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 transition zone and lower mantle, contain predominantly six-coordinated silicon. Silicon coordinated by five oxygen atoms is not normally found as a structural element in crystalline phases, but is nevertheless believed to play a central role in many dynamic processes that occur in silicates. 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 also believe to act as an intermediate activated state during oxygen diffusion in silicate minerals(7,8). Here we report the complete structure determination of an inorganic crystalline silicate-CaSi2O5-containing 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 pentacoordinate silicon, and the detailed geometry that we determine for the SiO5 group should provide a firm basis for characterizing and quantifying the role of pentacoordinate silicon in silicate melts and glasses.