Three pyroxene single-crystals with compositions NaCrSi2O6 (Ko100Di0), (Na0.75Ca0.25)(Cr0.75Mg0.25)Si2O6 (Ko75Di25) and (Na0.25Ca0.75)(Cr0.25Mg0.75)Si2O6 (Ko25Di75) have been studied by means of m>in situm> high-pressure X-ray single-crystal diffraction using a diamond anvil cell up to ~7.6 GPa. A third-order Birch-Murnaghan equation of state (BM3-EoS) was used to fit the m>P–Vm> data measured for the three samples. The bulk modulus, m>Km>T0, increases from 113.1(5) GPa for Ko25Di75 to 130.2(7) GPa for Ko100Di0, whereas the first pressure derivative, m>Km>′, decreases from 5.8 to 4.1 with increasing Ko content. The bulk modulus varies linearly with composition according to the expression: m>Km>T0 (GPa) = 107.4(1) + 0.228(1) × % molar Ko and the extrapolated value for pure diopside is in good agreement with the datum obtained from Brillouin scattering measurements and lies between data obtained from other X-ray diffraction studies. An inverse behaviour of volume compressibility and thermal expansion with composition is observed, m>i.em>. Na-richer compositions are stiffer and less expandable. Softening of the clinopyroxene structure with increasing diopside component is mainly due to the softening of the m>bm> direction and can be ascribed to the narrowing of the M1 octahedral chains which is greater in diopside than in kosmochlor.
