Oxy-dravite, Na(Al2Mg)(Al5Mg)(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup

Abstract

Oxy-dravite, Na(Al2Mg)(Al5Mg)(Si6O18)(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), ɛ = 1.620(5). Oxy-dravite is rhombohedral, space group R 3 m , with the unit-cell parameters a = 15.9273(2) and c = 7.2001(1) A, V = 1581.81(4) A3, Z = 3. Chemical characterization based on electron microprobe analysis, single-crystal structure refinement, Mossbauer, and optical spectroscopy, resulted in the empirical structural formula: ( Na 0.83 □ 0.15 K 0.02 ) X Σ 1.00 ( Al 1.34 Fe 0.58 3 + Mg 1.03 Fe 0.03 2 + Ti 0.02 ) Y Σ 3.00 ( Al 4.95 Mg 1.03 Fe 0.02 3 + ) Z Σ 6.00 ( Si 5.98 Al 0.02 O 18 ) T Σ 6.00 ( BO 3 ) B 3 ( OH ) V 3 ( O 0.76 OH 0.24 ) W Σ 1.00 . While the end-member formula of oxy-dravite may be formalized as NaY(Al3)Z(Al4Mg2)Si6O18(BO3)3(OH)3O, the most representative structural formula is NaY(Al2Mg)Z(Al5Mg)Si6O18(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 O2− 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 R 1 of 1.17% using 1586 equivalent reflections collected with Mo K α X-radiation. Oxy-dravite is chemically related to dravite (and fluor-dravite), NaMg3Al6Si6O18(BO3)3(OH)3(OH,F), by the heterovalent substitution Al3+ + O2− → Mg2+ + (OH,F)1−.