Five new mixed-framework trisilicates were synthesised using a high-temperature flux growth technique. Colourless, glassy plates of SrY2Si3O10 crystallise in space group m>Pm>1̅, with m>am> = 6.757(1), m>bm> = 6.885(1), m>cm> = 9.273(2) Å, m>αm> = 72.42(3), m>βm> = 86.37(3), m>γm> = 88.37(3)°, m>Vm> = 410.38(12) Å3, m>Zm> = 2. The main building units of the new structure type represented by SrY2Si3O10 are slightly curved Si3O10 trimers and Y2O11 dimers (composed of YO6 octahedra sharing an edge with YO7 polyhedra), which are further edge-connected to adjacent dimers to form twisted zigzag chains parallel to [010]. BaREE2Si3O10 (REE = Gd, Er, Yb, Sc) form colourless small prisms, pseudohexagonal plates or isometric crystals, and crystallise in space group m>Pm>21/m>mm>, with respectively m>am> = 5.435(1) / 5.389(1) / 5.377(1) / 5.273(1), m>bm> = 12.241(2) / 12.163(2) / 12.117(2) / 11.918(2), m>cm> = 6.932(1) / 6.840(1) / 6.790(1) / 6.591(1) Å, m>βm> = 106.26(3) / 106.47(3) / 106.50(3) / 107.06(3)°, m>Vm> = 442.74(13) / 429.94(12) / 424.17(12) / 395.98(12) Å3, m>Zm> = 2. BaREE2Si3O10 (REE = Gd, Er, Yb, Sc) are isotypic with BaY2Si3O10. Their topology is characterised by horseshoe-shaped trisilicate (Si3O10) groups and zigzag chains of edge-sharing distorted m>Mm>O6 octahedra (m>Mm> = Gd, Er, Yb, Sc). Correlations between m>βm>–, Si–Si–Si angle and unit-cell volume and REE3+ ionic radii are discussed. The geometries of the Si3O10 and m>Tm>3O10 groups (m>Tm> = Ge, P, As, Al, Ga, V) in non-silicates are briefly reviewed, with special focus on narrow Si–Si–Si angles.
