The crystal structure of ferberite, FeWO4

The crystal structure of ferberite has been determined and refined by projections to R values of 7.20/0 for hkO reflections and 5.4% for hOl reflections. The structure is based on Wand Fc octahedra forming infinitc zig-zag chains in the direction of thc c axis. Each chain contains just one type of cation and each octahedron is joined to the next by a common edge. Each W-octahedral chain is attached by common corners to four Fe chains, and each Fe chain is also surrounded by four W chains. The atomic configuration is then different from the structure reported for MgW04, which was believed to be isostructural with the members of the wolframite group, even though the cation positions are proved to be correct. Introduction Ferberite is an end member of the isomorphous series of the wolframites, whose chemical formula is (Fe,Mn)W04. The different names of the members of the series apply to the particular ranges of composition given on Table 1. * Present address: Instituto de Biofisica, Escuela de Medicina, U niversidad Austral, Valdivia, Chile. 62 HILDA Cm-DREsDNER and CARMEN ESCOBAR Table 1. Iron and manganese content in the wolframite series Name Atomic per cent substitution Fe Mn Corresponding weight percentages FeO MnO Hiibnerite W olframitc Ferberite o to 20 20 to 80 80 to 100 100 to 80 80 to 20 20 to 0 o to 4.8 4.8 to 18.9 18.9 to 23.6 23.4 to 18.7 18.7 to 4.7 4.7 to 0 The minerals of the group are monoclinic, space group P2/c with two formula units per cell. The first x-ray study of these compounds was made by BRaCH 1 in 1929 but the axial ratio and the monoclinic character had been reported by Des Cloizeaux2 as early as 1850. BRacH made an x-ray study of several tung states of the wolframite-type structure and of the scheelite-type structure. He proved by x-ray methods that the isomorphism of the wolframites and MgW04 found by optical methods by MACHATSCHKI3was correct. He determined the cation positions for MgW04 and by comparison of the x-ray data he showed that the same structure was valid for the Zn, Mn, Fe, Co and Ni tungstates and for the minerals hiibnerite, wolframite an ferberite. His results are resumed on Table 2. BRacH made no assumptions on the oxygen positions, but in the Strukturbericht, Vol. II, page 85, the following approximate coordinates are suggested for the oxygen atoms: Xl ~ 0.20 Yl ~ 0.04 Zl ~ 0.10 X2 ~ 0.20 Y2 ~ 0.40 Z2 ~ 0.40. These parameters yield W04 tetrahedra in which W -0 = 1.73 A, and Mg in octahedral coordination with Mg-O = 2.24 A. Crystals of a wolframite from Liquinaste, Argentina4, were kindly provided by PABLO KITTL from the National Atomic Energy Comis1 EINAR KLAUMANN BROCH, Untersuchungeniiber Kristallstrukturen des W olframittypus und des Scheelittypus. N orsk Akad., Oslo, Mat. -nat. Kl. Skrifter No.8 (1929) 4-61. 2 A. DES CLOIZEAUX, Sur les formes cristallines du wolfram. Ann. chim. physique [3J 28 (1850). 3 FELIX MACHATSCHKI, Uber die Kristallgestalt des Magnesiumwolframates. Z. Kristallogr. 67 (1928) 163-165. 4 ERWIN KITTL, Sobre la Wolframita de Liquinaste, Jujuy. Revista Minera 20 (1951) 56-61. Table 2. The wolframite-type-structure tungstates