Homogeneous garnet Fe3Cr2Si3O12 and its solid solutions with garnet Mg3Cr2Si3O12 (four intermediate samples) were synthesized at high pressure. Garnet Mg3Cr2Si3O12 was not synthesized in pure form. The sample contained traces of the two phases, MgSiO3 and Cr2O3. The products were investigated by means of X-ray powder diffraction and differential scanning calorimetry. Molar volumes of the solid solutions as a function of composition fit linear regression with an accuracy of 0.2%. No difference in heat capacity within the limits of experimental error was detected. In thermodynamic evaluations, the garnet solid solutions Fe3Cr2Si3O12-Mg3Cr2Si3O12 may be treated as ideal solid solutions. (C) 2002 Elsevier Science Ltd. All rights reserved.