Chemical compositions of spinet found in natural rocks such as peridotite and lunar rock and synthesized or reequilibrated in simple and complex systems show that, at mid-dle-low pressure, spinet coexisting with olivine may contain some Si whose solubility increases with temperature and pressure. Using least squares regression, we fitted the experimental data in a simple system to a simple thermodynamic model and obtained the relation between variation in free energy for reaction Mg2SiO4(Ol)= Mg4SiO4 (Sp) and temperature. The result obtained is very similar to that deduced by Navrotsky and Akaogi(1984) from calorimetric and high pressure phase equibrium data. This good agree-went proves that, at middle-low pressure, Si may exist in the form of silicate spinel (Mg2SiO4) in picotite, and the activity of Mg2SiO4 component in MgCr2O4-Mg2SiO4 spi-net solid solution can be expressed as aspMg2sio4=(NspMg/2)2.(Nspsi)2.(Nspi represents num- ber of Mg and Si cations in unit structure formula of spinet on the basis of 402-).This implies that MgCr2O4-Mg2SiO4 spinet solid solution has perfect short range order.The experimental and modeled results indicate that the solubility of Si in chrome spinel is very sensitive to temperature and hence may serve as a potential geothermometer.