CRYSTAL-STRUCTURE REFINEMENT OF SYNTHETIC FE-STAUROLITE AND MG-STAUROLITE BY RIETVELD ANALYSIS OF X-RAY POWDER-DIFFRACTION DATA

The crystal structure of Fe-staurolite, synthesized at 5 and 25 kbar a nd 680 degrees C, and of Mg-staurolite, synthesized at 25 kbar and 700 degrees C, has been refined from X-ray powder-diffraction data using the Rietveld method in the space group Ccmm. The study was aimed to ge t insight into the distribution of Fe2+, Mg and Al at the tetrahedral T2 and the octahedral M4 sites. Fe-staurolite synthesized at 25 kbar d iffers from that at 5 kbar by a lower Fe2+ content (2.84 instead of 3. 12 pfu) and higher Al content (0.60 instead of 0.32 pfu) at T2. Mg-sta urolite synthesized at 25 kbar differs from the 25-kbar Fe-staurolite by higher occupancy of Mg at M4 (0.88 pfu), compared with 0.20 Fe2+ pf u in the case of Fe-staurolite, accompanied with less Mg at T2 (2.12 p fu) and only a small amount of Al at T2 (0.16 pfu).