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, long-prismatic to acicular crystals up to 0.1 x 2 mm, combined in sheaf-like clusters up to 1 x 2 mm embedded in natrolite. The empirical formula based on sum of all metals and P = 8 apfu and O₁₂(F,OH,O)₁, is: (Sr_4.47Ca_0.33Ba_0.12Na_0.02La_0.02Ce_0.01Th_0.01)_∑4.98P_3.02O₁₂[F_0.56(OH)_0.35O_0.09]_∑1. The idealized formula is Sr₅(PO₄)₃F. The crystal structure is refined to R = 0.0305. Stronadelphite is hexagonal, space group P63/m; a = 9.845(7), c = 7.383(4) Å, V = 619.7(7) ų, Z = 2. It is isostructrural with fluorapatite. The mineral is named after the chemical element strontium and αδελφοσ, Greek for "brother", as the full strontium analogue of fluorapatite, the most widespread member of the apatite supergroup.