The structure of the silicate Rb-10[Si6O17] containing a novel dimeric cyclotrisilicate anion is reported. The compound is formed by the reaction of a mixture of SiO2 and Rb at temperatures above 700 degrees C. Systematic investigations by means of differential thermal analysis and temperature- dependent powder X-ray diffraction experiments revealed that the new compound evolved from Rb-6[Si3O9], which occurred as an intermediate product. Thus, the dimeric anion [Si6O17](10)- is formed by condensation of the monomeric cyclotrisilicate [Si3O9](6-). For both silicates, [Si6O17](10-) and [Si3O9](6-), the characteristic ring vibration modes were observed in the IR spectrum. The structure of Rb-10[Si6O17] was solved and refined from single-crystal X-ray diffraction data in the orthorhombic space group Pbca (No. 61). Synthesis and structure determination of Rb-10[Si6O17] bridge the gap to show that the recently reported structures of Rb-14[Si-4][Si6O17] and Rb-14[Ge-4][Si6O17] are indeed fascinating intergrowth structures of the stable oxide Rb-10[Si6O17] and the Zintl phases RbSi (Rb4Si4) and RbGe (Rb4Ge4), respectively.