across the most stunning and abundant pisolites just inside the entrance of Carlsbad Caverns National Park, where this one was photographed. Removing rocks, artifacts, plants, and animals is prohibited in national parks, so all of the pieces seen that day were left where there were found, although I was able to collect a lesser specimen for myself from another location.

Check out the gallery below for more!

Pisolites with mechanical pencil for scale. This piece was left in the field.