Phys. Chem. Minerals 6, 16%186(1980) PHYSICS [ CHEMISTRY [ MINERALS © by Springer-Verlag 1980 Structure, Formation, and Decomposition of APB's in Calcic Plagioclase H.R. Wenk and Y. Nakajima Department of Geolog)~ and Geophysics,Universityof California, Berkeley, California 94720, U.S.A. Abstract. In calcic plagioclase (Ca, Na) [(A1, Si) A1Si208] A1-Si ordering produces superstructures with periodic and non periodic antiphase bound- aries (APB's). Crystals growing at high temperature close to the melting point and cooling fairly rapidly order by nucleation of ordered domains which grow, resulting in an irregular pattern of curved APB's (b plagioclase). A modulated structure with periodic APB's forms by continuous ordering at large undercooling below the critical ordering temperature (e plagioclase). During annealing APB's are eliminated by pairwise recombination of adja- cent APB's to reduce strain energy along the boundaries thereby transforming nonstable e plagioclase into stable b plagioclase without change in chemical composition. This process is often accompanied by a chemical phase separa- tion with APB's providing favorable surfaces for diffusion. Transformations are documented by transmission electron microscopy (TEM) micrographs illustrating the variation in morphology of APB patterns in igneous and metamorphic plagioclase. They are in agreement with Korekawa et al.'s (1978) model of intermediate plagioclase which relies on periodic stacking of basic units rather than wavelike modulations. The paper includes observa- tions of a new type of satellite in Stillwater bytownite ('h' satellites) which are due to fine lamellar exsolution, Introduction Minerals belonging to the plagioclase solid solution series are the most common constituents of the earth's crust. Two substitutions, A1- Si coupled with Ca - Na, allow for both exsolution and ordering. Diffusion is slow and difficult to achieve in the laboratory. An interpretation of subsolidus phase relations has therefore to rely largely on comparative studies of geological samples which formed under a variety of conditions ranging from rapid cooling from high temperatures within a few seconds in volcanic rocks to crystallization at medium temperatures and annealing for millions of years in metamorphic rocks. Grove (1977) and 0342-1791/80/0006/0169/$03.60