Fluorcalciobritholite, (Ca,REE)5[(Si,P)O4]3F, a new mineral: description and crystal chemistry

The new mineral fluorcalciobritholite, ideally Ca3Ce2(SiO4)2(PO4)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/cm3. Optically, it is uniaxial (-) with omega 1.735(5), epsilon 1.730(5). Electron microprobe gave the following empirical formula based on [Si+P+S] = 3 apfu: [Ca2.80(Ce0.93La0.54Nd0.26Y0.18Pr0.08Sm0.03Gd0.03Dy0.02Yb0.02Er0.01)sum=2.12 Th0.04Mn0.03 Sr0.02]sum=4.99 [(Si1.94P1.06)sum=3 O12] [F0.76O0.22Cl0.01]sum=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 P63/m, with a = 9.580(7), c = 6.985(4) Å, V = 555.2(7) Å3. The crystal structure was refined from single-crystal X-ray diffraction data to RF = 0.029. Fluorcalciobritholite, whose simplified formula is (Ca,REE)5[(Si,P)O4]3F, differs from fluorbritholite in having Ca > sumREE, and differs from fluorapatite in having Si 8 P. Its compositional field falls within the limits Ca2.5REE2.5(SiO4)2.5(PO4)0.5F (boundary with fluorbritholite) and Ca3.5REE1.5(SiO4)1.5(PO4)1.5F (boundary with fluorapatite). Both the mineral and its name have been approved by the IMA Commission on New Minerals and Mineral Names