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]_·yH₂O are described. Phyllotungstite (A=predominantly Ca) is hexagonal, , , space group P6₃/mmc. Pittongite, a new secondary tungsten mineral from a wolframite deposit near Pittong in Victoria, southeastern Australia (A=predominantly Na) is hexagonal, , , space group P-6m2. The structures of both minerals can be described as unit-cell scale intergrowths of (111)_py pyrochlore slabs with pairs of hexagonal tungsten bronze (HTB) layers. In phyllotungstite, the (111)_py blocks have the same thickness, 6 Å, whereas pittongite contains pyrochlore blocks of two different thicknesses, 6 and 12 Å. The structures can alternatively be described in terms of chemical twinning of the pyrochlore structure on (111)_py oxygen planes. At the chemical twin planes, pairs of HTB layers are corner connected as in hexagonal WO₃.