Natromelansonite, Na 3 Zr[Si 7 AlO 19 ]⋅4–5H 2 O, a new member of the rhodesite mero-plesiotype series from Mont Saint-Hilaire, Quebec, Canada

Natromelansonite, Na 3 Zr[Si 7 AlO 19 ]⋅4–5H 2 O, was found at the Poudrette (Demix) quarry, Mont Saint-Hilaire, Quebec, Canada in a highly altered pegmatite together with a clay mineral, steacyite, polylithionite and rhodochrosite. It occurs as an outer zone of tabular crystals to 0.1 × 0.3 × 1 mm in size flattened on (001). The inner zone is made of melansonite. The mineral is grey with white powder colour and vitreous lustre. The cleavage is parallel to {010}, perfect. The Mohs hardness is 3.5. The mineral has green fluorescence under short-wave ultraviolet light. The D calc is 2.31 g/cm 3 . The infrared spectrum is reported. The composition (wt.%, average of 8 analyses) is Na 2 O 10.08, K 2 O 1.72, CaO 0.24, BaO 0.32, MnO 0.10, Al 2 O 3 6.86, Y 2 O 3 0.35, Yb 2 O 3 0.55, SiO 2 56.23, ZrO 2 14.78, H 2 O 9.52, total 100.75. The empirical formula calculated on the basis of O = 23 apfu and H = 8 apfu is Na(□Na 0.38 Ca 0.02 Mn 0.01 ) 2 (Na 0.70 K 0.28 Ba 0.02 ) Σ1.00 (Zr 0.91 Y 0.02 Yb 0.02 ) Σ0.95 (Si 7.08 Al 1.02 ) Σ8.10 O 19 ⋅4H 2 O. The mineral is monoclinic, P 2 1 /m , a = 6.5156(3) Å, b = 24.061(1) Å, c = 6.9759(6) Å, β = 90.453(5)° and V = 1093.61(9) Å 3 and Z = 2. The strongest reflections of the powder X-ray diffraction pattern [ d ,Å( I )( hkl )] are: 12.02(100)(020), 6.97(89)(001), 6.51(39)(100), 3.416(37)(160), 3.062(42)( $\bar{1}$ 61, 102, 161 and $\bar{1}$ 12), 3.018(38)(230 and 042), 2.864(40)(240 and 132). The crystal structure, solved and refined from single-crystal X-ray diffraction data ( R 1 = 0.042), is based on a double sheet of tetrahedra (T) and a sheet of octahedra (O) that alternate along the [010] direction forming a TOT structure typical for members of the rhodesite mero-plesiotype series.