Avdeevite, a Na-dominant alkali beryl: definition as valid mineral species and new data

Agakhanov A. A., Stepanenko D. A., Zubkova N. V., Pautov L. A., Pekov I. V., Kasatkin A. V., Karpenko V. Yu., Agakhanova V. A., Škoda R., Britvin S. N., Pushcharovsky D. Yu.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2020. V. 149. N 6. P. 1–19

https://doi.org/10.31857/S0869605520060027

Language: Russian

Abstract

In this paper the first Na-dominant member of the beryl group named avdeevite is described. The sample, which became its holotype, was found in a granitic pegmatite at the Kat Chay (Palelni) mine near Momeik township, Kyaukme district, Shan State, Myanmar. Both avdeevite and beryl form sheaf-like aggregates up to 4 cm composed by 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). New mineral is pink and transparent with vitreous lustre. The Mohs hardness is 8, D_meas= 2.89(2) and D_calc = 2.875 g/cm³. Avdeevite is optically uniaxial (–), nm = 1.601(2), np = 1.594(2); in transmitted light the mineral is colourless and non-pleochroic. The chemical composition of holotype (electron microprobe, Be, Li and H by ion microprobe, wt %): SiO₂ 61.06, Al₂O₃ 17.40, BeO 8.58, Na₂O 1.77, K₂O 0.09, Cs₂O 6.44, Rb₂O 1.38, Li₂O 2.51, H₂O 0.42, total 99.63. The empirical formula is (Na_0.34Cs_0.27H₂O_0.14Rb_0.09K_0.01)_Σ0.85(Be_2.04Li_1.00)_Σ3.04Al_2.03Si_6.03O₁₈. Taking into consideration previously published data for the samples with compositions corresponding to avdeevite, the simplified formula of this mineral can be most correctly written as: [(Na,R')_0.5-1(H₂O)_y]{(Be,Li)₃(Al,M²⁺)₂Si₆O₁₈}, y ≤ 1, R' = Cs, Rb, K, Ca, M²⁺ = Fe, Mg, Mn. Crystal structure was studied by single-crystal X-ray diffraction, R = 0.0322. The mineral is hexagonal, P6/mcc. Unit-cell parameters are: a = 9.2287(4) Å, c = 9.2610(3) Å, V = 683.07(5) Å3, and Z = 2. The strongest lines of the powder X-ray diffraction pattern are [d, 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 structure type. This mineral differs from beryl sensu stricto in that the sum of large cations in the channels exceeds 0.5 apfu and Na dominates among them. Avdeevite is named after the outstanding Russian chemist and mining engineer Ivan Vasil’evich Avdeev (1818-1865) who contributed a lot to the studies of natural and synthetic compounds of beryllium. The paper also contains data on the history of studies of Na-bearing beryls including earlier described samples which correspond to avdeevite.

Keywords: avdeevite, new mineral, alkali beryl, beryl group, crystal structure, granitic pegmatite, Kat Chay, Momeik, Myanmar