The low-temperature heat capacity, C p , of the crystalline and glassy forms of the mineral grossular Ca 3 Al 2 Si 3 O 1 2 are distinctly different. The effect has been linked to differences in coordination of Al and the resulting structural roles of the ion in each phase. Inelastic neutron-scattering measurements for the glassy phase reveals an excess density of states below 30 meV and C p , calculated from the vibrational density of states agree with thermodynamic measurements. Static neutron-scattering data confirm that Al is tetrahedrally coordinated in the glass as distinct from octahedral coordination in the crystal.