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

• Published in: The American mineralogist Vol. 98; no. 2-3; pp. 485 - 492
• Main Authors: Bacik, Peter, Cempirek, Jan, Uher, Pavel, Novak, Milan, Ozdin, Daniel, Filip, Jan, Skoda, Radek, Breiter, Karel, Klementova, Mariana, Duda, Rudolf, Groat, Lee A
• Format: Journal Article
• Language: English
• Published: Mineralogical Society of America 01.02.2013
• Subjects: Central Europe; chemical composition; crystal structure; crystal-structure refinement; Czech Republic; electron microanalysis; electron probe data; Europe; formula; gneisses; lattice parameters; metamorphic rocks; mica group; mineral localities; Mineralogy; Mossbauer spectra; muscovite; new mineral; new minerals; optical properties; orthogneiss; Oxy-schorl; Pribyslavice Czech Republic; Přibyslavice; refinement; ring silicates; schorl; sheet silicates; silicates; space groups; spectra; tourmaline group; tourmaline-supergroup minerals; type localities; X-ray diffraction data; Zlata Idka; Zlatá Idka
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am.2013.4293

Oxy-schorl (IMA 2011-011), ideally Na(Fe2+2Al)Al6Si6O18(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup, is described. In Zlata 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 Pribyslavice, 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 (Zlata Idka) and 3.19(1) and 3.198 g/cm3 (Pribyslavice), respectively. In plane-polarized light, oxy-schorl is pleochroic; O = green to bluish-green, E = pale yellowish to nearly colorless (Zlata Idka) and O = dark grayish-green, E = pale brown (Pribyslavice), uniaxial negative, ω = 1.663(2), ε = 1.641(2) (Zlata Idka) and ω = 1.662(2), ε = 1.637(2) (Pribyslavice). Oxy-schorl is trigonal, space group R3m, Z = 3, a = 15.916(3) Å, c = 7.107(1) Å, V = 1559.1(4) Å3 (Zlata Idka) and a = 15.985(1) Å, c = 7.154(1) Å, V = 1583.1(2) Å3 (Pribyslavice). The composition (average of 5 electron microprobe analyses from Zlata Idka and 5 from Pribyslavice) 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 Zlata Idka and Pribyslavice (in parentheses). A combination of EMPA, Mossbauer spectroscopy, and crystal-structure refinement yields empirical formulas (Na0.591Ca0.103[]0.306)Σ1.000(Al1.885 Fe2+1.108Mn0.005Ti0.002)Σ3.000(Al5.428Mg0.572) Σ6.000(Si5.506Al0.494)Σ6.000O18 (BO3)3(OH)3(O0.625OH0236F0.136Cl0.003)Σ1000 for Zlata Idka, and (Na0.586Ca0.017K0.006[]0.391)Σ1.000(Fe2+ 1.879Mn0.015 Al1.013Ti0.093)Σ3.00(Al5.732Mg0.190Fe3+0.078) Σ6.000(Si5.944Al0.056)Σ6.000O18(BO3)3(OH)3( O0.579F0.307OH0.115)Σ1.000 for Pribyslavice. 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 (Zlata Idka), and R2+ and R3+ cations ordered in the Y and Z sites and O disordered in the V and W sites (Pribyslavice).