Lucchesiite, CaFe2+3Al6(Si6O18)(BO3)3(OH)3O, is a new mineral of the tourmaline supergroup. It occurs in the Ratnapura District, Sri Lanka (6°35′N, 80°35′E), most probably from pegmatites and in Mirošov near Strážek, western Moravia, Czech Republic, (49°27′49.38″N, 16°9′54.34″E) in anatectic pegmatite contaminated by host calc-silicate rock. Crystals are black with a vitreous lustre, conchoidal fracture and grey streak. Lucchesiite has a Mohs hardness of ∼7 and a calculated density of 3.209 g/cm3 (Sri Lanka) to 3.243 g/cm3 (Czech Republic). In plane-polarized light, lucchesiite is pleochroic (O = very dark brown and E = light brown) and uniaxial (–). Lucchesiite is rhombohedral, space group R3m, a ≈ 16.00 Å, c ≈ 7.21 Å, V ≈ 1599.9 Å3, Z = 3. The crystal structure of lucchesiite was refined to R1 ≈ 1.5% using ∼2000 unique reflections collected with MoKα X-ray intensity data. Crystal-chemical analysis for the Sri Lanka (holotype) and Czech Republic (cotype) samples resulted in the empirical formulae, respectively: X(Ca0.69Na0.30K0.02)Σ1.01Y(Fe2+1.44Mg0.72Al0.48Ti4+0.33V3+0.02Mn0.01Zn0.01)Σ3.00Z(Al4.74Mg1.01Fe3+0.25)Σ6.00 [T(Si5.85Al0.15)Σ6.00O18](BO3)3V(OH)3W[O0.69F0.24(OH)0.07]Σ1.00 and X(Ca0.49Na0.45□0.05K0.01)Σ1.00Y(Fe2+1.14Fe3+0.95Mg0.42Al0.37Mn0.03Ti4+0.08Zn0.01)Σ3.00Z(Al5.11Fe3+0.38Mg0.52)Σ6.00[T(Si5.88Al0.12)Σ6.00O18] (BO3)3V[(OH)2.66O0.34]Σ3.00W(O0.94F0.06)Σ1.00.
Lucchesiite is an oxy-species belonging to the calcic group of the tourmaline supergroup. The closest end-member composition of a valid tourmaline species is that of feruvite, to which lucchesiite is ideally related by the heterovalent coupled substitution ZAl3+ + O1O2– ↔ ZMg2 + + O1(OH)1–. The new mineral was approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA 2015-043).