Popugaevaite, Ca3[B5O6(OH)6]FCl2·8H2O, a new phylloborate mineral

The new mineral popugaevaite Ca3[B5O6(OH)6]FCl2·8H2O was found at the Internatsional’nyi diamond mine, Internatsional’naya kimberlite pipe, Sakha (Yakutia) Republic, Russia. It belongs to the low-temperature hydrothermal mineral assemblage formed in the contact zone between kimberlite and a boron-bearing halite rock. Popugaevaite occurs as veinlets in massive aggregates of ekaterinite and crusts (up to 0.7 mm thick and up to 1 cm × 4 cm in area) on ekaterinite nodules embedded in halite. Other associated minerals are Fe-rich szaibélyite, serpentine, dolomite, pyrrhotite and chalcopyrite. Crude prismatic crystals of popugaevaite are up to 0.3 × 1 mm. The mineral is transparent, colourless, with vitreous lustre and perfect {010} cleavage. It is optically biaxial (–), α 1.502(2), β 1.523(2), γ 1.530(2) and 2Vmeas = 50(10)°. The chemical composition (wt.%, electron-microprobe, boron by ICP-MS, H2O calculated by stoichiometry) is: CaO 28.54, B2O3 28.62, F 3.19, Cl 11.50, H2O 32.83, O = (F,Cl) –3.94, total 100.74. The empirical formula, calculated based on 23 O+F+Cl and 22 H atoms per formula unit, is Ca3.07B4.96O6.03(OH)6F1.01Cl1.96·8H2O. Popugaevaite is monoclinic, space group Pn, a = 8.7055(11), b = 8.1025(11), c = 14.812(2) Å, β = 91.367(7)°, V = 1044.5(2) Å3 and Z = 2. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I,%)(hkl)] are: 8.12(100)(010), 4.058(27)(020), 3.577(15)( $\bar 1$ 21), 2.936(10)(123), 2.834(16)(301, $\bar 1$ 05) and 2.283(10)(133). The crystal structure was solved based on single-crystal XRD data and refined on powder data by the Rietveld method, Rwp = 0.0058, Rp = 0.0043 and Robs = 0.0241. Popugaevaite is an isostructural analogue of brianroulstonite Ca3[B5O6(OH)6](OH)Cl2·8H2O with F– instead of the OH– group non-bound with boron. The structure is based upon the layers of twelve-membered rings of alternating BO3 triangles and BO2(OH)2 tetrahedra. The mineral is named in honour of the Russian geologist Larisa Anatol’evna Popugaeva (1923–1977), one of the principal discoverers of diamondiferous kimberlite pipes in Yakutia.