High-pressure crystal chemistry of Fe3+-wadsleyite, β-Fe2.33Si0.67O4

Abstract

The crystal structure of Fe3+-wadsleyite, (Fe2+1.67Fe3+0.33)(Fe3+0.33Si0.67)O4, was determined by single-crystal X-ray techniques at six pressures to 8.95 GPa. The isothermal bulk modulus is K T 0 = 173(3) GPa [ K T 0′ = ∂ K T /∂ P = 5.2(9)], which is identical within error to bulk moduli observed for normal wadsleyites [β-(Mg,Fe)2SiO4]. Compression of Fe3+-wadsleyite is significantly more isotropic than for β-(Mg,Fe)2SiO4 because Fe3+ substitutes into both Si4+ tetrahedral sites and (Mg,Fe2+) octahedral sites. Ferric iron thus reduces the contrast between tetrahedral and octahedral compressibilities, which in turn reduces the compressional anisotropy. Bond distance analysis and octahedral compressibilities of the three symmetrically distinct octahedral sites reveal that Fe3+ orders preferentially into M1 and M3, while M2 occupancy is close to pure Fe2+.