Synthesis and water contents of Fe2+-bearing cordierites
Boberski, Cornelia; Schreyer, Werner
European Journal of Mineralogy Volume 2 Number 5 (1990), p. 565 - 584
published: Oct 4, 1990
manuscript accepted: May 11, 1990
manuscript received: Dec 6, 1989
ArtNo. ESP147050205003, Price: 29.00 €
Abstract Cordierites with compositions ranging from the Mg- to the Fe2+-end member were synthesized hydro thermally at 700°C/2 kbar/3 days or 650°C/1.5 kbar/ 3 days under the oxygen fugacity of the wuestite/iron buffer. Mossbauer spectra indicate that Fe2+ occupies only the octahedral site M in the cordierite structure. In ESR-spectra no Fe3+ was detected. The mean refractive index nmof the dehydrated synthetic cordierites increases linearly with F = Fe2+/(Fe2++Mg). The additional increase of nmwith rising water contents for Fe2+- bearing cordierites cannot be distinguished from that for pure Mg-cordierite because of the blurring effect of water-bearing voids in all cordierites synthesized hydrothermally. Cell edges a and b increase, whereas c decreases linearly with increasing F. For a given F, cell volume is independent of water content. Based on IR-data, the synthetic MgFe-cordierites are Al,Si ordered. Their distortion indices Δ vary from 0.22° to 0.29°. Infrared spectra show molecular water only with orientation of the H-H vector parallel to the crystallographic c-axis (Type I), which is typical for cordierites with no cations in the channel. Mg-cordierite was hydrated at 600°C/2 kbar using hematite/magnetite-(HM), quartz/fayalite/magnetite- (QFM), wuestite/iron-(WI), quartz/fayalite/iron-(QFI) and Mn/MnO-buffers in order to determine the influence of different water activities (aH2O) on the water content in cordierite. With decreasing aH2O only a slight decrease of water content in cordierite was observed, but its extent is smaller than the theoretically calculated decrease of aH2O m the fluid phase under these conditions. The equilibrium water contents of synthetic cordierites with F = 0.00, 0.25, 0.50, 0.75 and 1.00 (at WI buffer) were determined in the range of 500°C-750°C and 2-9 kbar. As in pure Mg-cordierite, the water content in Fe2+-bearing cordierite increases with increasing pressure and decreases slightly with increasing temperature. Within the error of measurement no systematic change of the water content in relation to F was found.