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 (Mg0.9Fe0.1)2SiO4, Fo90). The principal iron-containing intermediates, pentacarbonyliron and a low-valent, anionic metal carbonyl, produced by the reduction of Fe2(CO)9 by magnesium metal and Mg(OMe)2 in 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. H2O2 served 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-Fo90 powders at 1200-1300-degrees-C in a CO/CO2 atmosphere 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 H-2 and CO2 demonstrated that firing temperatures as low as 850-degrees-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-degrees-C and fully crystalline at 1000-degrees-C.