HYDRATION OF SYNTHETIC Mg-SILICATES (CIRCUMSTELLAR DUST ANALOGS) IN A HUMID CHAMBER: AN X-RAY DIFFRACTION STUDY

Introduction: Synthetic Mg-and Fe-silicate smokes simulate material in the dusty circumstellar envelope of asymptotic giant branch stars [1], presumably a raw material for planet formation. When immersed in water, Mg-silicate smokes react within days to produce proto-phyllosilicates observable under the electron microscope (EM) [2-4]. Water-immersed Fe-silicate smokes react more slowly [3, 4] although hydration in a humid chamber produces changes detectable by Mössbauer spectros-copy [5]. Annealing of Mg-and Fe-silicate smokes shows a similar difference in reaction rate [6]. This report presents preliminary results of changes in Mg-silicate smoke under hydration in a humid chamber. Additional innovations are: precautions to eliminate oxidation as a reaction mechanism, including preliminary sample treatment under flowing dry nitrogen; analysis by x-ray diffractometry (XRD) of bulk samples. [1] and stored at room temperature. 20 mg of sample was tapped into a 0.5-mm-deep XRD sample holder and subjected to a flow of 5 cm 3 /min of dry N 2 at 33 o C for 10-12 days. The sample in its holder was transferred to the upper plate of a desiccator that served as a humidification chamber with a layer of deoxygenated deionized water at the bottom. The chamber was maintained at 33 o C for 8 days, after which the sample, still under N 2 , was either set into a sealed vessel for transport to the x-ray diffractome-ter or emptied into a vial for future EM analysis. Sample color was visually observed before and after humidification. For XRD the sample was removed from its nitrogen environment and scanned with Cu K α radiation. Results and Discussion: The humidification brought about a color change from a rich brown to white, not unlike the progression from dark grey to white reported by Chizmadia et al. [3]. The significance of the color change is obscure. The preliminary flowing nitrogen treatment had no significant effect on the XRD pattern, which produced fairly sharp peaks. Preliminary analysis suggests the presence in the starting material of periclase, brucite, silicon, and possibly forsterite, but no appreciable enstatite. This result is roughly consistent with those of Rietmeijer et al. [2] obtained by EM-electron diffraction. The XRD pattern after humidification was very different, two broad peaks with maxima at about 0.29 and 0.16 nm spacings, and a shoulder to the 0.29-nm peak at 0.22 nm. This pattern likely signifies the disappearance of Mg oxides leaving only nanocrystals of Mg-silicates.