RIETVELD REFINEMENT OF CHAPMANITE SBFE2SI2O8OH, A TO DIOCTAHEDRAL KAOLINITE-LIKE MINERAL

Chapmanite, Sb(OH)Fe-2(SiO4)(2) is a rare mineral that generally occur s as yellow coatings on rocks from antimony mines. The very small dime nsions of the crystals prevented X-ray, single-crystal study of the mi neral whose structure was derived through high voltage electron diffra ction (Zhukhlistov and Zvyagin, 1977). The present study confirm the c orrectness of the structure determined by those authors and also shows the effectiveness of the Rietveld method in the case of highly dilute d phases. The structure of chapmanite is similar to that of kaolinite with two major differences: (a) the dioctahedral sheet of chapmanite c ontains Fe3+ instead of Al3+; (b) chapmanite has Sb3+ cations grasped to the octahedral sheet, on the opposite side with respect to the tetr ahedral sheet. (C) 1998 International Centre for Diffraction Data.