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Home > Mineral Abstracts > CanMin Vol. 44, pp. 105-115 (2006)




Vol. 44, pp. 105-115 (2006)



Johnsenite-(ce): a new member of the eudialyte group from Mont Saint-Hilaire, Quebec, Canada


Joel D. Grice and Robert A. Gault


ABSTRACT

     Johnsenite-(Ce), ideally Na12(Ce,La,Sr,Ca,M)3Ca6Mn3Zr3W(Si25O73)(CO3)(OH,Cl)2, is a new member of the eudialyte group from Mont Saint-Hilaire, Quebec, and is the W analogue of zirsilite-(Ce). It occurs as deeply etched, skeletal crystals to 4 mm and aggregates of crystals to 1 cm. Associated minerals include, albite, calcite, pectolite, aegirine, fluorapophyllite, zirsilite-(Ce), a burbankite-group phase, dawsonite, rhodochrosite, epididymite, galena, molybdenite, pyrite, pyrrhotite, quartz, an amphibole-group mineral, sphalerite, stillwellite-(Ce), titanite, cerite-(Ce), tuperssuatsiaite, steacyite, catapleiite, zakharovite, natrolite and microcline. It is transparent to translucent with a vitreous luster and white streak. It is brittle with a Mohs hardness of 5 - 6. It has no discernable cleavage or parting and an uneven fracture. It is uniaxiail negative with v 1.648(1) and ' 1.637(1). It is trigonal, space group, R3m, a 14.237(3) and c 30.03(1) Å, V 5271(2) Å3M, Z = 3.

     The eight strongest X-ray powder-diffraction lines, measured for johnsenite-(Ce) [d in Å (l)(hkl)] are:

11.308(95)(101)
9.460(81)(012)
4.295(34)(205)
3.547(36)(220)

3.395(38)(131)
3.167(75)(217)
2.968(100)(315)
2.849(81)(404)



     The infrared spectrum of johnsenite-(Ce) is given. An average result of eight electron-microprobe analyses, including three on the grain used for the structure determination, gave:

Na2O - 10.47
K2O - 0.25
CaO - 8.98
SrO - 1.60

MnO - 5.43
FeO - 1.61
Y2O3 - 0.70
La2O3 - 1.56

Ce2O3 - 3.01
Pr2O3 - 1.14
Nd2O3 - 0.89
Sm2O3 - 0.12

Gd2O3 - 0.33
Dy2O3 - 0.14
TiO2 - 0.73
ZrO2 - 9.60

HfO2 - 0.04
Nb2O5 - 0.82
WO3 - 5.23
SiO2 - 43.16

Cl - 0.77
CO2 - (1.27)
H2O - (0.32)
O = Cl -0.17

total 97.99 wt. %


     Concentrations of CO2 and H2O were calculated by stoichiometry based on results from the crystal-structure analysis. The empirical formula of johnsenite-(Ce), based on 29 (Si4+, Zr4+, Ti4+, Hf4+, Nb5+, W6+) cations and 1 (CO3) as determined in the crystal-structure analysis, is:


Na11.74 ([Ce0.64La0.33Dy0.03]S1.00Sr0.54Ca0.51Y0.22K0.19)S2.46


(Ca5.06[Pr0.24Nd0.18Gd0.06Sm0.02]S0.50Mn0.44)S6(Mn2.22Fe0.78)S3


(Zr2.71Ti0.32Hf0.01)S3.04(W0.78Nb0.21)S0.99Si24.97O73(CO3)(OH,Cl0.75)S2.


Dmeas. = 3.24(3) g/cm3; Dcalc. = 3.23 g/cm3.

     The structure consists of four layers. The layering is disctinctly polar, as indicated by the X-ray-diffraction intensity statistics, leyer composition and electrostatic potential of individual crystals. The new species is named after Ole Johnsen, of the Geological Museum, University of Copenhagen, Denmark.

Keywords: johnsenite-(Ce), eudialyte, new mineral species, crystal structure, Mont Saint-Hilaire, Quebec.



© 2006  The Canadian Mineralogist