Lucchesiite, CaFe²⁺₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₃O, a new mineral species of the tourmaline supergroup

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 ų, 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(Ca_0.69Na_0.30K_0.02)_Σ1.01^Y(Fe²⁺_1.44Mg_0.72Al_0.48Ti⁴⁺_0.33V³⁺_0.02Mn_0.01Zn_0.01)_Σ3.00^Z(Al_4.74Mg_1.01Fe³⁺_0.25)_Σ6.00 [^T(Si_5.85Al_0.15)_Σ6.00O₁₈](BO₃)₃^V(OH)₃^W[O_0.69F_0.24(OH)_0.07]_Σ1.00 and ^X(Ca_0.49Na_0.45□_0.05K_0.01)_Σ1.00^Y(Fe²⁺_1.14Fe³⁺_0.95Mg_0.42Al_0.37Mn_0.03Ti⁴⁺_0.08Zn_0.01)_Σ3.00^Z(Al_5.11Fe³⁺_0.38Mg_0.52)_Σ6.00[^T(Si_5.88Al_0.12)_Σ6.00O₁₈] (BO₃)₃^V[(OH)_2.66O_0.34]_Σ3.00^W(O_0.94F_0.06)_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⁺ + ^O1O^2– ↔ ^ZMg^2 +  + ^O1(OH)^1–. The new mineral was approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA 2015-043).