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The crystal structure of monoclinic magnesium astrophyllite has been redetermined. The samples were from Khibina alkaline massifs, Russia. K2NaNa(Fe,Mn)4Mg2Ti2[Si4O12]2(OH)4(OH,F)2, Mr = 1202.31, monoclinic, A2, a = 10.370 (3), b = 23.129 (5), c = 5.322 (1) Å, β = 99.55 (2)°, V = 1258.8 (5) Å3, Z = 2, Dx = 3.173 Mg m−3, Mo Kα, λ = 0.71069 Å, μ = 3.74 mm−1, F(000) = 1179, T = room temperature, R = 0.057 for 1720 reflections. The determination indicated that the coordination number of Ti is five. In tetrahedra of astrophyllite the open-branched Si—O chain connected with Ti—O semi-octahedra by corner-sharing formed a continuous sheet. Na atoms have two types of coordination. Their coordination numbers are eight and six. The Na atoms with coordination number eight are situated between layers. Other types of Na atoms are situated within the layers and form Na—O octahedra with partial occupancies.
