STRUCTURE REFINEMENT OF MONOCLINIC ASTROPHYLLITE

The crystal structure of monoclinic magnesium astrophyllite has been r edetermined. The samples were from Khibina alkaline massifs, Russia. 2 NaNa(Fe,Mn)(4-)Mg2Ti2[Si4O12](2)(OH)(4)(OH,F)(2), M-r-1202.31, monocli nic, A2, a = 10.370 (3), b = 23.129 (5), c = 5.322 (1) Angstrom, beta = 99.55(2)degrees, V = 1258.8(5)Angstrom(3), Z= 2, D-x =3.173 Mg m(-3) , Mo K alpha, gamma = 0.71069 Angstrom, mu = 3.74 mm(-1) F(000) = 1179 , T= room temperature, R = 0.057 for 1720 reflections. The determinati on indicated that the coordination number of Ti is five. In tetrahedra of astrophyllite the open-branched Si-O chain connected with Ti-O sem i-octahedra by corner-sharing formed a continuous sheet. Na atoms have two types of coordination. Their coordination numbers are eight and s ix. The Na atoms with coordination number eight are situated between l ayers. Other types of Na atoms are situated within the layers and form Na-O octahedra with partial occupancies.