The site symmetry and magnitude of the crystal field splitting of metal atoms trapped in rare gas matrices may be determined by a combination of the temperature and magnetic field dependence of magnetic circular dichroism (MCD) and magnetic linear dichroism (MLD). General theoretical expressions are derived for the MCD and MLD considering the Zeeman effect and the matrix (i.e., crystal field) as perturbations on the free atom JM(J)] states. These expressions are applied to the observed MCD and MLD T- and B-saturation curves for Fe atoms isolated in krypton and xenon matrices in the range 0-1.5 T and 2.3-22 K. An octahedral crystal field model accounts well for all four independent sets of data (MCD vs B, MCD vs 1/T, MLD vs B-2, and MLD vs 1/T-2) for two different, isolated electronic transitions. A crystal field parameter of +0.06+/-0.05 cm(-1) has been determined, corresponding to an overall electronic ground-state splitting of 3.2+/-2.5 cm(-1). This splitting is less than the optical bandwidth and is shown to be consistent with previous studies of rare gas matrix-isolated Fe atoms by Mossbauer and laser-excited emission spectroscopies.