Low-Temperature Syntheses of Olivine and Forsterite Facilitated by Hydrogen Peroxide

The use of an iron carbonyl, in combination with metal alkoxides, produced a new route to precursors for olivine ((Mg_0.9Fe_0.1)₂SiO₄, Fo₉₀). The principal iron-containing intermediates, pentacarbonyliron and a low-valent, anionic metal carbonyl, produced by the reduction of Fe₂(CO)₉by magnesium metal and Mg(OMe)₂in the presence of Si(OEt)₄, were soluble in methanol. The iron-containing components were smoothly incorporated into the sol when the mixture of alkoxides was simultaneously hydrolyzed and condensed by the addition of aqueous hydrogen peroxide. H₂O₂served at least two important functions: (1) it sequestered magnesium, thereby preventing precipitation of magnesium hydroxide; (2) it partially oxidized the iron carbonylate intermediates. After calcination, firing of pre-Fo₉₀powders at 1200–1300 °C in a CO/CO₂atmosphere readily converted them to single-phase, crystalline olivine. Hot-pressing the resulting powder gave fine-grained, dense, solid specimens. DTA data and separate firings in an atmosphere of H₂and CO₂demonstrated that firing temperatures as low as 850 °C were sufficient to produce crystalline olivine. Without iron, synthetic forsterite, Mg₂SiO₄, was the sole product in a process that required no added strong acid or base as a catalyst for hydrolysis of Si(OEt)₄. Calcined forsterite precursor powders were converted to single-phase forsterite that was partially crystallized at 750 °C and fully crystalline at 1000 °C.