Home > Mineral Abstracts > CanMin Vol. 39, pp. 1295-1306 (2001)




Vol. 39, pp. 1295-1306 (2001)



Natrolemoynite, a new hydrated
sodium zirconosilicate
from Mont Saint-Hilaire, Québec:
description and structure determination


Andrew M. McDonald and George Y. Chao


ABSTRACT

     Natrolemoynite is a new member of the lemoynite group found in altered or unaltered pegmatites cutting nepheline syenite at the Poudrette quarry, Mont Saint-Hilaire, Québec. In altered pegmatites, it is associated with microcline, lemoynite, lepidocrocite, galena, sphalerite, calcite and pyrite, and in unaltered pegmatites, with biotite, microcline, albite, magnetite, a chlorite-group mineral, a burbankite-group mineral, an unidentified donnayite-(Y)-like mineral, zircon and pyrochlore. Crystals are colorless to white (occasionally with a slightly pink to red tinge), bladed to prismatic, elongated along [001], with maximum widths of 1 mm and lengths of 2 mm. The mineral typically develops in compact radial aggregates and spheres 4 mm in maximum diameter. It is transparent to translucent, with a vitreous to subadamantine luster and a white. The transparent to translucent mineral has a vitreous to pearly luster and a white streak. The Mohs hardness is 3. Perfect {100} and {010} cleavages and a poor {001} cleavage are observed.

     Dmeas is 2.47(1) g/cm3 and Dcalc, 2.50 g/cm3. Natrolemoynite is biaxial negative, a 1.533(1), b 1.559(1), g 1.567(1), 2Vmeas = 63(1)°, 2Vcalc = 57(1)°; dispersion is weak, with r and v crossed. The optical orientation is
X = b, Z > a = 41° (measured in the obtuse angle B).

     The average result of nineteen electron-microprobe analyses gave:

Na2O - 7.47
K2O - 1.29
CaO - 0.37
MnO - 0.12

Al2O3 - 0.04
SiO2 - 54.51
TiO2 - 0.38
ZrO2 - 21.97

Nb2O5 - 1.01
H2O(calc) - 14.72
Total 101.88 wt.%



     corresponding to:


(Na2.66K0.30Ca0.07Mn0.02)S3.05 (Zr1.96Nb0.08Ti0.05)S2.09


(Si9.99Al0.01)S10O25.79 · 9H2O on the basis of 10(Si + Al) or,


ideally, Na4Zr2Si10O26 · 9H2O.

     It is monoclinic, C2/m, a 10.5150(2), b 16.2534(4), c 9.1029(3) Å, B 105.462(2)°, V 1499.4(1) Å3, Z = 2. The structure was refined to R = 4.2% for 2133 reflections [Fo > 4s(Fo)]. Six-membered rings of SiO4 tetrahedra are linked along [001] by ZrO6 octahedra to form a zirconosilicate framework. Channels parallel to [001] are occupied by Na and H2O. Evidence is provided for limited Na H3O+ substitution. Crystal-chemical and field relations indicate that paragenetically, natrolemoynite is one of the last sodium zirconosilicate hydrate phases to develop.

Keywords: natrolemoynite, new mineral species, zirconosilicate, crystal structure, Mont Saint-Hilaire, Québec.



© 2001  The Canadian Mineralogist