The activation of MgCl2 is an essential step for preparing performant Ziegler-Natta (ZN) catalysts for olefin polymerization, either by mechanical grinding of pristine MgCl2 or by chemical conversion of a precursor. Depending on the adopted activation process, different nanostructures are obtained in terms of disorder and morphology, which in turn influence the overall catalytic performance. In this work, we focused on the morphology of delta-MgCl2 nanoparticles (i.e. the relative extension of the exposed surfaces), by investigating with vibrational spectroscopies a series of mechanically and chemically activated MgCl2 samples, and by building up a comprehensive set of model structures (both ordered and disordered) and relative surfaces in order to highlight nanosizing effects and achieve an accurate description of MgCl2 particles at an atomistic level. We found that both mechanical and chemical activation promote the expression of MgCl(2 )surfaces exposing strongly acidic Mg2+ sites, which are those mostly involved in the catalysis, together with an increase of inter-surfaces edges, which have been recently considered as the main responsible for the ZN catalysts stereo-selectivity. (C) 2020 Elsevier Inc. All rights reserved.