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

Abstract The new mineral popugaevaite Ca 3 [B 5 O 6 (OH) 6 ]FCl 2 ·8H 2 O 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 2V meas = 50(10)°. The chemical composition (wt.%, electron-microprobe, boron by ICP-MS, H 2 O calculated by stoichiometry) is: CaO 28.54, B 2 O 3 28.62, F 3.19, Cl 11.50, H 2 O 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 Ca 3.07 B 4.96 O 6.03 (OH) 6 F 1.01 Cl 1.96 ·8H 2 O. 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) Å 3 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, R w p = 0.0058, R p = 0.0043 and R obs = 0.0241. Popugaevaite is an isostructural analogue of brianroulstonite Ca 3 [B 5 O 6 (OH) 6 ](OH)Cl 2 ·8H 2 O with F – instead of the OH – group non-bound with boron. The structure is based upon the layers of twelve-membered rings of alternating BO 3 triangles and BO 2 (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.