The unit-cell parameters evolution along the jadeite – aegirine solid solution was determined up to pressures between 6.5 and 9.7 GPa and room temperature by single-crystal X-ray diffraction. The pressure-volume data have been fitted using a third-order Birch-Murnaghan equation of state. The bulk modulus, KT0, is 134.0(7) GPa for pure jadeite and 116.1(5) for pure aegirine. Its evolution with composition along the join is not linear and can be described by the following weighted 2nd order polynomial: KT0 = 116.2(5) + [0.25(3)*(mol % Jd)] - [0.0008(3)*(mol % Jd)2] (1) The equation (1) is not affected by the values of the first pressure derivative K’. Such EoS coefficient shows no dependence upon composition and values between 3.9 and 4.5. The higher compressibility is always observed along the a axis, bnt compression along the b axis is most affected by compositional changes. Our unit-cell volume vs composition at room temperature data indicate a slight but significant deviation from linearity and, therefore, from the ideal solid solution between jadeite and aegirine reported by previous investigations.
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