The crystal structure of hollandite, the related manganese oxide minerals, and α-MnO2

Hollandite cryptomelane, coronadite and alpha-MnO 2 form an isostructural series of the general formula A 2-y B 8-z X 16 , A being large ions such as Ba 2+ , Pb 2+ , and K + , B small and medium-sized ions such as Mn 4+ , Fe 3+ , Mn 2+ , and X in the cases investigated O 2- and OH - ions. The unit cell, which contains one formula unit, is tetragonal or pseudotetragonal, in the latter case monoclinic. Approximate dimensions of the tetragonal unit cell are a=9.8 and c=2. 86 A. When deformed the short axis will be the b axis of the monoclinic cell. The deviation of beta from 90° is 0.5-1.5°, and the difference a-c is 0. 1-0.2 A. The space group of the tetragonal cell is C 4h 5 -I4/m and the atomic positions are (0, 0, 0; 1/2, 1/2, 1/2) + (2-y A in 2 (b): 0, 0, 1/2); (8-z B in 8 (h): x 1 , y 1 , 0; y 1 , x 1 , 0; y 2 , x 2 , 0); (8 X in 8 (h): x 2 , y 2 , 0; etc.); (8 X in 8 (h): x 3 , y 3 , 0, etc.). For a hollandite specimen the parameters were calculated to be x 1 =0.348, x 2 =0.153, x 3 =0.542, y 1 =0.167, y 2 =0.180, y 3 =0.167. The A ion is surrounded by eight oxygen ions at a distance of 2. 74 A forming a cube and at a greater distance (3.31 A) by four oxygen ions forming a square at the same z level as the A ion. The B ion is surrounded by six oxygen ions forming a octahedron, and with a mean distance of B-O=1.98 A. In the cases investigated, the variable y in the general formula varies from 0.8 and 1.3 and z from 0.1 to 0.5.