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Theoretical examination of possible nonuniform spin configurations in metallic iron indicates that circular spin (CS) is the lowest-energy nonuniform arrangement. The upper grain size limit (d0) to single-domain (SD) behavior is thus defined by the SD to CS transition. Superparamagnetic (SP) behavior marks the lower grain size limit to the stable SD range, and the SP to SD threshold size (ds) can be determined by Néel's relaxation theory. Calculations of d0 and ds for spherical metallic iron particles at 290°K indicate that d0 ( = 173 Å) < ds ( =260 Å), and no stable SD range exists. A stable SD range does exist for prolate ellipsoids of elongation q > 1.1 but remains very constricted. For a prolate ellipsoid of q = 1.67, a stable SD range occurs between the SP critical length LS = 150 Å and d0 = 360 Å. Both d0 and ds increase with temperature, but the stable SD range decreases. The size and shape criteria for the stable SD behavior of metallic iron help to explain (1) the low SD content of lunar samples, (2) the widespread occurrence of SP behavior and viscous magnetization in lunar soils and low metamorphic grade breccias, (3) the changes in the magnetic properties of breccias during annealing, and (4) the increased SD content of shocked breccias. The narrow grain size limits for SD behavior also suggest that magnetostatic interaction between metal grains in the solar nebula is not a viable mechanism for iron-silicate fractionation.