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

2013; Mineralogical Society of America; Volume: 98; Issue: 8-9 Linguagem: Inglês

10.2138/am.2013.4441

1945-3027

Ferdinando Bosi, Henrik Skogby,

X-ray Diffraction in Crystallography

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), 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: