Alflarsenite, a new beryllium-silicate zeolite from a syenitic pegmatite in the Larvik plutonic complex, Oslo Region, Norway

Alflarsenite is a new beryllium-silicate zeolite with chemical composition close to NaCa₂Be₃Si₄O₁₃(OH)·2H₂O. It is a late-stage hydrothermal mineral from a syenitic pegmatite in the Tuften larvikite quarry, Tvedalen, Larvik, Vestfold, south Norway. Closely associated minerals are calcite, analcime and K-feldspar. Alflarsenite is monoclinic, P2₁, with a = 7.1222(4), b = 19.8378(11), c = 9.8071(5) Å, β = 111.287(1)°, V = 1291.1(2) ų, Z = 4. The crystal structure was refined from single-crystal X-ray data to R1 = 0.045 for 3283 observed reflections. The zeolite structure is novel, consisting of intersecting channels: 5- and 8-membered channel rings along [010] and 3-, 6- and 8-membered channel rings along [100]. The strongest eight reflections of the X-ray powder-diffraction pattern [d(obs.) in Å (I) (hkl)] are: 9.095 (100) (001), 6.279 (42) (−111, 110), 4.189 (32) (−122, 121), 3.972 (76) (−141, 140), 3.205 (37) (−113, 112), 2.964 (70) (−232, 230), 2.915 (92) (−133, 132), 2.757 (33) (−242, 240). Alflarsenite crystals are colourless (in aggregates pale beige) and bladed; they are flattened on (010) and elongate on [100]. Identified crystal forms are {010}, {0–10}, {001} and {20–1}. Cleavage was not observed. The mineral is biaxial (+) with refractive indices α 1.578(1), β 1.580(1), γ 1.583(1), measured at 590 nm. 2V (meas.) = 82(5)° from extinction curves and = 76(5)° by the Kamb method; 2V (calc.) = 79°. The optical orientation is Z = b, X ^ a = 45(2)°. The Mohs hardness is ~ 4; D (calc.) = 2.605 g/cm³.