Variation of refractive index of synthetic Mg-cordierite with H2O content


The mean refractive index of grain mounts has been measured for 50 synthetic end-member Mg-cordierites with water contents varying from 0.83 to 2.9O weight percent. A linear relationship appears adequate to describe the variation of the mean refractive index with water content and can be formulated as W : [n(m) n(mo)]/0.00762 (esd:0.13 weight percent H2O) where W : HzO content in weight percent; n(m) : mean refractive index of hydrated Mgcordierite; and n(mo) : mean refractive index of the anhydrous starting material. The calibration curve is useful to determine water contents of Mg-cordierites in multiphase reaction products of the appropriate experimental systems. No change in the distortion index upon hydration of anhydrous Mg-cordierite has been observed, so the Stout model of cordierite hydration cannot be applicable to end-member Mg-cordierite. Introduction Pure Mg-cordierite, MgrALSi'O,s'nH2O, is an essential and common phase in the synthetic system MgO-AlrOr-SiOr-HrO, which is an important model, or critical subsystem of nodels, for most pelitic rocks (e.g. Schreyeq1976). As a result, Mg-cordierite plays an important part in many experimental and theoretical studies directed toward the investigation of progressive metamorphism in pelitic sediments, so that a detailed knowledge of the stability relations of this mineral is critical. From studies of natural cordierites (Leake, 1960; Iiyama, 1960; Schreyer, 1965), as well as experimental investigations of the system Mg-cordierite-H'O (Schreyer and Yoder, 1964; Mirwald and Schreyer, l9?7), cordierite is known to incorporate significant and varying amounts of water. Most investigators concur that this water is held in the molecular state in the open channels of this framework structure (Smith and Schreyer, 1962; Schreyer and Yoder, 1964; Farrell and Newnham, 1967; Goldman et al., 1977),but the question ofwhether or not hydrogenbonding to the silicate framework occurs is still debated (Stout, 197 5, 197 6; Langer and Schreyet, 197 6; Schreyer et al.,1979). Variable water content has a signiflcant effect on 0003-004x/80/030,H367$02.00 the stability relations of cordierite (Newton, 1972; Mirwald and Schreyet, 1917).In order to assess these relations both experimentally and thermodynamically, the equilibrium water content should be known for a given temperature and pressure. As discussed by Schreyer and Yoder (1964) and Newton (19'12), however, the direct determination of equitbrium HrO in quenched Mg-cordierite samples meets with serious problems, because of the extremely rapid exchange of water between the cordierite channels and the aqueous vapor phase during quenching' During our investigations on the hydration behavior of Mg-cordierite (Mirwald et al., 1979), we have found that with a quench of l0 seconds or less it is of mean refractive index vs. water content given by Schreyer and Yoder (1964), and thus provide a basis for routinely measuring equilibrium water contents of Mg-cordierite even in multiphase reaction products. The above relationship also forms a basic part

4 Figures and Tables

Cite this paper

@inproceedings{Medenbach2007VariationOR, title={Variation of refractive index of synthetic Mg-cordierite with H2O content}, author={Olaf Medenbach and WALTER V. MnneSCH and PETER W. MmwnLD}, year={2007} }