Epitaxial CoSi2 layers, which are phase pure but contain {111} twins, are grown on Si(001) at 700 degrees C by reactive deposition epitaxy. Transmission electron microscopy analyses show that the initial formation of CoSi2(001) follows the Volmer-Weber mode characterized by the independent nucleation and growth of three-dimensional islands whose evolution we follow as a function of deposited Co thickness t(Co) in order to understand the origin of the observed twin density. We find that there are two families of island shapes: inverse pyramids and platelets. The rectangular-based pyramidal islands extend along orthogonal < 110 > directions, bounded by four {111} CoSi2/Si interfaces, and grow with a cube-on-cube orientation with respect to the substrate: (001)(CoSi2)parallel to(001)(Si) and [100](CoSi2)parallel to[100](Si). Platelet-shaped CoSi2 islands are bounded across their long < 010 > directions by {111} twin planes (i.e. {111}(001)CoSi2 parallel to{111}(Si)) and their narrow < 010 > directions by {151}(CoSi2)parallel to{111}(Si) interfaces. The top and bottom surfaces are {2 (2) over bar1}, with {2 (2) over bar1}(CoSi2)parallel to(001)(Si), and {(11) over bar1}, with {(11) over bar1}(CoSi2)parallel to{1 (1) over bar1}(Si), respectively. The early stages of film growth (t(Co) <= 13 angstrom) are dominated by the twinned platelets due to a combination of higher nucleation rates resulting from a larger number of favorable adsorption sites in the Si(001)2 x 1 surface unit cell and rapid elongation of the platelets along preferred < 110 > directions. However, at t(Co) >= 13 angstrom island coalescence becomes significant as orthogonal platelets intersect and block elongation along fast growth directions. In this regime, where both twinned and untwinned island number densities have saturated, further island growth becomes dominated by the untwinned islands. A continuous epitaxial CoSi2(001) layer, with a twin density of 2.8 x 10(10) cm(-2), is obtained at t(Co) = 50 angstrom. (c) 2006 Elsevier B.V. All rights reserved.