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[PPS22-04] Hydrothermal and heating experiments: Implications for formation process of nepheline in carbonaceous chondrite
Keywords:nepheline, melilite, aqueous alteration, hydrothermal experiments, dehydration heating experiments, carbonaceous chondrite
We used i) synthetic melilite (gehlenite, Ca2Al2SiO7) + SiO2, and ii) natural plagioclase (An48) as starting materials. Four reaction solutions of 1N (mol/l)-NaOH (pH 0), H2O (pH 7), 0.1N-NaOH (pH 13) and 1N-NaOH (pH 14) were reacted with three different water/rock ratios (10, 100 and 1000). Na+ concentration in all solutions was maintained at 1 mol/l by addition of NaCl. Hydrothermal alteration experiments were performed with a PTFE reaction vessel loaded into steel autoclave and were carried out at temperature at 200 ℃ and pressure of 1.5 MPa for run duration time of 168 hours. The recovered samples were analyzed by XRD, SEM, TEM, and TG-DTA.
In the experiments using melilite+SiO2 with W/R = 100 and pH 7-14 solutions, hydrogrossulor (Ca3Al2[SiO4]1.6[O4H4]1.3) formed as a secondary phase with low SiO2 contents, while analcime (NaAlSi2O6H2O), nepheline-hydrate (NaAlSiO4H2O), basic-cancrinite (Na8Al6Si6O24[OH]2•2H2O) and tobermorite (Ca5Si6O16[OH]2•4H2O) formed with high SiO2 contents. Although no secondary phase was observed at high W/R (=1000), analcime and basic-cancrinite occurred at low W/R (=10). Under the pH 0 solution, melilite was dissolved and no secondary phases formed. In the experiments using plagioclase with W/R = 100 and pH 0-7 solutions, no secondary phases formed. On the other hand, under pH 13-14 solutions, analcime and nepheline-hydrate formed with W/R = 10 and 100.
A TG-DTA of nepheline-hydrate at heating rate 5 ℃/min in air showed that the dehydration started at ~100 ℃ and finished at ~800 ℃. From the DTA and XRD data of this condition, nepheline-hydrate is considered to be transformed into nepheline at ~800 ℃. From the TG-DTAs of nepheline-hydrate with different heating rates, we estimated that nepheline-hydrate could alter to nepheline for at least ~103 years at 400 ℃. A TG-DTA of analcime at heating rate 2 ℃/min in air showed that dehydration started at ~120 ℃ and finished at ~500 ℃. The XRD data showed analcime is transformed into nepheline and amorphous material below 1000 ℃, although we failed to estimate the required time of the transformation because of the DTA curves of analcime were too complicated.
The present results show melilite and plagioclase are easily altered into hydrous minerals like nepheline-hydrate and analcime in Na+-rich high alkali solutions. Because the experimental condition demonstrated in the present study seems to be comparable to actual carbonaceous chondrite parent body, we infer that nepheline in carbonaceous chondrite formed initially as intermediate products like nepheline-hydrate and analcime during aqueous alteration, and these hydrous minerals was subsequently dehydrated to nepheline.