Raman and X-Ray Investigation of Pyrope Garnet (Mg _0.76 Fe _0.14 Ca _0.10)₃Al₂Si₃O₁₂ under High Pressure

The compressional behaviour of natural pyrope garnet is investigated by using angle-dispersive synchrotron radiation x-ray diffraction and Raman spectroscopy in a diamond anvil cell at room temperature. The pressure-induced phase transition does not occur under given pressure. The equation of state of pyrope garnet is determined under pressure up to 25.3GPa. The bulk modulus K_T0 is 199GPa, with its first pressure derivative K'_T0 fixed to 4. The Raman spectra of pyrope garnet are studied. A new Raman peak nearly at 743cm^-1 is observed in a bending vibration of the SiO₄ tetrahedra frequency range at pressure of about 28GPa. We suggest that the new Raman peak results from the lattice distortion of the SiO₄ tetrahedra. All the Raman frequencies continuously increase with the increasing pressure. The average pressure derivative of the high frequency modes (650--1000cm^-1) is larger than that of the low frequency (smaller than 650cm^-1). Based on these data, the mode Gruneisen parameters for pyrope are obtained.