STRUCTURAL CHARACTERIZATION OF PENTACOORDINATE SILICON IN A CALCIUM SILICATE

IN silicate minerals formed at pressures typical of the Earth's crust, the silicon is usually coordinated by four oxygen atoms. In contrast, silicates formed at higher pressures, typical of the Earth's transiti on zone and lower mantle, contain predominantly six-coordinated silico n. Silicon coordinated by five oxygen atoms is not normally found as a structural element in crystalline phases, but is nevertheless believe d to play a central role in many dynamic processes that occur in silic ates. For example, pentacoordinate silicon is probably a component of aluminosilicate melts and glasses at mantle temperatures and pressures (1,2), where it will dominate their transport properties(3-6); it is a lso believe to act as an intermediate activated state during oxygen di ffusion in silicate minerals(7,8). Here we report the complete structu re determination of an inorganic crystalline silicate-CaSi2O5-containi ng SiO5 groups. Our results confirm the previous attribution(1,2,9) of peaks in the Si-29 NMR spectrum of this material to the presence of p entacoordinate silicon, and the detailed geometry that we determine fo r the SiO5 group should provide a firm basis for characterizing and qu antifying the role of pentacoordinate silicon in silicate melts and gl asses.