TOURETITE, LiAl4Be4(B11Be)O28, A NEW BORATE MINERAL FROM THE AMBALABE PEGMATITE, MADAGASCAR

formula unit from the 51 chemical analysis determined by LA-TOF-ICP-MS, is 3 (Li0.38Cs0.24K0.14Rb0.03Na0.02Pb0.01[]0.18)Σ1.00(Al4.00Si0.05Fe2+ 0.03Mn0.02)Σ4.10(Be3.60Li0.40)52 Σ4.00(B11.2 53 8Be0.72)Σ12.00O28. The cubic unit-cell parameters, determined by single-crystal X-ray 54 diffraction methods, are a = 7.31202(15) Å, V = 390.942(14) Å3. The structure refinement 55 was performed in space group P-43m from 226 unique reflections, to the final R1 value of 56 0.0444. The structure of touretite contains clusters of four edge-sharing AlO6 octahedra 57 located around the origin of the unit-cell, and linked to the BeO4 and BO4 tetrahedra by 58 corner-sharing. The alkali cations occupy the Li/Cs site in a large cavity at the center of the 59 unit-cell and have a coordination number of 12. The coordination polyhedron can be 60 described as a truncated tetrahedron, that shares its small triangular faces with faces of the 61 BeO4 tetrahedra. The species was accepted by the IMA-CNMNC under number IMA 2023- 62 003a, and the name was chosen to honor Jacques and Lydie Touret, for their contribution to 63 mineralogical sciences.