Avdeevite, a Na-Dominant Alkali Beryl: Determination as Valid Mineral Species and New Data

—This paper describes the first Na-dominant member of the beryl group, named avdeevite. The sample, which became its holotype, was found in a granitic pegmatite at the Kat Chay (Palelni) mine near the town of Momeik, Kyaukme district, Shan State, Myanmar. Both avdeevite and beryl form sheaflike aggregates up to 4 cm, consisting of hexagonal prismatic crystals up to 1 сm long and up to 1 mm thick. These crystals consist of epitactically intergrown avdeevite (core) and beryl (rim). The new mineral is pink and transparent with vitreous luster. The Mohs hardness is 8, D meas = 2.89(2) and D calc = 2.875 g/cm 3 . Avdeevite is optically uniaxial (–), ω = 1.601(2), ε = 1.594(2); in transmitted light, the mineral is colorless and nonpleochroic. The chemical composition of holotype (electron microprobe, Be, Li and H by ion microprobe) is, wt %: 61.06 SiO 2 , 17.40 Al 2 O 3 , 8.58 BeO, 1.77 Na 2 O, 0.09 K 2 O, 6.44 Cs 2 O, 1.38 Rb 2 O, 2.51 Li 2 O, 0.42 H 2 O; the total is 99.63. The empirical formula is (Na 0.34 Cs 0.27 H 2 O 0.14 Rb 0.09 K 0.01 ) Σ0.85 (Be 2.04 Li 1.00 ) Σ3.04 Al 2.03 Si 6.03 O 18 . Taking into account the previously published data for samples with compositions corresponding to avdeevite, the simplified formula of this mineral may be most correctly written as: [(Na, R ') 0.5–1 (H 2 O) y ]{(Be,Li) 3 (Al, M 2+ ) 2 Si 6 O 18 }, y ≤ 1, R ' = Cs, Rb, K, Ca, M 2+ = Fe, Mg, Mn. The crystal structure has been studied by single-crystal X-ray diffraction, R = 0.0322. The mineral is hexagonal, space group P 6 /mcc . The unit-cell parameters are: a = 9.2287(4) Å, c = 9.2610(3) Å, V = 683.07(5) Å 3 , and Z = 2. The strongest reflections of the XRD pattern are [ d meas , Å ( I , %) ( hkl )]: 7.96 (82) (100), 4.60 (31) (002), 3.99 (102) (200), 3.26 (100) (112), 3.02 (37) (202) (210), 2.866 (84) (211), 1.742 (19) (304) (410). Avdeevite belongs to the beryl structural type. This mineral differs from beryl sensu stricto in that the total large cations in channels exceeds 0.5 apfu and Na dominates among these cations. Avdeevite is named after Ivan Vasil’evich Avdeev (1818–1865), the renowned Russian chemist and mining engineer, who led the major studies of natural and synthetic beryllium compounds. The paper also contains data on the history of Na-bearing beryls including previously described samples, which correspond to avdeevite.