Bulk modulus variation along the diopsidekosmochlor solid solution

Abstract

Three pyroxene single-crystals with compositions NaCrSi 2 O 6 (Ko 100 Di 0 ), (Na 0.75 Ca 0.25 )(Cr 0.75 Mg 0.25 )Si 2 O 6 (Ko 75 Di 25 ) and (Na 0.25 Ca 0.75 )(Cr 0.25 Mg 0.75 )Si 2 O 6 (Ko 25 Di 75 ) have been studied by means of in situ 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 P–V data measured for the three samples. The bulk modulus, K T0 , increases from 113.1(5) GPa for Ko 25 Di 75 to 130.2(7) GPa for Ko 100 Di 0 , whereas the first pressure derivative, K ′, decreases from 5.8 to 4.1 with increasing Ko content. The bulk modulus varies linearly with composition according to the expression: K 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, i.e . 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 b direction and can be ascribed to the narrowing of the M1 octahedral chains which is greater in diopside than in kosmochlor.