Original paper

Bulk modulus variation along the diopsidekosmochlor solid solution

Boffa Ballaran, Tiziana; Nestola, Fabrizio; Tribaudino, Mario; Ohashi, Haruo

Abstract

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 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, KT0, increases from 113.1(5) GPa for Ko25Di75 to 130.2(7) GPa for Ko100Di0, 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: KT0 (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.