The growth and form of crystalline micrometer-long BaSO4 fibers, synthesized by unstirred room temperature (18-22 degrees C) reaction of Ba(AOT)(2) reverse micelles with NaAOT microemulsions containing sulfate anions at w = 10 and [Ba2+]:[SO42-] molar ratios between 5:1 and 1.4:1, was studied principally by transmission electron microscopy. The fibers consist of twisted bundles of BaSO4 nanofilaments, elongated along their [010] axis and held together by interdigitated surfactant bilayers. Similar experiments at 4, 30, and 40 degrees C produced short straight filament bundles, highly curved and cone-shaped structures, and spindle-shaped aggregates, respectively. The filamentous structures develop within transient aggregates consisting of 5-nm-size surfactant-nanoparticle clusters that precipitate from the reaction medium after several hours at room temperature. The first stage of transformation involves the nucleation of a single 5-nm-wide BaSO4 filament, followed by further filaments parallel to the primary thread. Coalignment of the filaments results in coiling of the bundle to give a self-terminating spiral-shaped structure, several hundred nanometers in size. Unidirectional linear outgrowth from the open end eventually gives rise to macroscopic fibers that often become splayed into cone-shaped features. A mechanism is discussed in which the primary surfactant-nanoparticle aggregates are specifically formed by a self-terminating process under conditions of molar excess of Ba2+ ions and evolve into the filamentous structures through structural reconstruction driven by the interplay of membrane curvature and inorganic lattice energy.