Such substitution is common for this mineral. Modern Geology, v. 4, p. 85-106. See additional spectra of grossular garnets in library for comparison. This sample has been washed in HCl acid to remove carbonate contaminants. Some of the thinner grains are isotropic, but the thicker grains have mottled extinction suggesting intense internal fracturing.
Grains have high relief, high refrative index, rough fracture and no cleavage.
Apart from the mottled extinction this sample resembles garnet. Sample tested with HCl acid and gave no fizz.