The diffusion of manganese in manganese silicate, Mn2SiO4. in equilibrium with MnSiO3 was experimentally studied as a function of crystallographic orientation, oxygen activity and temperature using high purity synthetic single crystals of manganese silicate. The oxygen activity dependence of the manganese cation diffusion in Mn2SiO4 along the three principle orientations at 1200 degrees C suggests that, at high oxygen activities, manganese cations move via different types of vacancies, most likely isolated manganese vacancies, (VMn2+)(n), and possibly neutral associates, {2(Mn-Mn2+(3+))(center dot) center dot(VMn2+)(n)}(x), the latter being minority defects. At lower oxygen activities, the oxygen activity dependence of the manganese tracer diffusion becomes smaller than that at higher oxygen activities and eventually becomes negative at very low oxygen activities. The latter is attributed to an increased concentration of manganese interstitials at lower oxygen activities. The temperature dependence of the manganese tracer diffusion along the three principle orientations was experimentally investigated between 1150 and 1250 degrees C at log a(O2) = -2.7 (a(O2) = P-O2/P-O2(0), with P-O2(0) = 1 atm) and between 1150 and 1300 degrees C at log a(O2) = -9.8. The results obtained suggest that the orientation dependence of the diffusion of manganese in Mn2SiO4 does not vary very significantly with temperature. When using the space group Pbnm to describe crystal orientations, the ratio found for the manganese tracer diffusion coefficients at log(O2) = -2.7 is approximately D-Mn[001](center dot):D-Mn[010](center dot):D-Mn[100](center dot) = 5.3:2.2:1. (C) 2012 Elsevier B.V. All rights reserved.