Thermal barrier coatings (TBCs) are thermally insulating ceramic layers added to the surfaces of actively cooled superalloy gas turbine components that enable them to operate more efficiently at high temperatures. Durability of TBCs depends on their tolerance to thermal cycling, which results from a porous microstructure that induces a low in-plane elastic modulus.  However, siliceous debris ingested with the intake air deposit on the coating surfaces, melt at peak temperatures in the engine cycle and penetrate the porosity, stiffening the coating and degrading their strain tolerance. The infiltration can be arrested by selecting a TBC composition that reacts rapidly with the penetrating melt, forming solid phases that fill the near-surface porosity precluding further flow into the coating. This investigation explores the stability of the garnet phase in reactions between yttrium-aluminum/iron garnets in systems consisting of Ca, Mg, Fe, Al silicate melts.  Reactants were produced by precipitation from precursor solutions, pyrolyzed into powder, pressed and heat treated in the range 1200-1400°C. The equilibrated microstructures were analyzed utilizing scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction to determine the extent of garnet formation and phase compositions.  The results are incorporated into a larger effort aimed at developing a thermodynamic database on garnet formation to inform novel TBC development.