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/cm<sup>3sup>. 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 R3m, with the unit-cell parameters a = 15.9273(2) and c = 7.2001(1) Å, V = 1581.81(4) Å<sup>3sup>, Z = 3. Chemical characterization based on electron microprobe analysis, single-crystal structure refinement, Mössbauer, and optical spectroscopy, resulted in the empirical structural formula: ![]()
alt="Formula" src="1442/embed/mml-math-1.gif" />
While the end-member formula of oxy-dravite may be formalized as Na<sup>Ysup>(Al3)<sup>Zsup>(Al4Mg2)Si6O18(BO3)3(OH)3O, the most representative structural formula is Na<sup>Ysup>(Al2Mg)<sup>Zsup>(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 O<sup>2−sup> 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), NaMg3Al6Si6O18(BO3)3(OH)3(OH,F), by the heterovalent substitution Al<sup>3+sup> + O<sup>2−sup> → Mg<sup>2+sup> + (OH,F)<sup>1−sup>.
