Elasticity of tetragonal end-member majorite and solid solutions in the system Mg4Si4O12-Mg3Al2Si3O12

The adiabatic elastic moduli of polycrystalline En(50)Py(50) and En(80)Py(20) majorite-garnet solid solutions (where En=4MgSiO(3), Py=Mg3Al2Si3O12) and the end-member En(100) tetragonal majorite were determined at ambient conditions using Brillouin spectroscopy. The adiabatic bulk modulus, K, and shear modulus, mu, of En(50)Py(50) were found to be K=173.1 (20) and mu=92.3 (10) GPa, and are indistinguishable from those of the end-member pyrope, Py(100). The moduli of the more majorite-rich samples are significantly lower and are virtually identical to each other (K=162.6(11) and mu=85.7(7) for En(80)Py(20); K=166(5) and mu=88(2) for En(100)). In combination with previously reported moduli for this system, we conclude that both K and Ct are constant over the compositional range from Py(100) to a majorite content of about 70-80%, whereupon the moduli decrease substantially. For compositions ranging from En(80)Py(20) to the end-member majorite, the moduli are also approximately independent of composition, but at a lower value. An alternative model with a continuous decrease in moduli with increasing majorite content cannot be excluded, within the uncertainties of existing measurements. The contrast in moduli between aluminous pyrope garnet and Al-free majorite are small compared with the modulus changes accompanying the pyroxene - majorite phase transformation.