A mechanistic model of impact attrition of particulate solids, having a semi-brittle failure mode, has been developed. The model describes the chipping mechanism, where the material loss from the particle is due to the formation of the subsurface lateral cracks. Indentation fracture mechanics is used to analyse the propagation of these cracks in order to provide a base to estimate the rate of attrition. A dimensionless attrition propensity parameter is derived from the above approach, whereby the extent of breakage is related to the material properties and impact conditions. It has the form: [GRAPHICS] where rho is the particle density, upsilon is the impact velocity, l is a characteristic particle size, H is the hardness and K-c is the fracture toughness. The experimental evaluation of the above model is presented in Part 2, where the impact attrition of three materials has been quantified and compared to the model predictions.