Ertlite, NaAl3Al6(Si4B2O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup

Abstract Ertlite, ideally NaAl3Al6(Si4B2O18)(BO3)3(OH)3O, is a new, very B-rich mineral of the tourmaline supergroup. It was found at two localities: the holotype in the Sahatany Valley, central Madagascar, and a cotype specimen from Sakangyi, Mogok Township, Mandalay Region, Myanmar. At both localities, the mineral occurs as a late-stage hydrothermal phase in open pockets within highly fractionated, B-rich granitic pegmatites. The holotype occurs as pink to brownish gray or near colorless euhedral crystals and aggregates, up to 10 mm in diameter, with vitreous luster, conchoidal fracture, and white streak. The mineral is uniaxial (−). Holotype ertlite has a Mohs hardness of ca. 7–8, a calculated density of 3.128 g·cm−3 and an superior compatibility index (1 − KP/KC = 0.018). Cotype ertlite has a Mohs hardness of ca. 7–8, a calculated density of 3.135 g·cm−3 and a superior compatibility index (1 − KP/KC = 0.001). Ertlite has trigonal symmetry, space group R3m, with a = 15.6509(8) Å, c = 7.0406(5) Å, V = 1493.55(19) Å3 (holotype) and a = 15.590(2) Å, c = 7.009(1) Å, V = 1475.3(4) Å3 (cotype, with the lowest unit-cell volume ever recorded for a natural tourmaline); Z = 3. The crystal structures were refined to R1 = 2.01% and 3.74%, respectively, using room-temperature data sets collected with MoKα radiation. Crystal-chemical analysis obtained using electron probe microanalysis, laser ablation inductively coupled plasma mass spectroscopy, Raman spectroscopy, and crystal structure refinement resulted in the following empirical holotype and cotype formulas, respectively: X(Na0.573Ca0.220□0.207)Σ1.000Y(Al2.760Li0.187Mn0.051Ti0.002)Σ3.000Z(Al6.000) T(Si4.526B1.419Al0.055)Σ6.000O18B(BO3)3V(OH2.669O0.331)Σ3.000W(O0.786OH0.188F0.026)Σ1.000 (holotype); X(Na0.743Ca0.109□0.146K0.002)Σ1.000Y(Al2.836Li0.144Mn0.002Fe0.010)Σ2.992Z(Al6.000) T(Si4.053B1.955)Σ6.008O18B(BO3)3V(OH)3.000W(O0.716OH0.245F0.039)Σ1.000 (cotype). Ertlite is an oxy-species that belongs to the sodic group of the tourmaline supergroup. The closest end-member compositions of valid tourmaline species are: olenite by the coupled substitution TSi4+ + VO2− → TB3+ + VOH− accompanied by the disorder exchange VO2− + WOH− → VOH− + WO2−, alumino-oxy-rossmanite by the coupled substitution X□ + TSi4+ + TAl3+ → XNa+ + 2(TB3+), and darrellhenryite by the coupled substitution YLi+ + 2(TSi4+) → YAl3+ + 2(TB3+). Ertlite was approved by the IMA-CNMNC (IMA 2023-086); the name honors the tourmaline specialist Andreas Ertl-Winand. Ertlite is closely related to synthetic excess-boron tourmalines. Holotype ertlite formed during a late stage of open pocket crystallization in a highly fractionated granitic pegmatite dike of lithium-cesium-tantalum-enriched type (LCT-type) of likely Pan-African age. Cotype ertlite crystallized in a very similar pegmatite environment.