ABSTRACT

     Ferrokentbrooksite, ideally Na₁₅Ca₆(Fe,Mn)₃Zr₃NbSi₂₅O₇₃(O,OH,H₂O)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, sérandite, 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 w 1.6221(3) and 1.6186(3). It is trigonal, space group R3m, a 14.2099(7) and c 30.067(2) Å, V 5257.7(3) ų, Z = 3. The strongest nine X-ray powder-diffraction lines [d in Å(I)(hkl)] are:

     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:

O F, Cl - 0.38

     The amount of H₂O 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_13.05REE_0.99K_0.32Ca_0.23Sr_0.15)_S14.74(Ca_4.59Mn_1.24Y_0.17)_S6


(Fe_2.39Mn_0.61)_S3 (Zr_3.00Ti_0.04Hf_0.03)_S3.07(Nb_0.64Si_0.23Zr_0.07Ta_0.02)_S0.96


(Si_24.93Al_0.07)_S25O₇₃(O,OH,H₂O)_S2.47(Cl_0.89F_0.71OH_0.40)_S2.


D_meas = 3.06(3) g/cm³, D_calc = 3.06 g/cm³.

     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.

Keywords: ferrokentbrooksite, eudialyte, new mineral species, crystal structure, Mont Saint-Hilaire, Québec.