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

¹ Faculty of Geology, Moscow State University, Vorobievy Gory, 119992 Moscow, Russia
² Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy
³ Fersman Mineralogical Museum, Leninskiy prospect 18-2, 117901 Moscow, Russia
⁴ Institute of Problems of Chemical Physics, 142432 Chernogolovka, Moscow Oblast, Russia
⁵ Institute of Geochemistry and Analytical Chemistry, Kosygin ul. 19, 117975 Moscow, Russia
⁶ Geological Institute of Kola Science Centre, Fersman ul. 14, 184200 Apatity, Murmansk Oblast, Russia
⁷ NPO Regenerator, 3^rd Passage of Mar’ina Roshcha 40, 127018 Moscow, Russia

^* Corresponding author, e-mail: pasero{at}dst.unipi.it

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 1.735(5), 1.730(5). Electron microprobe gave the following empirical formula based on [Si+P+S] = 3 apfu: [Ca_2.80 (Ce_0.93La_0.54Nd_0.26Y_0.18Pr_0.08Sm_0.03Gd_0.03Dy_0.02Yb_0.02Er_0.01)_2.12 Th_0.04Mn_0.03 Sr_0.02] _4.99 [(Si_1.94P_1.06) ₃O₁₂] [F_0.76O_0.22Cl_0.01]_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) ų. 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)O₄]₃F, differs from fluorbritholite in having Ca > REE, and differs from fluorapatite in having Si > P. Its compositional field falls within the limits Ca_2.5REE_2.5(SiO₄)_2.5(PO₄)_0.5F (boundary with fluorbritholite) and Ca_3.5REE_1.5(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.

Key-words: fluorcalciobritholite, new mineral, crystal structure, Khibiny alkaline complex, britholite group, apatite group.