Rietveld refinement of chapmanite SbFe2Si2O8OH, a TO dioctahedral kaolinite-like mineral

Chapmanite, Sb(OH)Fe2(SiO4)2 is a rare mineral that generally occurs as yellow coatings on rocks from antimony mines. The very small dimensions of the crystals prevented X-ray single-crystal study of the mineral whose structure was derived through high voltage electron diffraction (Zhukhlistov & Zvyagin, 1977). The present study confirm the correctness of the structure determined by those authors and shows the effectiveness of the Rietveld method also in the case of highly diluted phases. The structure of chapmanite is similar to that of kaolinite with two major differences: a) the dioctahedral sheet of chapmanite contains Fe3+ instead of Al3+; b) chapmanite has Sb3+ cations grasped to the octahedral sheet. on the opposite side with respect to the tetrahedral sheet.