Single‐crystal X-ray structure refinements of crystals with variable compositions in the Ni2SiO4‐Mg2SiO4 join indicate substantial deviations from Vegard’s law, primarily in their anomalous values of the b cell edge, consistent with previous observations by Matsui and Syono (1968). The preferential stabilization of Ni in M1 arises primarily from repulsive and dispersive interactions and secondarily from the contribution of crystal‐field stabilization. Excess entropy‐of‐mixing terms arising from ordering on sites are counterbalanced by negative heat‐of‐mixing terms. The mixture is thus not far from ideal in terms of Gibbs free energy over a wide range of temperatures, in agreement with the experimental observations of Campbell and Roeder (1968) and more recent findings (Seifert and O’Neill, 1987).

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