Orbicular rocks, which are worldwide in occurrence, have been described in more than one hundred localities. Orbicules are found in igneous, metamorphic, and migmatitic terrains and are not restricted to unusual or limited compositions. Concentric shells of contrasted texture and mineralogy about a central core compose orbicule structure, which is often characterized by radially and/or tangentially oriented minerals.
Hypotheses of orbicule genesis include both magmatic and metamorphic origins for these rocks; however, no single hypothesis provides a general explanation. Orbicules are compared with other natural and artificial rhythmically layered structures (Liese-gang rings and layered igneous rocks). Spacing of orbicule shells reflects stability of the formational environment. Some orbicule shells result from exchange of material between core and matrix. Conditions that cause rhythmic layering in igneous rocks may result in orbicule formation if crystallization is localized about scattered centers. Cores of such orbicules serve as crystallization centers and need not have specific or limited compositions. No correlation has been demonstrated between orbicule structure, chemical composition, or gross geologic setting.
Standard orbicule terminology is suggested: orbicular rocks, orbicules, cores, shells, matrix, and country rock. Descriptive classification of occurrences of orbicular rocks is made according to composition (country rock type), structure (proto-, single-shell, and multi shelled [regular or irregular]), texture (granular, tangential, and radial), and core type (cores similar and dissimilar to matrix).