Publication details
A potentially new tourmaline with tetrahedrally coordinated boron
| Authors | |
|---|---|
| Year of publication | 2024 |
| Type | Appeared in Conference without Proceedings |
| Citation | |
| Description | Tourmaline with tetrahedrally coordinated boron, ideally Na(Al3)Al6(Si4B2)(BO3)3O18(OH)3(O), is a potentially new tourmaline species from B-rich granitic pegmatite, in the Sahatany Valley, central Madagascar. It crystallised during a late stage of open pocket crystallisation in a highly fractionated granitic pegmatite dike of lithium-caesium-tantalum-enriched type (LCT-type) of likely Pan-African age. Pink crystals of the tourmaline are in part rimmed by, or intergrown with fibrous prisms in parallel growth of yellowish-brown tourmaline, and are associated with behierite, quartz and Kfeldspar. The tourmaline forms euhedral crystals ca. 5-10 mm in diameter, pink with a vitreous to transparent lustre, conchoidal fracture and white streak. It has the Mohs hardness of ca. 7, the calculated density of 3.038 g.cm-3 and is uniaxial with ? = 1.659(2), ? = 1.639(2) at 589.3 nm white LED light, pleochroism is light grey in the omega direction and colourless in the epsilon direction. The tourmaline has trigonal symmetry, space group R3m, a = 15.6509(8) A, b = 15.6509(8) A, c = 7.0406(5) A, V = 1493.55 (19) A3 and Z = 3. The crystal structure was refined to R1 = 1.99 % using 2192 unique reflections collected with MoK? X-ray source. The structure was refined using the SHELXL-2013 program [1] with employing neutral scattering factors. Occupancies of X-, Y-, Z- and Tcation sites were refined as free variables. Occupancies of X-, Y-, and T- sites were refined using two major elements located in the respective sites, i.e., Na vs Ca at the X-site, Al vs Li at the Y-site, and Si vs B at the T-site. The hydrogen atoms were located in a residual electron-density map. The H1 and H3 site isotropic displacement parameters were constrained to be equal to 1.2 times of that of the O1 and O3 site; their distances from the donor oxygen atoms were constrained to 1.02(5) and 0.98(5) A, respectively. The empirical formula of the potentially new tourmaline was calculated on the basis of Y+Z+T+B = 18 (apfu) and (OH-+F-+Cl-+O2-) = 4 apfu. The structural refinement showed full occupancies of the Y-, Z-, and T-sites, resulting in the final empirical formula: X(Na0.573Ca0.220?0.207)?1.000 Y(Al2.760Li0.187Mn0.051Ti0.002)?3.000 Z(Al6.000)T(Si4.526B1.419Al0.055)?6.000 B(B3.000)O27 V(OH2.857O0.143)W(O0.974F0.026) The ideal formula of the potentially new tourmaline conforms to both the dominant-valency and dominant-constituent rules [2]. The dominant end-member approach [3] provided an identical result. Based on the analysis, the structural formula contains 57.3 mol. % of the end-member (limited by the 0.573 apfu Na at the X-site). The closest end-member compositions of valid tourmaline species are olenite, alumino-oxyrossmanite, and darrellhenryite, related by the substitutions: TSi4+ + VO2– › TB3+ + VOH– accompanied by the disorder exchange VO2- + WOH- › VOH- + WO2-; X? + TSi4+ + TAl3+ › XNa+ + 2(TB3+); and YLi+ + 2(TSi4+) › YAl3+ + 2(TB3+), respectively. |