Am. Mcdonald et Gy. Chao, Natrolemoynite, a new hydrated sodium zirconosilicate from Mont Saint-Hilaire, Quebec: Description and structure determination, CAN MINERAL, 39, 2001, pp. 1295-1306
Natrolemoynite is a new member of the lemoynite group found in altered or u
naltered pegmatites cutting nepheline syenite at the Poudrette quarry, Mont
Saint-Hilaire, Quebec. In altered pegmatites. it is associated with microc
line, lemoynite, lepidocrocite, galena, sphalerite, calcite and pyrite, and
in unaltered pegmatites, with biotite, microcline, albite, magnetite, a ch
lorite-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, elon
gate along [001], with maximum widths of 1 mm and lengths of 2 mm. The mine
ral typically develops in compact radial aggregates and spheres 4 mm in max
imum diameter. It is transparent to translucent, with a vitreous to subadam
antine luster and a white streak. The Mohs hardness is 3. Perfect {100} and
{010} cleavages and a poor {001} cleavage are observed. D-means is 2.47(l)
g/cm(3) and D-calc, 2.50 g/cm(3). Natrolemoynite is biaxial negative, alph
a 1.533(l), beta 1.559(l). 1.567(l), 2V(meas), = 63(1)degrees, 2V(calc). =
57(1)degrees; dispersion is weak, with r and v crossed. The optical orienta
tion is X = b, Z /\ a 41 degrees (measured in the obtuse angle beta). The a
verage 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, Nb
2O5 1.01 and H2O (calc.) 14.72. total 101.88 wt.%, corresponding to (Na2.66
K0.30Ca0.07Mn0.02)Sigma3.05 (Zr1.96Nb0.08Ti0.05)Sigma 2.09 (Si9.99Al0.01)Si
gma 10O25.79H(2)O on the basis of 10(Si + Al) or, ideally, Na4Zr2Si10O26. 9
H(2)O. It is monoclinic, (C-2/m, a [0.5150(2), b 16.2534(4), c 9.1029(3) An
gstrom,beta 105.462(2)degrees, V 1499.4(1) Angstrom (3), Z= 2. The structur
e was refined to R = 4.2% for 2133 reflections [F-o > 4 sigma (F-o)]. Six-m
embered rings of SiO4 tetrahedra are linked along [001] by ZrO6 octahedra t
o form a zirconosilicate framework. Channels parallel to [001] are occupied
by Na and H2O. Evidence is provided for limited Na <----> H3O+ substitutio
n. Crystal-chemical and field relations indicate that paragenetically, natr
olemoynite is one of the last sodium zirconosilicate hydrate phases to deve
lop.