Fluorbritholite-(Y), (Y,Ca,Ln)_5[(Si,P)O_4]_3 F, a new mineral of the britholite group

摘要

The new britholite-group mineral fl uorbritholite-(Y) was found in pegmatites related to alkaline granites. In Hamaroy, Nordland, Norway, it occurs in Lagmannsvik [the holotype, grains up to 1 mm with Y-rich fl uorite, allanite-(Ce), quartz, bastnasite-(Ce), britholite-(Y), gadolinite-(Y), hundholmenite-(Y), etc.] and Krakmo [rims around fl uorapatite, with quartz, fl uorite, allanite-(Ce), britholite-(Y), fl uorcalciobritholite and tengerite-(Y)]. At Mt. Vyuntspakhk, Western Keivy, Kola Peninsula, Russia, the mineral forms coarse, hexagonal, prismatic to tabular crystals and irregular grains up to 2.5 × 4 cm, with quartz, magnetite, microcline, zircon, fergusonite-(Y) and thorite. Fluorbritholite-(Y) is transparent, from light pinkish-brown to dark brown. Mohs’ hardness is 5.5, no cleavage was observed, fracture is uneven to conchoidal. D_(calc). for the holotype is 4.61 g/cm~3. Optically, it is uniaxial (+), ω 1.784(2), ε 1.789(3). The IR spectrum is given. Average values for four point microprobe analyses of the holotype are (wt.%): CaO 12.21, MnO 1.06, Y_2O_3 30.80, La_2O_3 1.13, Ce_2O_3 7.12, Pr_2O_3 0.69, Nd_2O_3 6.85, Sm_2O_3 2.15, Gd_2O_3 2.23, Dy_2O_3 2.80, Er_2O_3 2.61, Yb_2O_3 3.75, SiO_2 24.18, P_2O_5 0.29, F 2.01, – O = F_2 –0.85; total 99.03. The empirical formula based on 13 anions is: [(Y_(2.013)Ce_(0.320)Nd_(0.300)Yb_(0.140)Dy_(0.111)Er_(0.101)Gd_(0.091)Sm_(0.091)La_(0.051)Pr_(0.031)_(Σ3.249)Ca_(1.607)Mn_(0.110)]_(Σ4.966) [(Si_(2.970)P_(0.030))_(Σ3)O_(12)][F_(0.781)O_(0.210)(OH)_(0.009)]_(Σ1). The simplifi ed formula is: (Y,Ca,Ln)_5[(Si,P)O_4]_3 F. The crystal structure was refi ned to R = 4.46 % from single-crystal X-ray diffraction data. Fluorbritholite-(Y) is hexagonal, P6_3/m; a = 9.4437(2), c = 6.8169(2) A, V = 526.50(2) A~3, Z = 2. It is isostructural with fl uorapatite. The strongest lines of the powder X-ray diffraction pattern [d, A (I, %) (hkl)] are: 4.104 (27) (200), 3.160 (27) (102), 3.102 (29) (210), 2.826 (100) (121), 2.775 (58) (112), 2.737 (46) (300), 1.948 (25) (222), 1.839 (28) (123). The mineral is named as the fl uorine-dominant analogue of britholite-(Y). The type specimen is deposited in the Fersman Mineralogical Museum, Moscow.