REFINEMENT OF THE STRUCTURE OF NATURAL FERRIPHLOGOPITE

Two ferriphlogopite-1M crystals with a composition )(Sigma=2.99)[(Fe0. 953+Si3.05)(Sigma=4.00)O-10.17] (OH)(1.79)F-0.04 (sample S1) and e0.95 3+Si3.05)(Sigma=4.00)O-10.18](OH)(1.75)F-0.07 (sample S2) occur within an alkali-carbonatic complex near Tapira, Belo Horizonte, Minas Gerai s, Brazil. Each crystal was studied by single-crystal X-ray diffractio n. The least-squares refinements of space group C2/m resulted in R val ues of 0.031 for S1 and 0.025 for S2. Results showed that Fe3+ substit utes for Si within the tetrahedral sites and that the Fe distribution is fully disordered. The octahedral sites are preferentially occupied by Mg. The presence of Fe3+ within the tetrahedral sheet produces incr eased cell edge lengths. For sample S1, a = 5.362 Angstrom, b = 9.288 Angstrom, c = 10.321 Angstrom and the monoclinic beta angle was: beta = 99.99 degrees. For sample S2, a = 5.3649 Angstrom, b = = 9.2924 Angs trom, c = 10.3255 Angstrom and the monoclinic beta angle was: beta = 9 9.988 degrees. The tetrahedral rotation angle of alpha = 11.5 degrees is necessary for tetrahedral and octahedral sheet congruency. The enla rged tetrahedral sites are regular, with cations close to their geomet ric center. Ferriphlogopites have identical mean bond lengths for M1 a nd M2 sites within standard deviation. The M1-O3 and M2-O3 bond length s are longer than the mean so that O3 may articulate with the tetrahed ra.