THE ELASTIC CONSTANTS OF MONOCLINIC SINGLE-CRYSTAL CHROME-DIOPSIDE TO 1,300 K

Abstract

Values of the complete adiabatic elastic tensor for single-crystal chrome-diopside (a monoclinic pyroxene mineral) are presented from 298 to 1,300 K. The data were obtained using resonant ultrasound spectroscopy (RUS). They are the first published results for the temperature T dependences of the 13 individual elastic constants C ij of any clinopyroxene mineral. Each C ij is appropriately described by a linear function in T throughout the range of T. Values for each (∂C ij /∂T) P in GPa K−1 are as follows: C 11, −0.0291; C 22, −0.0248; C 33, −0.0179; C 44, −0.0103; C 55, −0.0077; C 66, −0.0152; C 12, −0.0119; C 13, −0.0064; C 23, 0.0000; C 15, 0.0025; C 25, 0.0022; C 35, −0.0046; and C 46, 0.0026. Values of (∂M/∂T) P in GPa K−1, where M represents an isotropic bulk property calculated from the C ij data, are as follows: adiabatic bulk modulus K S , −0.0123; isothermal bulk modulus K T , −0.0178; and shear modulus G, −0.00998. Some diopside derivatives, notably (∂K S /∂T) P , (∂K T /∂T) P , and (∂V P /∂T) P , where V P is the compressional wave velocity, have smaller magnitudes than all other minerals of importance in Earth’s mantle, thus, confirming predictions from systematics studies. We find several dimensionless quantities for this monoclinic mineral have normal values compared to other mantle minerals. Further, αK T (α is the volume coefficient of thermal expansion) for diopside is approximately independent of both T and volume V at elevated temperature, so its equation of state is accurately expressed in simplified form.