Elasticity of single crystal pyrope and implications for garnet solid solution series

The elastic moduli of a synthetic single crystal of pyrope (Mg₃Al₂Si₃O₁₂) have been determined using a technique based on Brillouin scattering. These results are used in an evaluation of the effect of composition on the elastic properties of silicate garnet solid solution series (Mg, Fe, Mn, Ca)₃ (Al, Fe, Cr)₂ Si₃O₁₂. In the pyralspites (Mg FeMn aluminum garnets), for which a large amount of data is available, this analysis indicates that the bulk modulus K is independent of the Fe²⁺/Mg²⁺ ratio, which is similar to the behavior observed in olivines and pyroxenes. However, the shear modulus μ of the garnets increases by 10% from the Mg to the Fe end member, in contrast to the decrease of μ with Fe content which is observed in olivines and pyroxenes. This contrasting behavior is most probably related to the oxygen coordination of the cation site occupied by Mg²⁺ and Fe²⁺ in these different minerals.