Thermal conductivity of stishovite and of a homogeneous lower mantel composed of stishovite and periclase

Abstract

The cylindrical probe method of measuring the thermal conductivity of minerals (Horai and Simmons, 1969) is tested for quartz type Ge0 and found to be useful for measuring the thermal conductivity of germanates. By this method the thermal conductivities of quartz type Ge02 and rutile type Ge02 are 10.1 mcal/ cm-sec-°C and 45.5 mcal/cm-sec-°C, respectively. The thermal conductivity of stishovite at room temperature and pressure was found by isostructural extrapolation to be approximately 50 mcal/cm-sec-°C, which is an order of magnitude greater than the average thermal conductivity of igneous rocks. If forsterite (Mg2 SiO4 ) decomposes to the denser form of periclase (Mg0), which has a thermal conductivity of 110 mcal/cm-sec-°C at ambient conditions, plus stishovite (Si02) in the lower mantle, then this region has a higher lattice thermal conductivity than previously expected. An approximate calculation considering pressure and temperature effects shows the lattice thermal conductivity of stishovite to remain nearly constant at about 25% of its ambient value in the lower mantle, while the lattice thermal conductivity of periclase increases considerably with depth in the lower mantle from a low of about 25% of its ambient value. The net effect is an increase of lattice thermal conductivity with depth in the lower mantle.