FERROKENTBROOKSITE, A NEW MEMBER OF THE EUDIALYTE GROUP FROM MONT SAINT-HILAIRE, QUEBEC, CANADA

Abstract

Ferrokentbrooksite, ideally Na 15 Ca 6 (Fe,Mn) 3 Zr 3 NbSi 25 O 73 (O,OH,H 2 O) 3 (Cl,F,OH) 2 , is a new member of the eudialyte group from Mont Saint-Hilaire, Quebec; it is the ferrous-iron-dominant analogue of kentbrooksite. It occurs as reddish brown to red, pseudo-octahedral crystals to 1 cm in diameter. Associated minerals include microcline, nepheline (partially altered to natrolite), fluorite, fluorapatite, natrolite, gonnardite, rhodochrosite, aegirine, albite, calcite, serandite, ancylite-(Ce), and catapleiite. It is transparent with a vitreous luster and a white streak. It is brittle, with a hardness of 5–6 (Mohs scale). It has no cleavage, no parting, and an uneven to conchoidal fracture. It is uniaxial negative with ω 1.6221(3) and e 1.6186(3). It is trigonal, space group R 3 m , a 14.2099(7) and c 30.067(2) A, V 5257.7(3) A 3 , Z = 3. The strongest nine X-ray powder-diffraction lines [ d in A( I )( hkl )] are: 7.104(38)(110), 5.694(50)(202), 4.300(43)(205), 3.955(31)(214), 3.391(51)(131), 3.207(31)(208), 3.155(31)(217), 2.968(100)(315) and 2.847(98)(404). The infrared spectrum of ferrokentbrooksite is given. An average result of two electron-microprobe analyses of the grain on which the structure was refined gave Na 2 O 11.96, K 2 O 0.44, CaO 7.99, MnO 3.88, FeO 5.08, SrO 0.45, Al 2 O 3 0.11, Y 2 O 3 0.58, La 2 O 3 1.51, Ce 2 O 3 2.51, Nd 2 O 3 0.53, Sm 2 O 3 0.11, Gd 2 O 3 0.17, SiO 2 44.70, TiO 2 0.09, ZrO 2 11.20, HfO 2 0.17, Nb 2 O 5 2.51, Ta 2 O 5 0.16, F 0.40, Cl 0.93, H 2 O 0.35, O ≡ F, Cl –0.38, for a total of 95.45 wt.%. The amount of H 2 O was calculated by stoichiometry from the crystal-structure analysis. The empirical formula of ferrokentbrooksite, based on 77.47 anions as determined in the crystal-structure analysis, is: (Na 13.05 REE 0.99 K 0.32 Ca 0.23 Sr 0.15 ) ∑14.74 (Ca 4.59 Mn 1.24 Y 0.17 ) ∑6 (Fe 2.39 Mn 0.61 ) ∑3 (Zr 3.00 Ti 0.04 Hf 0.03 ) ∑3.07 (Nb 0.64 Si 0.23 Zr 0.07 Ta 0.02 ) ∑0.96 (Si 24.93 Al 0.07 ) ∑25 O 73 (O,OH,H 2 O) ∑2.47 (Cl 0.89 F 0.71 OH 0.40 ) ∑2 ; D meas = 3.06(3) g/cm 3 , D calc = 3.06 g/cm 3 . Ferrokentbrooksite possesses the eudialyte structure, and has Fe as the predominant element at M (2) replacing Mn. Fe is predominantly five-fold coordinated, and bond-valence calculations indicate it to be divalent.