Home > Mineral Abstracts > CanMin Vol. 41, pp. 55-60 (2003)




Vol. 41, pp. 55-60 (2003)



Ferrokentbrooksite, a new member
of the eudialyte group from
Mont Saint-Hilaire, Québec, Canada


Ole Johnsen, Joel D. Grice, and Robert A. Gault


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, sÚrandite, 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) Å3, Z = 3. The strongest nine X-ray powder-diffraction lines [d in Å(I)(hkl)] are:

7.104(38)(110)
5.694(50),(202)
4.300(43)(205)

3.955(31)(214)
3.391(51)(131)
3.955(31)(214)

3.207(31)(208)
3.155(31)(217)
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:

Na2O - 11.96
K2O - 0.44
CaO - 7.99
MnO - 3.88

FeO - 5.08
SrO - 0.45
Al2O3 - 0.11
Y2O3 - 0.58

La2O3 - 1.51
Ce2O3 - 2.51
Nd2O3 - 0.53
Sm2O3 - 0.11

Gd2O3 - 0.17
SiO2 - 44.70
TiO2 - 0.09
ZrO2 - 11.20

HfO2 - 0.17
Nb2O5 - 2.51
Ta2O5 - 0.16
F - 0.40

Cl - 0.93
H2O - 0.35
O F, Cl - 0.38
Total 95.45 wt.%



     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:


(Na13.05REE0.99K0.32Ca0.23Sr0.15)S14.74(Ca4.59Mn1.24Y0.17)S6


(Fe2.39Mn0.61)S3 (Zr3.00Ti0.04Hf0.03)S3.07(Nb0.64Si0.23Zr0.07Ta0.02)S0.96


(Si24.93Al0.07)S25O73(O,OH,H2O)S2.47(Cl0.89F0.71OH0.40)S2.


Dmeas = 3.06(3) g/cm3, Dcalc = 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.


© 2003  The Canadian Mineralogist