orbicule

An orbicule is a spherical body found in some granites and other rocks. These bodies can range in size from microscopic to over 10 feet in diameter and are characterized by concentric shells of their constituent materials.1

In geology, orbicular structures refer to rounded, onion-like masses with distinct concentric layering found within phaneritic rocks. The layers within each orbicule are typically thin, irregular, and sharply defined, differing in composition or texture from adjacent layers. Some layers may contain mineral grains oriented radially, similar to spherulitic layers in volcanic rocks. While the minerals in orbicules are usually the same as those in the surrounding rock, their proportions may differ. This concentric structure is thought to result from rhythmic crystallization around specific centers during the early stages of rock formation.2