Stronadelphite, Sr₅(PO₄)₃F, a new apatite-group mineral

The new apatite-group mineral stronadelphite was found in the Belovitovoye peralkaline pegmatite at the Kirovskii apatite mine, Mt. Kukisvumchorr, Khibiny alkaline complex, Kola peninsula, Russia. Associated minerals are natrolite, microcline, aegirine, pectolite, lamprophyllite, belovite-(Ce), belovite-(La), gaidonnayite, nenadkevichite, komarovite, manganokukisvumite, epididymite, sphalerite, neotocite, etc. Stronadelphite forms hexagonal, prismatic to acicular crystals up to 0.1 × 0.1 × 2 mm, combined in sheaf-like clusters up to 1 × 2 mm embedded in natrolite. The mineral is transparent, colourless with a pale greenish tint. Stronadelphite is brittle, with Mohs hardness 5, no cleavage observed. D_calc is 3.915 g/cm³. Optically, the new mineral is uniaxial (−), ω 1.630(1), ε 1.623(1). Average values for six point microprobe analyses (wt%) are: Na₂O 0.10, CaO 2.49, SrO 62.72, BaO 2.40, La₂O₃ 0.34, Ce₂O₃ 0.22, ThO₂ 0.28, P₂O₅ 29.02, F 1.45, H₂O(calc.) 0.54, −O=F₂ −0.61; total 98.95. The empirical formula based on 13 anions [O₁₂(F,OH)] is: (Sr_4.46Ca_0.33Ba_0.12Na_0.02La_0.02Ce_0.01Th_0.01)_∑=4.97P_3.01O₁₂[F_0.56(OH)_0.44]_∑=1. The idealized formula is Sr₅(PO₄)₃F. Stronadelphite is hexagonal, space group P6₃/m; a = 9.845(7), c = 7.383(4) Å, V = 619.7(7) Å ³, Z = 2. Its crystal structure has been refined to R = 0.0305 using single-crystal X-ray diffraction data. Stronadelphite is isostructural with fluorapatite. The strongest lines of the powder diffraction pattern [d in Å (I) (hkl)] are: 3.71 (30) (002), 3.21 (40) (120, 210), 2.940 (100) (211, 121, 112), 2.823 (35) (300, 202), 2.009 (50) (222, 312, 132), 1.955 (45) (213, 123), 1.500 (30) (151, 511, 332). The mineral is named after the chemical element strontium and αδελφóσ, Greek for “brother”, as the full strontium analogue of fluorapatite, the most widespread member of the apatite supergroup. Both the mineral and its name have been approved by the IMA CNMNC. The type specimen is deposited in Fersman Mineralogical Museum, Moscow, with the registration number 3693/1.