- journal_title：American Mineralogist
- Contributor：Ferdinando Bosi ; Henrik Skogby
- Publisher：Mineralogical Society of America
- Date：2013-08-01
- Format：text/html
- Language：en
- Identifier：10.2138/am.2013.4441
- journal_abbrev：American Mineralogist
- issn：0003-004X
- volume：98
- issue：8-9
- firstpage：1442
- section：Articles
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 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: 1">1442/embed/mml-math-1.gif" />
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 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 Al3+ + O2− → Mg2+ + (OH,F)1−.