| Research Abstract |
Rodingite derived from hornblende gabbro occur in serpentinite of the Mineoka tectonic belt, Chiba. Plagioclase in the original gabbro changes progressively to thomsonite, prehnite and hydrogrossular, hornblende to cummingtonite and chlorite, and orthopyroxene to talc. Four metasomatic zones such as the least altered zone, thomsonite zone, prehnite-hydrogrossular zone and hydrogrossular zone are recognized from gabbro to serpentinite. The necessary amount of calcium for the formation of rodingite is supplied from both of hornblende in the original gabbro and surrounding peridotite. The water content of hydrogrossular increases with the metasomatism: i.e. The 2X value of Ca_3Al_2[SiO_4]_<3-X>[O_4H_4]_X increases to 1.2-2.1 in the prehnite-hydrogrossular zone, and 1.9-2.4 in the hydrogrossular zone.
The process of rodingitization has been experimentally revealed by hydrothermal annealing of grossular which is the important mineral in rodingite. When the glass having the grossular composit
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ion is hydrothermally treated, the product is hydrous grossular whose water content decreases with the increase of run temperature. Such hydrous garnet is considered to be a metastable phase due to a short run duration. As stated below, however, hydrous grossular is formed from water-free garnet by hydrothermal annealing. This implies that hydrogrossular is not metastable but stable at low temperatures.
Water-free grossular was hydrothermally synthesized from the glass at 660ﾟC and 695ﾟC for various durations (0minute, 5 minutes, 2 hours, 14 hours and 6 days) under a fluid pressure of 100 MPa using a test-tube type apparatus. The products were starting materials for the study of the hydration of garnet. Each material was hydrothermally annealed at 450ﾟC or 350ﾟC under the same fluid pressure. Materials whose holding time are longer than 2 hours do not show remarkable change in the d_<420>values at a low temperature in the treatment of such short duration. On the contrary, remarkable increase of the d_<420>values was recognized in the products derived from the materials of 0 and 5 minutes holding. The ratio of [O_4H_4]/([SiO_4] +[O_4H_4] is inferred to be 12 % at 450ﾟC and 18 % at 350ﾟC, respectively. The IR pattern of these annealed materials also show that [SiO_4] thtrahedons are replaced by [O_4H_4] ones. Less
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