Equation of state of pyrope–almandine solid solution measured using a diamond anvil cell and in situ synchrotron X-ray diffraction

•In situ x-ray diffraction on three synthetic pyrope-almandine garnets was conducted.•Pressure-volume relations of three synthetic pyrope-almandine garnets were measured.•Bulk moduli of three synthetic pyrope-almandine garnets were derived.•Bulk modulus of pyrope-almandine joint follows K = 170 + 15x(almondine mole fraction).

The pressure–volume relations of three synthetic garnet samples along pyrope–almandine (Py–Alm) join were measured at ambient temperature and high pressures up to 7, 21 and 19 GPa for Py83Alm17, Py54Alm46 and Py30Alm70, respectively. The obtained P–V data were fitted to the third order Birch–Murnaghan equation of state (EOS). The ambient cell volumes V0 of the three samples were measured to be 1511(1), 1515(2), and 1526(1) Å3 respectively. With fixed pressure derivative of the isothermal bulk modulus K′ at 4.3, isothermal bulk moduli K0 of the three samples were determined to be 172(4), 174(2), and 183(2) GPa respectively. These results confirm that almandine content (iron substitution) increase the bulk modulus of the garnet join following a nearly ideal mixing model. The relation between bulk modulus and almandine mole fraction (n) in this garnet join is derived to be K0 = 170 + 15 n. These data can be used to contribute to construction of compositional model of earth mantle.