OSCILLATORY ZONING IN MINERALS - A COMMON PHENOMENON

Crystals characterized by oscillatory zoning show evidence for a quasi -cyclic alternation in the chemical composition of discrete growth-she lls from tens of nanometers to tens of micrometers in thickness. Oscil latory zoning is most commonly observed and studied in crystals of mag matic plagioclase, but sensitive imaging techniques (e.g., Nomarski in terference contrast microscopy, cathodoluminescence, back-scattered el ectron imaging, X-ray topographs) reveal its presence in many other mi nerals. Through the application of these techniques and an exhaustive review of the literature, oscillatory zoning is shown to occur in at l east 75 rock-forming and accessory minerals comprising most major mine ral classes: silicates (hydrous and anhydrous), sulfides, oxides, hali des, carbonates, phosphates, and sulfates. Such mineral zoning is a co mmon but often well-concealed phenomenon in magmatic rocks (particular ly alkaline ones), hydrothermally altered rocks, mineralized rocks, an d carbonate sequences. Mechanisms of oscillatory zoning are discussed, including the less-studied effects of adsorption and elastic stress i n near-surface growth-layers. Its ''non-equilibrium chemistry'' makes oscillatory zoning a rich subject for further research.