Pisoids

Pisoids are small, rounded carbonate concretions characterized by concentric lamination around a central nucleus. They occupy a size range between ooids and pisolites, and in many sources are described as grains larger than typical ooids but smaller than pisolites. Typical pisoids measure from about 0.5 millimeters to a few millimeters in diameter, though exact limits vary among authors. Their surfaces are usually smooth to slightly pitted, and the internal structure shows multiple concentric layers surrounding the core, which is often a sand grain or a fossil fragment.

Most pisoids are composed of calcite or aragonite, with possible coatings or impurities such as silica or iron oxides. They form through diagenetic precipitation of carbonate mineral around a nucleus in waters that are locally supersaturated with calcium carbonate. Repeated precipitation yields the laminated concentric structure, and the grains can become cemented into surrounding sediment. This process is similar to the formation of pisolites but operates at a smaller scale, resulting in micro- to small-mesoscale concretionary grains.

Pisoids are commonly found in carbonate-rich sediments such as beach and shoreface sands, tidal flats, lagoons, and shallow lacustrine deposits. Their presence often reflects episodes of carbonate supersaturation and active diagenesis under modest hydrodynamic energy, providing information about past water chemistry and sedimentary environments. Terminology for pisoids varies in the literature: some authors treat pisoids as the micro-scale members between ooids and pisolites, while others use the term more narrowly, leading to overlapping definitions across studies.