Oxy-dravite, Na(Al2Mg)(Al5Mg)(Si6O 18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup

Oxy-dravite, Na(Al2Mg)(Al5Mg)(Si6O 18)(BO3)3(OH)3O, is a new mineral of the tourmaline supergroup. The holotype specimen originates from the locality of Osarara (Narok district, Kenya) and occurs in quartz-muscovite schist. Crystals of oxy-dravite are dark red, partially translucent with a vitreous luster, a pink streak, and conchoidal fracture. It has a Mohs hardness of approximately 7, and a calculated density of 3.073 g/cm3. In plane-polarized light, oxy-dravite is pleochroic (O = orange and E = pink) and uniaxial negative: ω = 1.650(5), ε = 1.620(5). Oxy-dravite is rhombohedral, space group R3m, with the unit-cell parameters a = 15.9273(2) and c = 7.2001(1) Å, V = 1581.81(4) Å3, Z = 3. Chemical characterization based on electron microprobe analysis, single-crystal structure refinement, Mössbauer, and optical spectroscopy, resulted in the empirical structural formula: X(Na0.83□0.15K 0.02)∑1.00 Y(Al1.34Fe 0.58 3+Mg1.03Fe0.03 2+Ti0.02)∑3.00 Z(Al 4.95Mg1.03Fe0.02 3+) ∑6.00 T(Si5.98Al0.02O 18)∑6.00 B(BO3)3 V(OH)3 W(O0.76OH0.24) ∑1.00. While the end-member formula of oxy-dravite may be formalized as NaY(Al3)Z(Al4Mg 2)Si6O18(BO3)3(OH) 3O, the most representative structural formula is Na Y(Al2Mg)Z(Al5Mg)Si6O 18(BO3)3(OH)3O. The difference between these two formulas is solely in Al-Mg order-disorder, i.e., there is no difference in chemical composition. Although the Mg-Al disorder over the Y and Z sites is controlled by the short-range bond-valence requirements of O 2- at the O1 (≡ W) site, the amount of Mg at the Z site is a function of the degree of cation size mismatch at Z. The crystal structure of oxy-dravite was refined to statistical index R1 of 1.17% using 1586 equivalent reflections collected with MoKα X-radiation. Oxy-dravite is chemically related to dravite (and fluor-dravite), NaMg3Al6Si 6O18(BO3)3(OH)3(OH,F), by the heterovalent substitution Al3+ + O2- → Mg 2+ + (OH,F)1-.