The Lonesome Mountain area in the Beartooth Mountains, Montana and Wyoming, includes large outcrops of orbicular gneiss in a terrain of amphibolites, migmatites, and granitic gneisses. The orbicules have cores of chiefly plagioclase, biotite, or hornblende-biotite, surrounded by alternating plagioclase-rich and biotite- or hornblende-rich shells. Radially oriented biotite and plagioclase are common in the cores. Tangentially oriented biotite forms the biotite-rich shells. Field and laboratory evidence suggests that orbicules formed as abnormal by-products of granitization of amphibolites. Orbicule formation involves feldspathization without complementary quartz enrichment.
Metamorphic orbicules and Liesegang phenomena are compared. Both result from diffusion processes that produce discontinuous, periodic precipitation or crystallization.
Dimensional analyses of shell spacings aid in determining the mechanism of orbicule formation. Rhythmically banded phenomena fall into two groups according to spacing of their bands: (1) spacing in exponential progression—metamorphic orbicules, spherulites, Liesegang rings, some agates; (2) spacing in arithmetic progression—igneous orbicules, concretions, oölites, oscillatory-zoned plagioclase, stalactites, varved argillites.
Objects of group 1 form by internally controlled rhythmic processes, which are always disequilibrium phenomena. Those of group 2 form by externally controlled rhythmic processes and may be equilibrium or disequilibrium phenomena. Orbicules of the Lonesome Mountain area have shells exponentially spaced and result from rhythmic diffusion processes. Laboratory evidence suggests that orbicule formation involved migration of Ca+2 ions outwards from the cores of orbicules and simultaneous migration of Na+1 and Si+4 ions inwards toward the centers of orbicules.