Product contamination by particles nucleated within the processing environment often limits the deposition rate during chemical vapor deposition processes. A fundamental understanding of how these particles nucleate could allow higher growth rates while minimizing particle contamination. Here we present an extensive chemical kinetic mechanism for silicon hydride cluster formation during silane pyrolysis. This mechanism includes detailed chemical information about the relative stability and reactivity of different possible silicon hydride clusters. It provides a means of calculating a particle nucleation rate that can be used as the nucleation source term in aerosol dynamics models that predict particle formation, growth, and transport. A group additivity method was developed to estimate thermochemical properties of the silicon hydride clusters. Reactivity rules for the silicon hydride clusters were proposed based on the group additivity estimates for the reaction thermochemistry and the analogous reactions of smaller silicon hydrides. These rules were used to generate a reaction mechanism consisting of reversible reactions among silicon hydrides containing up to 10 silicon atoms and irreversible formation of silicon hydrides containing 11-20 silicon atoms. The resulting mechanism was used in kinetic simulations of clustering during silane pyrolysis in the absence of any surface reactions. Results of those simulations are presented, along with reaction path analyses in which key reaction paths and rate-limiting steps are identified and discussed.