In this work we explore the low energy complexions of the symmetrical tilt grain boundary (GB) 60.8°//[100](011) in forsterite through molecular dynamics and first principles calculations. Using conservative sampling, we find six stoichiometric complexions with energies ranging from 0.66 to 1.25 J.m−2. We investigate the segregation of MgO vacancy pairs, and find that in most cases it is more favorable for vacancies to lie within the GBs than in the surrounding crystals, leading to new atomic structures. From these results we infer that at finite temperature when vacancies are present in the system, GBs are likely to absorb them and to be non-stoichiometric. We find many GB complexions containing a free oxygen ion, which may have profound implications for geological processes.