An extensive search for the lowest energy structure of Si-12 has been carried out using a combination of simulated annealing studies based on tight-binding molecular dynamics and density functional based Car-Parrinello calculations. This investigation revealed three families of cluster structures that are low in energy. The potential energy surface in the vicinity of these structures has corrugated landscape, similar to that associated with the conformations of long chain polymers and proteins. The lowest energy structure is a hexacapped trigonal prism, which is a continuation of the growth pattern started at Si-6, whereby the faces of a trigonal prism or anti prism seed are terminated by adatoms. This finding reveals emergence of a nucleation pattern in the growth of silicon clusters in the 6-13 atom size range.