The chemistry and structure of redledgeite

ABSTRACT. Two distinct habits of redledgeite are described: small black bipyramidal crystals and yellow-green fibres. The mineral is a Ba Cr-Ti oxide rather than a Mg-Cr-Ti oxide as previously supposed. Electron microprobe analysis gave Bal.10(Cr1.82Tis.9s Fe0.10Vo.o8)x7.95016 and Bal.27(fr2.4.sTis.49Ee0.02)x7.99 O~6 for the black and yellow green forms respectively. The mineral is a monoclinic hollandite-type hase, space group I2/m, with a = 10.129(2); b = 2.95(1); c = 10.135(2) A; 13 = 90.05(11) ~ and Z = 1; calculated ensity for the black form is 4.413 g cm-3. The crystal structure was refined to R 6.3 ~, Rw 7.4 ~ using 1017 reflections with F> 3a(F) from a set of 1062 unique reflections. Electron diffraction studies revealed weak superlattice r flections, with a period of 2.24b due to tunnel cation ordering