Unit-cell intergrowth of pyrochlore and hexagonal tungsten bronze structures in secondary tungsten minerals

Structural relations between secondary tungsten minerals with general composition A(x)[(W,Fe)(O,OH)(3)]center dot yH(2)O are described. Phyllotungstite (A = predominantly Ca) is hexagonal, a = 7.31(3) angstrom, c = 19.55(l) angstrom, space group P6(3)/mmc. Pittongite, a new secondary tungsten mineral from a wolframite deposit near Pittong in Victoria, southeastern Australia (A = predominantly Na) is hexagonal, a = 7.286(l) angstrom, c = 50.49(l) angstrom, space group P-6m2. The structures of both minerals can be described as unit-cell scale intergrowths of (I I I)(py) pyrochlore slabs with pairs of hexagonal tungsten bronze (HTB) layers. In phyllotungstite, the (I I I)py blocks have the same thickness, 6 angstrom, whereas pittongite contains pyrochlore blocks of two different thicknesses, 6 and 12 angstrom. The structures can alternatively be described in terms of chemical twinning of the pyrochlore structure on (I I I)py oxygen planes. At the chemical twin planes, pairs of HTB layers are corner connected as in hexagonal WO3. (c) 2006 Elsevier Inc. All rights reserved.