Celleriite, (Mn2+2Al)Al6(Si6O18)(BO3)3(OH)3(OH), a new mineral species of the tourmaline supergroup

Celleriite, (Mn 2+ 2 Al)Al 6 (Si 6 O 18)(BO 3) 3 (OH) 3 (OH), is a new mineral of the tourmaline supergroup. It was discovered in the Rosina pegmatite, San Piero in Campo, Elba Island, Italy (holotype specimen) and in the Pikárec pegmatite, western Moravia, Czech Republic (co-type specimen). Celleriite in hand specimen is violet to gray-blue (holotype) and dark brownish-green (co-type) with a vitreous luster, conchoidal fracture and white streak. Celleriite has a Mohs hardness of approximately 7, and a calculated density of 3.13 g/cm and 3.14 g/cm for holotype and its co-type, respectively. In plane-polarized light in thin section, celleriite is pleochroic (O = pale violet and E = light grey-blue in holotype; O = pale green and E = colorless in co-type) and uniaxial negative. Celleriite has trigonal symmetry: space group R3m, Z = 3, a = 15.9518(4) and 15.9332(3) Å, c = 7.1579(2) and 7.13086(15) Å, V = 1577.38(9) and 1567.76(6) Å for holotype and co-type, respectively (data from single-crystal X-ray diffraction). The crystal structure of the holotype specimen was refined to R1 = 2.89% using 1696 unique reflections collected with MoKα X-ray intensity data. Structural, chemical and spectroscopic analyses resulted in the formulae: X( 0.58 Na0.42)Σ1.00 Y(Mn2+ 1.39 Fe2+ 0.16 Mg0.01 Al1.14 Fe3+0.01 Li0.28 Ti0.01)Σ3.00 Z Al6 [T(Si5.99 Al0.01)Σ6.00 O18] (BO3)3 (OH)3 W [(OH)0.65 F0.03 O0.32]Σ1.00 (for holotype) and X( 0.51 Na0.49)Σ1.00 Y(Mn2+0.90 Fe2+0.50 Al1.36 Fe3+0.04 Li0.17 Zn0.04)Σ3.00 ZAl6 [T (Si5.75 B0.25)Σ6.00 O18] (BO3)3 (OH)3 W [(OH)0.35 F0.17 O0.48]Σ1.00 (for co-type) Celleriite is a hydroxy-species belonging to the X-site vacant group of the tourmaline supergroup. The new mineral was approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association, proposal n. 2019-089. In the Rosina pegmatite, celleriite formed an overgrowth at the analogous pole of elbaite–fluor-elbaite–rossmanite crystals during the latest stage of evolution of pegmatite cavities, after an event of pocket rupture. In the Pikárec pegmatite, celleriite occurs as an intermediate growth sector of elbaite, princivalleite and fluor-elbaite