尖晶石中的Si含量及其存在形式

自然岩石及简单和复杂体系中实验合成或经重新平衡的尖晶石化学成分显示，在中－低压条件下，与橄榄石共存的尖晶石可含一定量的Ｓｉ，其含量随温度和压力的升高而增大。用最小二乘法对简单体系中 实验数据的热力学模拟发现，Ｓｉ在铬尖晶石中以硅酸盐尖晶石（Ｍｇ２ＳｉＯ４）形式存在，Ｍｇ２ＳｉＯ４组份在铬尖晶石中的活度ａＳＤＭＧ２ＳｉＯ４＝（ＮＳＰＭＧ／２）２＊（ＮＳＰＳｉ）２（ＮＳＰＳｉ代表以４个氧为基础的单位尖

Solubility and Mode of Occurrence of Si in Spinel

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.