The new mineral fluorcalciobritholite, ideally Ca 3 Ce 2 (SiO 4) 2 (PO 4)F, has been found at Mount Kukisvumchorr, Khibiny alkaline complex, Kola Peninsula, Russia, in veinlets which contains aggregates of orthoclase, nepheline, sodalite and biotite in association with grains of fayalite, gadolinite-(Ce), zircon, monazite-(Ce), zirconolite ("polymignite"), fluorapatite, fluorite, molybdenite, löllingite and graphite. Fluorcalciobritholite forms long-prismatic hexagonal crystals up to 0.5 x 10 mm; the main crystal form is the hexagonal prism {10-10}. The mineral is transparent, with a pale pinkish to brown colour and a white streak. The hardness (Mohs) is 5.5, and the observed density is 4.2(1) g/cm 3. Optically, it is uniaxial (-) with K 1.735(5), 5 1.730(5). Electron microprobe gave the following empirical formula based on [Si+P+S] = 3 apfu: [Ca 2.80 (Ce 0.93 La 0.54 Nd 0.26 Y 0.18 Pr 0.08 Sm 0.03 Gd 0.03 Dy 0.02 Yb 0.02 Er 0.01) 7 2.12 Th 0.04 Mn 0.03 Sr 0.02 ] 7 4.99 [(Si 1.94 P 1.06) 7 3 O 12 ] [F 0.76 O 0.22 Cl 0.01 ] 7 0.99 (Z = 2). The IR spectrum of metamict fluorcalciobritholite from Siberia showed a marked similarity with those of hydroxylbritholite-(Ce) and hydroxylbritholite-(Y). The strongest lines of the X-ray powder pattern [d in Å (I) (hkl)] are: 3.51 (45) 002, 3.15 (70) 102, 2.85 (100) 211, 121, 2.78 (60) 300. The mineral is hexagonal, space group P6 3 /m, with a = 9.580(7), c = 6.985(4) Å, V = 555.2(7) Å 3. The crystal structure was refined from single-crystal X-ray diffraction data to R F = 0.029. Fluorcalciobritholite, whose simplified formula is (Ca,REE) 5 [(Si,P)O 4 ] 3 F, differs from fluorbritholite in having Ca 8 7 REE, and differs from fluorapatite in having Si 8 P. Its compositional field falls within the limits Ca 2.5 REE 2.5 (SiO 4) 2.5 (PO 4) 0.5 F (boundary with fluorbritholite) and Ca 3.5 REE 1.5 (SiO 4) 1.5 (PO 4) 1.5 F (boundary with fluorapatite). Both the mineral and its name have been approved by the IMA Commission on New Minerals and Mineral Names.
