Oxy-schorl, Na(Fe₂ ²⁺Al)Al₆Si₆O₁₈(BO₃)₃(OH)₃O, a new mineral from Zlatá Idka, Slovak Republic and Přibyslavice, Czech Republic

Abstract Oxy-schorl (IMA 2011-011), ideally Na(Fe₂²⁺Al)Al₆Si₆O₁₈(BO₃)₃(OH)₃O, a new mineral species of the tourmaline supergroup, is described. In Zlatá Idka, Slovak Republic (type locality), fan-shaped aggregates of greenish black acicular crystals ranging up to 2 cm in size, forming aggregates up to 3.5 cm thick were found in extensively metasomatically altered metarhyolite pyroclastics with Qtz+Ab+Ms. In Přibyslavice, Czech Republic (co-type locality), abundant brownish black subhedral, columnar crystals of oxy-schorl, up to 1 cm in size, arranged in thin layers, or irregular clusters up to 5 cm in diameter, occur in a foliated muscovite-tourmaline orthogneiss associated with Kfs+Ab+Qtz+Ms+Bt+Grt. Oxy-schorl from both localities has a Mohs hardness of 7 with no observable cleavage and parting. The measured and calculated densities are 3.17(2) and 3.208 g/cm3 (Zlatá Idka) and 3.19(1) and 3.198 g/cm³ (Přibyslavice), respectively. In plane-polarized light, oxy-schorl is pleochroic; O = green to bluish-green, E = pale yellowish to nearly colorless (Zlatá Idka) and O = dark grayish-green, E = pale brown (Přibyslavice), uniaxial negative, ω = 1.663(2), ε = 1.641(2) (Zlatá Idka) and ω = 1.662(2), ε = 1.637(2) (Přibyslavice). Oxy-schorl is trigonal, space group R3m, Z = 3, a = 15.916(3) Å, c = 7.107(1) Å, V = 1559.1(4) ų (Zlatá Idka) and a = 15.985(1) Å, c = 7.154(1) Å, V = 1583.1(2) ų (Přibyslavice). The composition (average of 5 electron microprobe analyses from Zlatá Idka and 5 from Přibyslavice) is (in wt%): SiO₂ 33.85 (34.57), TiO₂ <0.05 (0.72), Al₂O₃ 39.08 (33.55), Fe₂O₃ not determined (0.61), FeO 11.59 (13.07), MnO <0.06 (0.10), MgO 0.04 (0.74), CaO 0.30 (0.09), Na₂O 1.67 (1.76), K₂O <0.02 (0.03), F 0.26 (0.56), Cl 0.01 (<0.01), B₂O₃ (calc.) 10.39 (10.11), H₂O (from the crystal-structure refinement) 2.92 (2.72), sum 99.29 (98.41) for Zlatá Idka and Přibyslavice (in parentheses). A combination of EMPA, Mössbauer spectroscopy, and crystal-structure refinement yields empirical formulas (Na_0.591Ca_0.103□_0.306)_Σ1.000(Al_1.885Fe²⁺ _1.108Mn_0.005Ti_0.002)_Σ3.000(Al_5.428Mg_0.572)_Σ6.000(Si_5.506Al_0.494)_Σ6.000O₁₈ (BO₃)₃(OH)₃(O_0.625OH_0.236F_0.136Cl_0.003)_Σ1.000 for Zlatá Idka, and (Na_0.586Ca_0.017K_0.006□_0.391)_Σ1.000(Fe²⁺_1.879Mn_0.015 Al_1.013Ti_0.093)_Σ3.00(Al_5.732Mg_0.190Fe³⁺_0.078)_Σ6.000(Si_5.944Al_0.056)_Σ6.000O₁₈(BO₃)₃(OH)₃(O_0.579F_0.307OH_0.115)_Σ1000 for Přibyslavice. Oxy-schorl is derived from schorl end-member by the AlOFe_-1(OH)_-1 substitution. The studied crystals of oxy-schorl represent two distinct ordering mechanisms: disorder of R²⁺ and R³⁺ cations in octahedral sites and all O ordered in the W site (Zlatá Idka), and R²⁺ and R³⁺ cations ordered in the Y and Z sites and O disordered in the V and W sites (Přibyslavice).