High-pressure crystal chemistry of Fe (super 3+) -wadsleyite, beta -Fe (sub 2.33) Si (sub 0.67) O (sub 4)
High-pressure crystal chemistry of Fe (super 3+) -wadsleyite, beta -Fe (sub 2.33) Si (sub 0.67) O (sub 4)
American Mineralogist (June 2000) 85 (5-6): 778-783
A single crystal XRD study of Fe (super 3+) -wadsleyite [beta -(Fe (super 2+) (sub 1.67) Fe (super 3+) (sub 0.33) )(Fe (super 3+) (sub 0.33) Si (sub 0.67) )O (sub 4) ] showed that at room-P unit-cell parameters are a 5.8590, b 11.8557, c 8.3772 Aa and V 580.91 Aa (super 3) . These values differ significantly from those of a hypothetical beta -wadsleyite in that the substitution of 2Fe (super 3+) for Fe (super 2+) + Si increases a and c, while decreasing b. The bulk modulus (K (sub T) (sub 0) ) is identical within experimental error ( approximately 173 GPa) for the two phases; linear compressibilities, as reflected by compression of the three crystallographic axes, are significantly greater for beta -wadsleyite however. In both normal and Fe (super 3+) -wadsleyite the b and c axes are least and most compressible, respectively. The wadsleyite structure is based on four symmetrically independent cation-oxygen polyhedra: three octahedral sites M1, M2, and M3 and one tetrahedral site (T). Half of Fe (super 3+) substitutes for Si on the T site; the remaining 0.33 Fe (super 3+) plus 1.67 Fe (super 2+) cations per formula unit are distributed over the three octahedral sites. 22% occupancy of Fe (super 3+) in both M1 and M3 is consistent with the observation that the Fe-O distances for M1 and M3 are approximately 0.025 Aa shorter than predicted for a pure Fe (super 2+) phase; however, the M2 site incorporates little, if any ferric iron.