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

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.